lOkffl Withnell Bay VEGETATION lOOtn BCA 2 ' 500 m BCA 2 Withnell Bay SANDY TIDAL FLAT | SALT FLAT | HIGH TIDAL ALLUVIAL FAN JOURNAL OF THE ROYAL SOCIETY OF WESTERN AUSTRALIA VOLUM PART 2 1987 200 100 4.4 Av 2.8 Age 2.6t jtZ ■ ^ 'e -a* fc-- TRANSECT Cl o>»d for« »t I |Clo»«d tcrub Clo fd 10 op»n >c fub f < tH I Op»n h»tth i lOpgn hfth M.H.W.N M.a.L __i THE MANGROVES OF THE DAMPIER ARCHIPELAGO WESTERN AUSTRAL! Lpgandr* l•l•n Locality o ^ Qldity Island >> Angsl Island "C t Rosamary Island Dolphin Island Conzinc Bay Weal Lewii^ Island WIthnall Bay King Bs_yjg r ^ W /dampier Enderby Island A/ IMsat Intercourse Island HABITAT THE ROYAL SOCIETY OE WESTERN AUSTRALIA PATRON Her Majesty the Queen VICE-PATRON His Excellency Professor Ciordon Reid, Governor of Western Austral COUNCIL 1986-1987 President I. S. P. Beard, M.A„ B.Se., D. Phil. Vice-Presidents .1. T. Tippett, B.Sc., Ph.D. ,1. S. Pate, F.R.S., B.,Se., Ph.D. Past President S. .1. Hallani. M.A. Joint lion. Secretaries V. Hobbs, B.Se. (Hons.). Ph.D. K. W. Dixon, B.Se. (Hons.), Ph.D. llon.T reasurer W. A. Cowling, B.Agric.Sc.(Hons.), Ph.D. Hon. Librarian H. E. Balmc. M.A.. Grad. Dip. Lib. Stud. Won, Editor B. Dell, B.Sc. (Hons.). Ph.D. .1. Backhouse. B.Sc., M.Sc. A. E. Cockbain, B.Sc., Ph.D. S. J. Curry, M.A. E. P. Hopkins, B.Sc., Dip.For.Ph.D. L. E. Koch. M.Sc.. Ph.D. ,1. D. Majer, B.Sc., D.I.C., Ccrl.Ed., Ph.D. K. McNamara, B.Sc.,(Hons.), Ph.D. ,1. Webb, B.Sc., Ph.D., Dip.Ed. 1 CONTENTS Page ABSTRACT 1. INTRODUCTION 30 2. METHODS 31 3. REGIONAL SETTING Climate 32 Oceanography 34 Regional coastal morphology 34 4. PHYSICAL FRAMEWORK AND HABITATS Geomorphology 34 Substrates 39 Stratigraphy.. 40 Groundwater/Soilwater 42 5. BENTHIC FAUNA 44 6. CLASSIFICATION OF MANGROVE HABITATS IN THE DAMPIER ARCHIPELAGO The approach to classification 45 The mangrove habitats of the Dampier Archipelago 45 Synthesis 48 7. MANGROVE VEGETATION Species pool 48 Notes on species 50 Fioristic/slructure classification 50 The Mangrove assemblages 50 Description of assemblages 53 8. DESCRIPTION OF SELECTED AREAS King Bay 81 Withnell Bay 81 Conzinc Bay 8 1 Searipple Passage 83 Angel Island/Gidley Island 83 Malus Islands 83 Lewis Islands 84 Enderby Island 84 West Intercourse Island area 84 9. DISCUSSION 85 10. REFERENCES 87 51396-1 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. Mangroves of the Dampier Archipelago, Western Australia V. Semeniuk' and P. A. S. Wurm^ *21 Glenmerc Road. Warwick, W.A. 6024 -P.O. Box 186. North Perth. W.A. 6006 Manuscript received 17June 1986; accepted 20 October 1986. Abstract The mangrove habitats, vegetation assemblages and vegetation structural and florislic types along the coast of the Dampier Archipelago occur in definitive and discrete locations that arc related to coastal evolution, gcomorphology, and a range of physico-chemical and biological factors. I'hc disposition and evolution of coastal environments has been determined by the interaction of the oceanography/meleorology with the ancestral geomorphology of the Prccambrian rock hinterland. The large scale coastal geomorphic units which have developed are: straight sandy and rocky coasts; embayments and straits: and connective tidal lands. Medium scale, intertidal geomorphic units developed within the large scale setting include: rocky shores, gravelly, sandy and muddy tidal Hats, spits and cheniers, sandy beaches, tidal creeks, shoals, tidal lagoons, mid and high tidal alluvial fans, limestone pavements and the hinterland margin. Habitats for mangroves and an associated mangrove assemblage are developed on these geomorphic units in response to salinity, tidal level and substrate. Various selected areas in the region are described to provide an indication of the variability of mangrove habitats and vegetation. There is a heterogeneous distribution of habitats in each of the areas at the small and medium scale, with the embayment setting containing the most heterogeneous aggregate of habitats. However, while small scale heterogeneity is emphasised it is also noted that many habitats arc recurring. For instance, rocky shores are widespread units and may be considered to be ///c most common habitat in the region. Other units, while discrete in occurrence and limited in area, also are quite widespread when viewed regionally. Mangroves are most luxuriant, abundant, and diverse in embayments and connective tidal-lands. The most species rich, zoned mangal occurs in two main environments: (1) on some alluvial fans and (2) peripheral to sandy terrains, such as spits and beaches. Six mangrove species occur in the Dampier Archipelago area: Avicennia njarina. Ae.i'ialitts annulata, Acgiceras cornkiilalum. Hru^mcra exanstata, Ccriops ta^al and Rhizophora siylosa. Environmental forcing factors can only influence three aspects of mangrove populations; their composition, structure and maintenance. Since there is only a small species pool in the region, environmental gradients and difterences between habitats therefore arc reflected in a variety of structural and physiognomic types within the limited tloristic assemblage. Although the vegetation, comprised of structural/floristic units, that occurs on a given habitat may vary locally, generally recurring patterns of distribution can be determined. The muddy tidal flats support scrub of a seaward Rhizophora zone and a landward Avicennia zone. Sandy tidal flats support a physiognomically-zoncd Avicennia, grading from forest at its seaward edge, through scrub to heath or low shrubland. Rocky/bouldcry shores support a monospecific Avicennia shrubland or scrub. Spits and cheniers support zones of Avicennia shrubland. Ceriops scrub, Hruyuiera scrub and Avicennia scrub. Beaches support an Avicennia scrub which may be mixed with Ceriops, Hntyuiera, Aegialitis and Ae^^iceras, Avicennia scrub occurs along the hinterland margin, along high tidal dune margins, and along high tidal lagoons. Tidal creek banks are vegetated by Rhizophora and/or Avicennia forest or scrub. Small accretioncry shoals in tidal creeks support an Aeyiceras scrub with an undcrstorcy q( A eytalitis. Limestone pavements support an Avicennia shrubland. Gravelly tidal flats support an Avicennia scrub, fligh tidal alluvial fans support a mixed Ceriops and Avicennia shrubland or scrub. Mid-tidal alluvial fans support a physiognomically zoned Avicennia scrub with patches of Rhizophora scrub. The approach adopted in this study has tended to emphasise the physico-chemical factors that lead to coastal heterogeneity and habitat variability at various scales of reference. I'hus this paper provides a framework within which the distribution and internal heterogeneity of mangrove formations may be understood by resolution of the essential features of habitat evolution, maintenance and distribution. Mangroves then arc viewed simply as opportunistic inhabitants that respond to the variable environment by changes in their composition, physiognomy, structure and maintenance strategics. 29 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Introduction This paper reports on studies conducted over several years on the mangroves of the Dampier Archipelago (Fig. 1). Previous publications have dealt with regional geology (Geological Survey of W.A. 1980). climate (Bureau of Meteorology 1973. 1976). both marine and terrestrial flora and fauna (Wilson and Marsh 1974. Beard 1975, Blabcr cf a/. 1985, Storr 1984). The first most comprehensive published works on the natural history of the area dealt with the coastal landforms. habitats and biota, with brief description of the littoral vegetation (Burbidge and Prince 1972; Semcniuk c! af. 1982). Mangrove species in mangals (mangal = “mangrove community" after MacNae 1968) of the Dampier Archipelago have been listed by Semeniuk et al. (1978) as part of a regional study of Western Australian mangroves. Published data on mangrove environments of Dampier Archipelago also are provided by: Bridgewater (1982) who applied the classification approach of (’hapman (1977) to some localities in this region; Semcniuk (1983), in a regional study of freshwater seepage and its effect on mangrove distributions; and by Semcniuk (1985), in a regional study of ria type shorelines and their habitats. These studies do not provide information about the variability and detail of mangrove environments of the Dampier Archipelago to the level presented here. OCEANIC ZONE Dolphin Island Angel Island Searipple Passage DAMPIER ARCHIPELAGO Conzlnc Bay ^ West Lewis Island East Lewis Island ntercourse Island DAMPIER West Intercourse Island MAITLAND DELTA COMPLEX KARRATHA HINTERLAND .J ^VEST + , , c o astal + + TTTTT-r<’5“T^ + OCEANIC ZONE Tidal lands Bedrock and alluvial plains Legendre Island NtCKOL BAY COMPLEX Figure 1.— Locality diagram showing geographic features of the Dampier Archipelago mentioned in this paper, and location of the regional scale geomorphic units. 30 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. The objectives of this paper arc to provide: • a description of the Dampier Archipelago mangrove environments in terms of coastal morphology, stratigraphy, substrates and hydrology, all of which arc useful in understanding the physico-chemical basis of the mangrove systems: • a classification of mangrove habitats of the Dampier Archipelago; • a description of the distribution, structure and composition of mangrove assemblages; • a description of selected representative mangrove areas of the region. As a result, this paper provides a philosophy of approach in describing mangrove systems by classifying mangrove habitats as a prerequisite to analysing mangrove assemblages. Other studies elsewhere to dale have tended to emphasise the florislic or slruclurai/floristic attributes of mangrove formations but the approach adopted here has emphasised the physical basis of mangrove distribution and mangrove compositional and structural heterogeneity. Methods In documenting mangrove environments, it is necessary to use an interdisciplinary approach, utilising data on regional selling and processes with that of the Figure 2. — Map showing location of study sites. Insets show location of more detailed maps that arc presented later in the paper. 31 Journal of Ihe Royal Society of Western Australia, Vol. 69, Part 2, 1 987. local cnvironmenl. and integrating data on physiography, oceanography, hydrology, chemistry, scdimcnlology, soils, stratigraphy, and biology. Terminology in this paper for the scalar frames of reference and the geomorphic units therein is after Semeniuk (1986). In the documentation of mangrove environments of Dampier Archipelago aerial photographs and Held surveys were used to identify the main large-scale coastal types. The various phototones developed by Ihe different substrate types, habitat types and mangroves were then investigated by continuous traverses through the tidal zone and niangaf. Spot checks of numerous other sites that had distinctive and/or unusual pholotones on aerial photographs also w'ere carried out. The main mangrove and adjoining environments readily identified by pholotones and consequently investigated were: (I) mangroves on alluvial fans, (2) mangroves fringing the hinterland, (3) high-tidal mangroves fringing spits/cheniers, (4) stands of Ccriops, (5) stands of Rhizophora, (6) stands of Avicennia (with or without other species), (7) salt flats, (8) terrestrial parts of spils/cheniers, (9) mangroves on rocky shores, and (10) mangroves on mid tidal shoals. Additional geomorphic and vegetation units were identified on site. At the large scale, the entire coastline of the Dampier Archipelago was surveyed for this study by boat and by helicopter and information from these surveys, together with analyses of aerial photographs, forms the data base for regional distribution of mangrove types. Consequently the maps, cross sections and transect locations shown in the tlgurcs, presented later, represent only a selection of the entire data base upon which this paper is based. At the medium and small scales, some 20 selected areas and 70 transects were investigated in detail to study geomorphology. substrate. stratigraphy, soil/groundwater salinity, fauna and flora (Fig. 2). The selected areas were mapped in terms of habitats, mangrove assemblages and vegetation (structure and floristics). The mapping involved ground traverses and low altitude aerial surveys. Transects were then selected for more detailed study. The type of information obtained from transects is presented in Fig. 3. Mangrove composition, height, canopy structure and distribution were documented. At selected intervals, usually in the middle of a zone or pholotone, the composition and density (i.c. number of individuals) of the vegetation was quantified within five replicate 5 m x 5 m quadrats. In many locations ten. fifteen or twenty replicates were used because of low numbers of individuals or variability of composition, In total some 2 000 quadrats were documented. Fauna were sampled and counted using box cores, or were counted in 5 m x 5 m or 1 m x 1 m quadrats. Cryptic fauna were observed and collected from crevices, logs and mangrove trunks and branches. Conclusions pertaining to the function of fauna in mangrove environments were obtained by observing faunal behaviour and by field experimentation. Substrate and groundwatcr/soilwatcr were sampled at the same localities as vegetation study sites. Soil, excavated up to 100 cm deep, depending on the depth of the water table, was described in terms of colour, structure, fabric, texture and composition. Samples of the surface soil and the groundwater table were collected for salinity analyses. Soil samples were hermetically sealed, frozen, and thawed when ready for laboratory analysis. Cjroundwaler samples, and water extracted from soil samples, were filtered through a 0,045 /xm tiller and analysed for total dissolved solids by evaporation technique. Soil samples were subsequently sieved at 1 phi intervals for granulometric analyses. The stratigraphy of sites was investigated by pits (0.3- 1.2 m deep), auger (up to 3 m deep), by coring, and by probing with an extendable rod to 6 m deep. The following rock/sediment types could be differentiated in the subsurface by probing; (1) mud. (2) sand. (3) muddy sand, (4) inlerlaycd sandy and muddy sediment, (5) gravelly sediment, (6) rock. Terminology for the geometry of stratigraphic units is after Krynine (1948) and Fairbridge ( 1 968). Description of Dampier Archipelago Environment: Regional Setting Climate The Dampier Archipelago coastal cnvironmenl has a tropical arid climate (BWh of Kbppen 1936) with two major seasons; a wet summer season and a dry. warm winter season. Gcnlilli (1971) considered the area as "pscudo-monsoonaf*. Bureau of Meteorology (1973. 1975) data show that the area has a mean of some 33 rainy days per year w ith about 315 mm of precipitation. Mean maximum and minimum temperature in summer in February is 37.1 C and 26.4 (' respectively. Mean maximum and mean minimum temperatures in winter in July arc 26.3"C, and I4.0"C, respectively. Mean evaporation is 3.518 mm per year with a mean monthly minimum of 193 mm during July, and a mean monthly maximum of 384 mm during December. Wind originates from various sectors dependent on the lime of day and season (Bureau of Meteorology 1 973). The area may be considered to have 2 major wind seasons, winter and summer (Steedman 1985). The period between the major seasons is a time of fluctuating winds and general calms. In winter, a high pressure anticyclone belt controls the regional wind pattern with development of easterly through to southerly winds (Genliili 1971, 1972; Bureau of Meteorology 1973, 1976) . In summer strong southwesterly through to northwesterly winds arc generated by the interaction of a semi-permanent low pressure trough and the adjoining high pressure systems (Steedman 1985). Landbreezc/scabreeze effects occur throughout the year and interact w ith the seasonal regional wind pattern. For instance, landbrcezes emanating from southeasterly quadrants reinforce the winter regional paltern, but the winter scabree/cs moderate the regional wind paltern. In summer, sea breezes reinforce the westerly regional wind pattern and landbrce/cs moderate the westerly flow. As a result of interactions of regional wind patterns, winter is characterised by wind emanating from easterly, southeasterly to southerly directions, with moderate wind from northwest in the afternoon, whilst summer is characterised by strong northwesterly, westerly and southwesterly wind and moderated wind at night. The wind data, based on 15 yrs record (Bureau of Meteorology 1 983), are summarized in Figure 4. Summer is also the period of local thunderstorms and tropical cyclones which impinge on the coast. Coleman (1971) and Lorensz (1981) show that a cyclone passes within 100 km of this coast about every 10 years, and within 150 km about every 5-10 years. The cyclones are associated with destructive winds, substantial rainfall and storm surges, and arc effective agents in mobilising coarse sediments into storm ridges. 32 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 3. — Schematic profile showing the range of information (numbered i-7) gathered from field surveys along a transect. This diagram also indicates the style in which data are presented later in Figures 24-30. showing; A. gcomorphic, stratigraphic, vegetation structure and tlorisiics results: and location of sampling sites: the vegetation is drawn to scale; B, histograms of grain si/e distribution of the substrate; f', graph showing salinity of groundwater and soilwater at the various sampling sites across the transect; and D. information on the abundance of mangrove species at each sampling site; abundances arc expressed as mean standard deviation; the most abundant species at each site are highlighted by bolder and larger typeface. The mangrove names Avicenniu. Rhtzophora. i'enops. Hru^uicra. ar\6 Ir,?/a'rav are abbreviated tO -'Xvi. Rhi. ('. Br, Agl and Age in the later diagrams. 33 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Oceanography The tides of the Dampicr Archipelago are macrotidal; the maximum range is 6.3 m; the mean spring range is 5.6 m and the mean neap range is 1 .0 m (Australian Tide Tables 1985). Long period swell originates in the Southern Indian Ocean and is refracted by the regional bathymetry to approach the Dampier Archipelago mainly from northwesterly and northerly sectors. The swell subsequently is variably refracted and diffracted around the complex bathymetry and islands of the archipelago. Landbrec/c/scabrceze systems and thunderstorms generate local short period wind waves which are an important prevailing feature in the coastal zones. Wind waves follow the wind pattern and consequently there is a dominance of locally developed waves from west, northwest and southwest, through to southeast. As a result of the dominant directions of swell and windwaves the portions of coastline exposed to the wave trains have geomorphic features dominated by wave action (c.g. spits, beaches, sand bars, see Figure 4). Periodically and especially during cyclones there is discharge of freshwater from the adjoining rivers in the region into the marine environment. There is also input of freshwater from the surrounding hinterland by sheet flooding and by the numerous smaller creeks and streams. Subterranean freshwater seepage from the hinterland also continues as an important phenomenon well into the dry seasons (Semeniuk 1983). Regional Coastal Morphology Dampier Archipelago is a complex some I 000 krn- in area. The Dampier Archipelago region has been divided into five main units by Semeniuk et ai (1982). These units are; the Hinterland. Nickol Bay Complex. Maitland Delta Complex, Oceanic Zone and the Dampier Archipelago. Their distribution is shown in Figure 1. The units are described briefly below' to provide a context and definition of the Dampier Archipelago unit in its regional setting. The Hinterland is composed of rocky hills, ridges, colluvial slopes, alluvia! Oats, river courses, sandplains and subacrial portions of deltas. The bedrock geology of the hinterland consists of folded meta-scdimenlary and meta-volcanic rocks and irregularly ovoid domes ot granite (Geological Survey of Western Australia 1980). The margin of the hinterland within the Dampicr Archipelago may be irregular and diffuse where low- lying supratidal flats, extensive high tidal salt flats, alluvia! valleys and deltaic lobes form a broad zone of contact. The Nickol Bay Complex is a large shallow marine embaymcnl with shoreline tidal flats, dunes, limestone pavements and rocky shores. The contact with the Dampicr Archipelago is sharp and located at the eastern margin of the chain of bedrock islands. The Maitland Delta Complex is a coastal lowland comprised of deltaic landforms such as sand ridges, riverine and tidal channels and tidal Hats. The contact with the Dampier Archipelago is gradational and placed at the outer limit of the deltaic sand ridges. The Oceanic Zone characterised by deep oceanic environments (greater than 20m depth) has an abrupt contact with the Dampicr Archipelago marked by a sleep slope. The Dampicr .Archipelago is an inundated landmass similar in topography to the present hinterland. The ancient arid zone landforms of the region, consisting of elevated ranges, ridges, hills, hillocks, plains and alluvial fans, have determined to a large extent the configuration and disposition of coastal and marine morphology and environments within the Dampicr Archipelago. A gently inclined undulating submerged plain now occurs 5-20m below mean sea level (e.g. Mermaid Sound). Inundated hills and ridges rise up from this plain, the lowest forming subtidal rocky reefs, the highest forming the numerous islands of the archipelago. Inundated valleys form the embaymcnts, straits and channels of the archipelago. Much of the coastline is Precambrian igneous rock. Pleistocene limestone is locally plastered onto this rock. Overlying all rock types is a veneer of varying thickness of sand, gravel or mud. Fach of these substrate types occurs in a range of locations w hich vary according to height above or below mean sea level, topographical configuration and exposure to waves and currents. Consequently, numerous medium to small- scale geomorphic units can be differentiated within the Dampier Archipelago mainly on substrate type, geometry and substrate surface and tidal level. The units have developed by a combination of factors which include: erosion versus sedimentation, underlying stratigraphic sequence, tidal level, w'avc action and ancestral (pre-Holocene) physiography. The geomorphic units are: (I) Intertidal rocky shore, (2) Subtidal rocky shore. (3) Intertidal limestone pavements. (4) Subtidal limestone pavement. (5) Intertidal flats, (6) Intertidal beaches, (7) Subtidal shoals, and (8) Subtidal plains. A description of these medium scale geomorphic units, their distribution and their occurrence within regional coastal morphology types of the Dampicr .Archipelago have been presented in Table 2 of Semeniuk et al. (1982). PHYSICAL FRAMEWORK AND HABITATS Geoniorphology Prior to the post-glacial transgression, the Dampier Archipelago was a dissected terrain which had a subdued drainage, scattered subplanar lowland plains, linear ranges and ridges, scattered hillocks, knolls and hills, and riverine courses. Marine flooding and sedimentary infill of this terrain has resulted in the development of deep water sounds, narrow embayments, broad embaymcnts. islands and connective tidal-lands between these islands (Semeniuk et al. 1982). For purposes of describing mangrove habitats the shorelines of this island-dominated system al the large scale can be subdivided into the following inlergradalional units (Fig. 5): (1) Straight coasts, which are composed of open rocky- houlder shores or open sandy beach-dune shores. (2) Embavnients and straits, where valley courses have been flooded by the post-glacial transgression and infilled with sedimentary deposits: these cmbaymcnts/slraits vary from' narrow' to broad and from small (e.g. Noname Creek) to large (c.g. King Bay). Many of the embayments arc exposed to waves at their entrances such that rew'orking ot sediments has developed spils/chcnicrs. ‘ (3) Connective tidal-lands, which develop where hills of the original terrain have been surrounded and isolated by tidal deposits to form islands. Some islands, where exposed to wave action, have bordering spits but mostly they are protected and have muddy connective tidal-lands which Join with the mainland or other islands. The islands and connective tidal-lands are gradational in size from large-scale features such as the West Intercourse Island system to small hillocks on the tidal flat. 34 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 4. — Relationship of oceanography/metcorology to coastal features. A. Summary of surface wind analysis which incorporates 15 years data from Bureau of Meteorology (1983). B. Schematic diagram showing how various coastal features such as muddy embayments. spits and sandy coasts are developed by oceanographic/meleorologic factors. C. D. and E. The influence of oceanographic/metcorologic factors on the coastline of Searipple Passage (C). Enderby Island (D) and the western shore of Burrup ‘'Island” from King Bay in the south to Conzinc Bay in the north (E). 51396-2 35 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. I he medium-scale coastal geomorphology of the Dampier Archipelago area has been determined by the smallcr-scalc geomorphology of the ancestral bedrock topography and by processes of marine sedimentation and erosion. For example, protected embayments have accumulated a mud wedge (tidal flat) which is dissected locally by tidal creeks. Scmi-proiecled embayments have accumulated a sand sheet or mud wedge (tidal flat) with encroaching sand spits formed by wave reworking and tidal dispersion. Exposed straight coasts either have rocky shores winnowed free of major sedimentary accumulations, or have spits which develop muddy accumulations on their protected side, or have sandy beach shores. Subsequently, depending on the range of interacting physical processes, such as mud and sand sedimentation, tidal creek erosion, wave erosion, or conditions of sedimentary equilibrium, there has developed a wide range of medium scale tidal zone geomorphic units (Fig. 6). These units generally are developed either peripheral to the main shorelines along large islands, or circumferential to smaller islands. A description of the physical features and origin of the medium scale units follows. (1) Hinterland margin: a unit (10-50m wide) at the junction of the rocky hinterland and the tidal flat that is underlain by reworked colluvium or muddy sand washed off the hinterland; it is inundated only on the highest tide and is subject to freshwater seepage; it should be noted that this unit is distinct from a high tidal dune margin. (2) Alluvial fan: alluvial accumulations, fan to deltoid in shape, that form in mid to high tidal environments where supratidal, terrestrial creeks and streams debouch onto tidal flats; this unit occurs particularly at the mouths of large creeks; substrates are sandy and gravelly and mixed with mud; the fans arc subject to freshwater seepage. 36 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. MEDIUM SCALE TIDAL ZONE AND COASTAL GEOMORPHIC UNITS LEGEND Bedrock hinterland Muddy tidal flat Alluvial fan Spit Dunes Lagoon Tidal creek mouth fan Tidal creek Hinterland margin Rocky bouldery shore Gravel shore Sandy beach Mid tidal alluvial fan Figure 6.— Schematic diagram showing typical distribution of medium scale geomorphic units within embayment and straight coast settings. 37 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. (3) Tidal flats: broad ( 100 m to more than I km) gently inclined surfaces underlain by sand in low tidal levels and mud. muddy sand or sand in mid to high tidal levels; generally mud is the more common substrate in mid to high tidal levels but sand is common where tidal flats front a dune system or a spit or chenicr system; mid to high tidal flats arc vegetated by mangroves; tidal Hats have formed where shorelines have markedly accreted in the Holocene; a salt flat is developed at high tidal levels where mangroves are absent due to the development of a hypersalinc to brine groundwater/soilwater field; locally the mangrove-vegetated tidal flat is truncated at its seaward margin by a small (1-2 m) cliff cut into tidal-tlat muds. (4) Tidal creeks: these channels are erosional features (3 m-10 m wide, and approximately 2 m-3 m deep) that meander and bifurcate across tidal flats; small- scale shoals may develop in the creeks. (5) Spits/Cheniers: elongate, narrow (10-50m wide) sand and shelly sand deposits which are wave- developed features; spits typically emanate from exposed to semi-exposed headlands; cheniers are detached from headlands; although superficially similar, the inter-tidal margin of spits and cheniers is distinct from beaches in its relationship to the tidal gradient e.g. beaches arc parallel to shore and spits may transect the tidal gradient. (6) Sandy beaches: narrow, steeply-inclined surfaces of sand and shelly sand fronting exposed coastal situations. (7) Lagoons: high tidal, shallow drainage channels, or creeks, or depressions, up to 50 m wide and 500 m long, barred or impounded by dunes and beach ridges. (8) Rocky-bouldery shores: steeply inclined to cliffed, fissured rocky to bouldery shores comprised of rock boulders or bedrock; these shores generally are wave-exposed environments. (9) Gravel shores: steeply inclined shores comprised of rock gravel sheets; these shores also are generally wave-exposed. (\0) Shoals: accumulations of Icnsoid and linear mounds of sand and shelly sand (1 00 m or more in width) situated at mid tidal levels; the surfaces are plane to hummocky. {\\) Small islands: emergent bedrock prominences several metres to tens of metres across usually surrounded by tidal deposits. {\1) Dunes: supralidal sand ribbons to mounds 10 m to greater than I 000 m wide accumulated as onshore aeolian deposits. {\l>)Subtidal channels and bays: permanently-inundated environments that adjoin the tidal zone units listed above; the units arc underlain by rock, sand or mud depending upon which tidal zone unit is adjoining; these units are not discussed further in this paper. All the units except dunes, subtidal channels and bays are important as mangrove habitats and may support littoral vegetation in their mid tidal to upper tidal range. It should also be noted that this number of geomorphic units expands the list presented by Semeniuk (1985) for ria components in the Dampicr Archipelago region. The distribution and relative abundance of these medium- scale units in relationship to the large-scale geomorphology is summarised in Table 1. The disposition of the mcdtum-scale geomorphic units also is shown for selected small areas in the maps presented later in the text. TABLE 1 Distribution and Relative Abundance of Medium Scale Geomorphic Units Within Large Scale Geomorphic Setting MEDIUM SCALE GEOMORPHIC UNITS Large Scale Geomorphic Setting Hinterland Margin Alluvial Fan Tidal Flat Tidal Creek & a Lagoon BoX Gravel Shore Shoals Small Islands Dunes Straight coast: rocky/bouldery — P — — — — — c c — P — Straight coast: sandy — u u — u c p — P — P c Embayment c c c P c c p c P P P P Connective tidal land P P c c c p — — P P P — c common occurrence p may be present u uncommon occurrence — absent 38 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Substrates Substrates are imporianl in the developrnenl of distinct mangrove habitats because grain size and sediment type can directly or indirectly influence various aspects of hydrology and soil fertility. For instance, substrate type can influence internal drainage and accordingly affect various stages of water logging and salinity regimes, and the composition of substrates affects the nutrient uptake, retention or storage properties of a soil. There are a wide range of substrate types in the tidal zone of the Dampier Archipelago area. However, all can be related to geomorphology and oceanographic aspect and associated processes such as wave action, tidal action, sedimentation, erosion, etc. In the Dampier Archipelago area there are seven major settings for substrates and each has generated distinctive suites of substrates. The settings are listed below in order of decreasing wave exposure. ( 1 ) exposed rocky, bouldery and gravelly shores (2) exposed sandy shores (3) semi-prolccled embaymenls with spits/cheniers (4) sheltered sand shores (5) sheltered muddy shores (6) terrestrial/lidal Hat Junction (7) alluvial fan environments TABLE 2 Description of Substrates in the Mangrove Environments of the Dampier Archipelago Description Substrate Type Structure Fabric, Texture and Composition Colour Occurrence Bioturbated and root-structured mud bioturbated and root structured mudstone fabric of mud sized clay minerals, silica carbonates and organic matter grey to brown muddy tidal flat Bioturbated and root-structured sandy mud bioturbated and root structured wackestone fabric; quartz shell and lithoclasl sand in mud matrix grey to brown muddy tidal flat Homogeneous mud homogeneous mudstone fabric of mud sized clay minerals, silica carbonates and organic matter grey to brown muddy tidal flat Textured-mottled mud and sand mottled mud with patches of sand and sandy mud, or sand with patches of mud colour variegated: brown, buff and grey sandy tidal flat; spit margins; chenier margins; dune margins Homogeneous muddy sand homogeneous quartz, shell, lithoclast sand with interstitial mud brown to grey sandy tidal flat; spit margins; chenier margins; dune margins Root-structured muddy sand root structured quartz, shell, lithoclast sand with interstitial mud brown to grey sandy tidal flat; spit margins; chenier margins; dune margins Homogeneous sand homogeneous grain-supported sand of quartz, shell and lithoclast buff to brown sandy tidal flat; spit margins; chenier margins; shoals Laminated sand texturally layered grain-supported sand of quartz, shell and lithoclast buff to brown dunes; dune margins; beaches; spit margins; chenier margins Rock gravel structureless framework of gravel; interstitial sand or mud may be present interstitial sediment is grey rocky shores; tidal gravel flats; alluvial fans; hinterland margin Boulder deposits structureless to imbricated framework of boulders; interstitial sand or mud may be present interstitial sediment is grey rocky shores; hinterland margins Interlayered mud and sand texturally layered sand of quartz shell and lithoclast, alternating on a centimetre scale with mud layers brown to buff spit margins; chenier margins; shoals Limestone pavements — indurated calcarenile — limestone headlands/ pavements Bedrock pavements and slopes — fissured to smooth igneous rock substrates — rocky shores 39 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Within each of these sellings there arc a range of physical processes that influence the development of sediment bodies. The processes significant for the generation of the suites of substrates arc: (I) sediment supply, which determines sediment type as well as whether the shores are rocky or covered with sediment: (2) wave and tidal energy, which determine whether the shores are winnowed free of sediment or are covered by winnowed coarser sediments; (3) wave exposure, storm action, and frequency of flooding, which determine within sediment-filled embayments whether the shores are underlain by sand in low tidal zones or by mud in progressively higher tidal zones, and (4) fluviatilc input, which determines the amount of coarse sediment (sand and gravel) that is discharged along the hinterland edge and high tidal alluvial fans. The main components of substrates are mud. sand, shell and rock gravel, limestone and igneous bedrock. Mud is comprised of clay minerals, quartz silt and skeletal sill. Sand is comprised of quartz, rock fragments, or comminuted shell. The gravel component is comprised of shell, the accumulated tests of the marine benthos, and rock gravel which cither is eroded hinterland rocks or marine-reworked colluvium. The limestone is Pleistocene calcareniie or Holocene beachrock. Mela-igneous rock that crops out on the hinterland occurs as bedrock. The components of mud, sand and shell and rock gravel when accumulated and mixed develop a w'idc variety of substrate types. The effect of burrowing biota such as crabs further mixes the sediments and produces homogeneous and biolurbated or burrow-structured substrates. Vegetation contributes to the development of substrate variability by input of detritus and by root structuring. The main substrates encountered in this study include: (1) biolurbated and root-structured mud (2) biolurbated and root-structured sandy mud (3) homogeneous mud (4) Icxlurcd-mottlcd mud and sand (5) homogeneous muddy sand (6) rool-.struclured muddy sand (7) homogeneous sand (8) laminated sand (9) rock yrave/ (10) deposits (11) interlaycred mud and sand ( 1 2) limestone pavements ( 1 3) bedrock pavements and slopes. In cmbaymeni settings the muddy and sandy substrates arc dominant. In straight coastal exposed rock shore settings the gravelly and bouldcry substrates are dominant. A summary description of substrate types occurring in each setting is presented in Table 2. The occurrance of these substrate types and their granulometric analyses arc noted as surface soils on most profiles presented later in this paper. These units arc also encountered in the stratigraphic profiles. Straligraphy The Quaternary stratigraphy of coastal areas such as the Dampicr Archipelago is important for at least two reasons. Firstly, it provides an historical context for the origin and distribution of substrates and consequently habitats. I hus it provides information on the longevity and maintenance of mangrove- vegetated habitats since the sedimentary (biotic) lithotopes can form distinctive stratigraphic units. Secondly, the stratigraphy forms the basic framework for the tidal flat hydrology and the tidal flal/hinlerland hydrologic exchanges. Since groundwater salinity, recharge and mixing are important physico- chemical elements of mangrove ecosystems, it follows that the stratigraphic array of aquifers and aquatards is an important component of these systems. Stratigraphic units are differentiated on the basis of geometry, lithology and internal features such as structure and colour. The stratigraphic profile under the tidal zones contains the following units: (1) mud and muddy sand uedge: a thick wedge of biolurbated and root-structured mud which may grade into sandy mud and muddy sand (some formed under mangrove cover). It is at least 2-3 m thick at its seaward edge and pinches out to landward. This deposit occupies the major portion of an cmbaymeni. The wedge inlcrdigitaies with gravel/sand deposits developed along the hinterland edge, and with sand deposits of the spits/cheniers; (2) sand/shclly sand wedge: a thick wedge of biolurbated to layered sand, shelly sand and muddy sand. It is at least 2-3 m thick at its seaward edge and onlaps older stratigraphic units to landward. The unit occupies a major portion of exposed embayments. The wedge interdigitates with landward gravci/sand deposits and with dune deposits; (3) sand sheet: a thin sheet of sand 10-30 cm thick that forms a blanket on the salt flats. Contact with underlying unit, the mud wedge, is sharp, and it can intcrdigilale with adjacent sand bars: (4) muddy sand/gravel sheet or ribbon: a sheet or ribbon {c. 1 m thick) of muddy sand or gravel sandwiched between the main tidal flat wedge and the hinterland; this deposit is reworked buried soil or colluvium mixed with tidal mud, and has developed where the tidal flat sediments onlapped the hinterland: (5) grave! /boulder ribbon: a narrow and thin accumulation of gravel and boulders developed along rocky shores. The unit is locally muddy/sandy and it is 10-50 cm thick; (6) sand/grave! fan: a fan-shaped (to deltoid-wedge) deposit of sand, muddy sand or gravel, up to 2-3 m thick and lens of metres across, formed where terrestrial drainage has discharged sediment onto the high tidal fiats; the alluvial fan interdigitates with the muddy tidal flat wedge: (7) grave! fan: a thick fan-shaped lens, up to several metres thick and greater than 100 m across, formed where major streams have discharged coarse gravelly sediment onto the tidal flats. (8) sand bar: a shoe-string or ribbon-shaped sand or shelly sand unit, up to 100 m wide, I 000 m long and 3-4 m thick, formed as a (mid-) high-tidal to supratidal spit or chcnicr. The unit interdigitates with, and progrades over mud wedges; these deposits commonly are located at headland entrances to embayments or along wave-exposed portions of islands. (9) dune/beachridge: a ribbon to prism of fine and medium sand formed as a low coastal aeolian deposit. 40 Journal of the Royal Society of Western Australia, Vol. 69, Pan 2, 1987. Figure 7. — Schcmiic diagram of geometry and disposition of the main stratigraphic units. .Actual examples of these units are shown on the transect cross sections presented later in this paper. 41 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. {\0) hcachrock: locally cemented sheets and lenses of sand and shelly sand which through erosion has formed low cliffs and high tidal pavements. {W) limestone: a sheet of Pleistocene calcarcnite or shelly calcarenile which forms a pavement or ramp in the tidal zone. {\2) bedrock: the underlying Precambrian melavolcanic and Igneous rock. The geometric and spatial relationships between stratigraphic units are illustrated schematically in Figure 7. Specifically, the main tidal flat unit, which is best developed in embayments and in settings of intertidal land connections consists of a wedge of muddy sediment tapering landward and overlying igneous bedrock. This unit may be overlain by or mixed with sand where for example, a sand spit developed on top of the mud wedge has been subsequently geomorphically degraded to form a sand sheet. The contact between the narrow, landward edge of the muddy wedge and the gravelly soils of the hinterland is also represented in the stratigraphy as a muddy gravel sheet. Gravelly and coarse sand units penetrate the mud wedge where terrigenous alluvial deposition occurs (or has occurred) at the margin of the tidal zone. On sandy shores the main tidal wedge is sandy. On the more exposed coast the intertidal unit becomes a discontinuous ribbon of sediment forming patchy veneers, or occurring interstitial to boulders. Statigraphic profiles show that much of the tidal lands are infilled terrestrial lowlands. The stratigraphy records a gradual infilling during the Holocene by one or more of the lithologic/stratigraphic units described above. On the main tidal flat the sedimentary accumulation has resulted in a wedge of muddy sediment while on exposed sandy coasts the entire stratigraphic interval is composed of sand overlying bedrock. The consistent internal lithology and structure of muddy tidal fiat coasts and spil-lincd coasts show that these shore types are long-term stable features which, during the latter Holocene, do not appear to alternate between environmental extremes. For instance, on the spit-lined headlands the stratigraphic sections frequently show that a sandy (or muddy sand) spit has inlerdigitated with tidal flat muds as the shoreline accreted (eg. Figure. 25b). On the other hand some spit sections indicate that they arc only recent incursions of a sand bar into a muddy tidal fiat environment (Figure. 26C). Alluvial and colluvial discharge onto tidal zones from fiuviatilc and hinterland sources continue to accrete sediment and thus maintain the geomorphic unils/habitats associated with them (Figure 26A). Groundwater /Soilwater The occurrence, depth and salinity of the groundwater and soilwater systems on tidal fiats and adjoining hinterland arc iniporianl hydrologic factors that can regulate mangrove populations (MacNae 1968, Chapman 1976, Cinir6n et aL 1978, Semcniuk 1983, 1985). Indeed zonation of mangrove species documented by numerous authors can be related to groundwater and soilw'atcr hydrology. Soilwater and groundwater can be classified on occurrence and salinity, and the salinity of the various groundwater/soilwatcr units can be shown to be closely linked to stratigraphy, subtrale, recharge mechanisms and evapo-lranspiration. There are six main bodies of groundwater (Semeniuk, 1 985 and in prep.). These are: ( 1 ) hinterland and dune groundwater (freshwater) (2) hinterland and dune margin groundwater (salinc/mixed) (3) alluvial fan groundwater (saline/mixed) (4) tidal fiat groundwater (saline) (5) spil/chenier groundwater (saline/mixcd) (6) rocky shore groundwater (saline) Each of these groundwater and soilwater units has its own range of salinity as well as its own internal gradients of salinity and chemical composition. The prevailing mechanisms that result in the variable groundwater and soilwater units are: ( I ) seawater recharge. (2) freshwater recharge. (3) mixing and (4) evaporation. Ultimately there are only two main sources of groundwater: freshwater (discharged from the hinterland), and marine water (recharged daily, forinighily. varying to half- yearly). The disposition of aquifers, aqualards and aquicludes within the stratigraphic system, and the processes of marine recharge, groundwater migration, groundwater discharge, evaporation and transpiration interact to produce a gradient of increasing hypcrsalinity across tidal fiats. This gradient is complicated by a zone of mixing between tidal fiat groundwater (hypcrsaline) and freshwater seepage. Examples of this occur along the hinterland margin or alluvial fans. There is also mixing between seawater (seeping from spits and cheniers) and tidal flat groundwater. The resulting groundw-alcr bodies underlying the tidal zone units have various salinities as shown in Table 3. Soilwater frequently parallels the salinity gradients of groundwater because there is vertical mixing due to recharge by tidal waters and to bioturbalion by fauna. However soilwater salinity may differ from that of groundwater because of evaporation effects. A summary of aspects of groundwater and soilwater salinity is presented in Table 3 and Figure 8. The variability of salinity of groundwater and soilwater along a selected transect is illustrated in Figure 9. This indicates that the variability in salinity at any site is not large and that single samples of groundwater and soilwater are representative of gradient trends. Hinterland and dune groundwater — The groundwater under the hinterland and dunes is fresh to brackish. It seeps to seaward at the hinterland margin at soil, colluvium and rock interfaces and discharges onto the tidal flats in the subsurface. Where it infiltrates the tidal flat it mixes with, and dilutes tidal groundwater hypersalinity. The hinterland groundwater is recharged seasonally by meteoric water. Hinterland margin and dune margin groundwater/ soilwater — This groundwater and soilwater system resides in a sheet of muddy colluvial soil or in the ribbon of muddy sand, and forms a zone of mixture between tidal fiat hypersaline groundwater and freshwater seepage from the hinterland or dunes. As a result there is a gradient from saline water (r. 60”/oo) at landward margins to very hypersaline (90^/oo) where the system adjoins the main tidal flat. Recharge by seawater is infrequent (varying from monthly to twice-yearly); recharge by freshwater runoff and seepage is perennial, being marked in the wet season and less pronounced in the dry season (Semeniuk 1983). The water table is elevated relative to the adjoining high tidal flat system. 42 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 8. — Relalionship of groundwater saliniiv gradients to transects across various gcomorphic units (i-v). .A. Schematic diagram showing location of the transects (i-v) in relationship to the geomorphic units. B. C. D. E and F. are examples of the salinity gradients along transect i. ii. iii. iv and v respectively. Each transect is based on an actual field example. Alluvial fan groundwalcr/soilwater — The water table of the alluvial fans is elevated by freshwater discharge particularly during the wet season where there is combined surface runoff and subsurface seepage. During the dry season there is ongoing subsurface seepage and the water table is still elevated (0.3 m-1.0 m below the ground) relative to the adjoining high tidal flat. The salinity of groundwater and soilwatcr of alluvial fans is saline, being c. 45-60‘Voo at landw'ard parts of the system and grades via mixing with hypersalinc water up to values of 90‘Voo where the alluvial fans adjoin the main tidal flat. I’idal flat gronndwalcr/soilwalcr — (9n the main tidal flat groundwater generally occurs about 10 cm below the surface at seaward mangrove locations during low tide periods. Depending on substrate and tides (neap v.v spring), it is found progressively deeper until it may be greater than I m deep under salt flats. Ciroundwaler salinity is c. 40*^700 at seaward locations where it is recharged daily, and progressively increases up to highly hypersalinc values greater than 200Woo above MHWS where it is recharged fortnightly. Groundwater salinity values decrease at the frcshwaicr-innuenced zone along the hinterland margin and along the contact with alluvial fans. Soilwaler salinity shows a graded increase in salinity from c. 4()‘^/oo to 300*^/oo across the tidal flat, with a decrease in values along the frcshwatcr-influenccd hinterland edge and alluvial fans. Spit and chenicr groundwatcr/soilwatcr — The spit and chenier ground-w'atcr resides within the sand bar stratigraphic unit. The sand is well drained and the water table lies generally more than 0.5 m below the surface. The groundwater salinity approximates seawater where there is daily recharge at seaward parts of the spits and chenicr. Salinity generally grades up to values of 60-70"/oo at levels of HWS where recharge is fortnightly. On the wider emergent terrestrial spits and Cheniers there may be a thin lens of freshwater. Soilwaler salinity gradients generally follow that of groundwater. Rocky shore groundmitcr/soilwater — Ciroundwaler in the rocky shore environments resides in the fissures of rocky substrates or in the interstitial cavities of boulders, slabs and gravel deposits that occur along such shores, (icnerally, rocky shore environments arc well drained and where observed the water table is more than 0.5 m below the surface. The groundwater salinity appro.\imates seawater at levels of MSL and grades up to c. lOO^Vno at levels of MHWS. Soilwaler occurring as a pellicular film on rock surfaces and boulders is generally loo difficult to sample, but soilwaler collected from sediment interstitial to the frame of fissured rock or boulders shows a gradient of salinity similar to groundwater. 51396-3 43 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. TABLE 3 Groundwaicr/Soilwaier Description; Main Aquifers; Properties and TDS of Groundwater; Properties of Soilwater Aquifer (iroundwater Soilwater Type of Water Salinity of Water Recharge Mechanism Waterlogging of Soil Soil Salinity Hinterland and dune freshwater to brackish freshwater to brackish rainfall not described since soil docs not support mangroves not described since soil does not support mangroves Hinterland and dune margin shecl/ribbon contains water developed by mixing of hypersaline seawater and freshwater seepage from hinterland and frc-shwa ter/ seawater seepage from dunes up to 280 /.,. where there is no freshwater seepage; otherwise with freshwater or seawater seepage 50- 907... recharged infrequeiiily with seawater by highest tides; recharged perennially or annually by freshwater seepage which precludes any potential extreme hypcrsalinily waterlogged during wet season and after (infrequent) tidal flooding >3007;, but depending on freshw'ater seepage there may be a gradient of 507. up to 907.; Alluvial fan contains water developed by mixing ofhypersalinc seawater and freshwater seepage from hinterland 45-60'’/.. up to 90 / dependent on amount of freshwater seepage recharged infrequently w ith seawater by highest tides; recharged perennially or annually Ity freshwater seepage w hich precludes any potential extreme hypensaliniiy waterlogged during wet season, otherwise saturated in phreatic zone (usually-. 50cm deep) 407c. to 907.. Tidal flat wedge contains water approximating to seawater at seaward poilion. grades up to very hypcrsaline at landward" portion gradient; 40'-/. at MSL. 45 / - in mid Rlurophora zone. = 90/100 /. , at landward edge of mangal grading up to 2807 across salt Hats recharged daily to fortnightly at levels up to MHWN and MHWS respectively; evapotranspiraiion and evaporation combine to develop a gradient of increasing hypcrsalinity waterlogged most of the time gradient MSL = 407„. up to> 3007.oatHAT Spits/ Cheniers contains water approximating normal seawater grading up to hypersaline seawater 407 . varying to 60-707. depending on seawater and freshwater seepage depending on tidal level, recharged daily to fortnightly by seawater and seasonally by meteoric w'aler waterlogged during high tide: well-drained during ebb tide gradient of = 407.. to > 707., Rocky shores contains water approximating normal seawater grading up to liypersalinc seawater = 357,. at MSL to 90- 1007-, at MHWS depending on tidal level, recharged daily to fortnightly by seawater waterlogged during high tide; well-drained during ebb tide gradient of = 307... at MSL to 90-1007ooat MHWS. and > 2007.. at HAT Benthic fauna The benthic fauna of mangroves are important elements of the ecosystem. The animals function in several ways to facilitate the physico-chemical exchanges and biological processes that arc useful and essential to maintaining mangrove populations. The following information on function of the mangrove fauna is based on observations in the Dampier Archipelago mangrove environments and on literature (MacNae 1968; Saenger ct ai 1977;Milward 1 982; Hutchings and Rccher 1 982). The benthic fauna have a major function in bioturbation whereby soils are turned over rapidly relative to the life span of a mangrove tree. The bioturbation activities of fauna also aids in the recycling and mixing of sediment, such that groundwater- saturated soils are periodically brought to the surface to provide salinity exchange. In the same way both the mixing of soil by fauna and the burrow conduits themselves facilitate oxygen transfer. Benthic fauna such as crabs prey upon seedlings and therefore have a major impact on population regulation. Grazing fauna digest fresh and decaying mangrove leaves, leaf detritus and microbial products of mangrove decay, thus assisting cither in the fixing or export of nutrient. Fauna such as insects and boring “shipworm”, such as Teredo sp, contribute to mangrove destruction and thus arc a component of the processes of regulation and elimination. Faeces of fauna furthermore provide organically bound mineral particles that arc components of the nutrient breakdown and storage cycle. For these reasons, both burrowing and vagile benthic fauna are considered here to be important components of mangrove ecosystems. Information on types of fauna, their relative abundance, distribution and function is presented, according to habitat, in Table 4. The most diverse and abundant benthic fauna in mangrove environments occurs on muddy tidal flats where the fauna also is well zoned across the flat. The most important components of the fauna are the 44 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 9. — Transect showing variation in salinity of groundwater and soilwater across tidal flat in King Bay during one sampling survey. This transect is transect 3 of Semeniuk 1983. burrowing crustaceans {Thalassina anomala, Sesanna spp, Uca spp), peanut worms Phascolosoma. burrowing fish, together with grazing molluscs and predatory Sesarma. In the other mangrove habitats fauna is less abundant. In the gravelly environments of alluvial fans, hinterland margins and rocky shores the fauna resides in sediment interstitial to a gravel/boulder frame. Sandy beaches, sandy tidal Hats, spits and dune margins support the least abundant and least diverse fauna. However, in zones which are transitional between sand and mud, burrowing crustaceans (e.g. Thalassina and Metopograpsus) arc important in bioturbation and in developing water exchange conduits. Classification of Mangrove Habitats in the Danipier Archipelago Approach to classification The identification of mangrove habitats rests on the fact that mangroves inhabit gcomorphic units that have distinct morphology, substrate, salinity, etc. Adjoining areas, not vegetated by mangroves, of course, can be mapped to define the boundary and extent of a given mangrove formation. Once the occurrence of a mangrove formation is defined, the recognition of habitat types is achieved by identifying the physical and chemical features (such as spits and alluvial fans, etc.) within the system. Thus within an embayment with extensive well developed mangroves, for instance, there may be distinguished alluvial fans, spits, tidal creeks, etc., each with their influence on mangrove occurrence, abundance, composition and physiognomy. By not resolving and identifying the geomorphic units (or habitats) within the embayment. the heterogeneity evident in the mangrove formations tends to be viewed as mosaics of differing floristics and structure. The habitat approach adopted here (following Semeniuk 1985) attempts to reconcile, at the large and medium scales of reference, some of the variation in mangrove formations with the more obvious physico-chemical features in the environment. The mangrove habitats of the Dampier Archipelago The important factors in development of mangrove habitats are tidal level, substrate type, stratigraphy and salinity. These arc inter-related and their interaction develops distinct habitats for mangroves in this area. Substrates arc developed within a given gcomorphic unit and arc related to tidal and wave processes at a specific tidal level on the shore. The tidal level and substrate in turn influence recharge rate and evaporation, and hence salinity of groundwater and soilwater. Specific medium 45 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. TABLE 4 Occurrence. Abundance and Function ot Benthic Fauna in Mangrove Environments of the Dampier Archipelago Habitat Main Fauna Types* Abundance^ Function-^ Muddy tidal flat Crustacea: Sesanna spp. medium A C D E G 7 halaasina anomala low/medium ABCD Uca spp medium AC D Scv//a serrata low ACD h'pixanihm denlalus low A C D Metopo^rapsus fronlalis low A C D E G Atphem sp low ACD Siphunculid; Phdacolusoma high A C' D Mollusca: Teredinids high F Telcscopium U’lescopium low/medium 1 Serita lineata low/medium 1 Onchidium low/medium 1 Pisces: gobioid fish low/medium ACD Sandy tidal flat Crustacea; Sesarma spp low A C D E G I'ca sp low ACD Mollusca: Teredinids high F Tcrebralia sulcata medium/high 1 Tcrchralia palustris medium/high 1 Pisces; gobioid lish low ACD Rocky bouldery shore Crustacea: S'ictopograpsus frontalis low G Sesarma spp low ACDG Petrolithes teres low 7 Mollusca: Nerita spp low i vanous gastropods low H I Spits/cheniers Crustacea: Ocvpode spp low ACD Metopograpsus frontalis low ACDG Mollusca: Teredinids high F Tcrebralia sulcata low 1 Tcrebralia palustris low 1 Beach Crustacea: Ocypode spp low ACD Scopimcra sp medium/low ACD Mtetyris sp medium/low ACD Hinterland margin Crustacea: Sesarma sp low/medium A C D E G ( 'ca sp low ACD Gravel tidal flat Crustacea: Sesarma spp low A C D E G Petrolithes teres low ACD High tidal alluvial fan Crustacea: Sesanna spp low ACDG (’ca sp low ACD Tidal creek Crustacea; Uca spp medium ACD Macrophihahnus sp medium ACD Mollusca: Teredinids high F Pisces: gobioid fish low/medium ACD Mid tidal alluvial fan C rustacea: Sesarma spp low/medium A C' D E Ci Uca sp low/medium ACD Mollusca: Teredinids high F High tidal dune margin Crustacea; Sesanna spp medium A C D E G Uca low ACD Mollusca: Terebralia sulcata medium/high I lerehralia palustris medium/high 1 Pisces: gobioid fish low/medium ACD High tidal lagoon Crustacea: Sesanna spp medium A C D E G 1 'ca sp low/medium ACD Mollusca: 7 'erehraliii sulcata medium 1 Terebralia palustris medium 1 Pisces; gobioid fish low/medium ACD Shoals Crustacea; Miclvns sp medium/high ACD Scopimera sp medium/high ACD Uca sp low/medium ACD Limestone pavement no fauna relevant to mangroves — — I. fauna listed herein are those that have obvious direct relationship lo mangrove ecology; fauna such as encrusting barnacles, oysters or algae grazing gastropods although potentially abundant, are not included. An ifauna and insect fauna also are not included. 2. high abundance: density greater than 10/m“ medium abundance- density l-IO/m“ low abundance: density less than I/m" 3. A biolurbaiion: soil turnover affecting salinity and nutrients B bioturbation: soil mounding developing microscale soil salinity mosaics C aeration: function of open burrows and bioturbation structures lo facilitate gas exchange D generation of conduits; function of burrows and bioturbation structures lo facilitate hydrologic exchanges E seedling predation: predation ol' seeds, seedlings and saplings F wood boring Cj leal degradation H leaf grazing 1 faeces production 46 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. scale gcomorphic units with their distinctive substrate and stratigraphy also may function as discrete conduits for marine water flow or fresh water flow, both of which influence the salinity of the habitat. Thus habitats arc strongly linked to gcomorphic unit. As a result, a medium gcomorphic unit functions as a distinct habitat that is the product of tidal level, substrate and salinity. Within a gcomorphic unit (or habitat) there also are internal gradients. These occur because: (1) tidal level on a gradually rising shore provides a gradient of decreasing frcquency-of-inundation; (2) substrates may exhibit a gradient with grain-size or sediment abundance increasing or decreasing in a specific direction; (3) salinity of groundwater/soilwater also shows a graded change dependent on tidal level (and hence a change in recharge and evaporation) and proximity to recharge of marine water (to seaward) or fresh water (to landward). Mangrove habitats in the Dampier Archipelago may be classified at several scales (following Semeniuk 1985 p. 22). In this study region there are 2 appropriate scales i.e, large scale sellings and medium scale settings. Sheltered embayment mangal Mangal on connective tidal land Rocky shore mangal Undifferentiated subtidal to low intertidal Salt flat Rocky and alluvial hinterland Causeway Sandy shore mangal Undifferentiated mangal outside Dampier Archipelago Figure 10. — Distribulion at the large scale of mangal types in the Dampier Archipelago. In this figure the 2 mangal types along straight coasts are mapped as separate units, i.e. sandy coasts and rocky coasts. 47 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, ! 987. At the large scale 3 categories of mangrove-vegetated coastal types arc recognised (Fig. 10): (1) Embaymcnt types, which are probably the most important areas for mangroves in this region; (2) Connective tidal-land types, which are the second most important mangrove systems in this region; (3) Straight coasts composed of open rocky-bouldcry shore types and open sandy beach-dune shore types. Within each of the large scale mangrove settings there is the distinct suite of medium scale gcomorphic units described on p. 36-38. These various gcomorphic units arc treated herein as distinct mangrove habitats since each gcomorphic unit essentially has its ovyn suite of physico-chemical parameters such as tidal level (frcqucncy-of-inundation), substrate and salinity. However, boundaries between some gcomorphic units also may function as discrete mangrove habitats because of the development of specitlc /ones of mixing or overlap in factors such as sediment type or fresh water seepage or slope. Thus, for instance, where the salt flat on a muddy tidal flat adjoins a dune terrain, the zone of contact between the two units is a discrete mangrove habitat: — to seaward there is a salt flat devoid of mangrove, to landward there is the dune terrain with its terrestrial vegetation, and along the zone of contact there is a narrow mangrove habitat. In some cases a given gcomorphic unit may occur at difTcreni tidal levels ol the shore e.g. alluvial fans which predominantly occur in high tidal environments but some may penetrate seaward to mid tidal and low tidal shore levels. In these cases gcomorphic units at the various tidal levels arc distinguished as different mangrove habitats. Thus high tidal alluvial fans arc distinguished from mid-tidal alluvial fans. The mangrove habitats recognised in this study listed in order of their abundance and importance in supporting mangroves arc as follows: 1. Muddy tidal Hat 2. Sandy tidal Hat 3. Rocky-bouldery shore 4. Spit and chenier 5. Beach 6. Hinterland margin 7. Gravel tidal flat 8. High tidal alluvial fan 9. Tidal creek 10. Mid-tidal alluvial fan 1 1 . High tidal dune margin 12. High-tidal lagoon 13. Shoal 1 4. Limestone pavement A description of these units and their features important to mangroves is provided in Table 5. Synthesis The edaphic features of mangrove environments in the Dampicr Archipelago are the product of several interacting physical, chemical and faunal factors. These arc: ( 1 ) ancestral topography (2) oceanographic aspect (3) modern processes of sedimcnlation/erosion (4) tidal slope (5) stratigraphic sequence and substrates (6) salinity regimes (7) fauna effects. These factors contribute to development of habitats that have distinct tidal level, substrates, salinity and fauna, all of which arc significant in maintaining and regulating mangrove populations. A summary of the development of habitats may also be approached from the point of view of marine processes and terrestrial processes acting on the archipelago shore. The main marine processes include daily to monthly tidal flooding, wave and tidal-current action, and fauna cfl'ccts. These processes arc influential in shaping environmental factors up to levels of mean high water spring tide. Thus, they influence the development of substrate types, gradients in substrate variation, sediment transport, the evolving stratigraphic sequence, frequency of flooding, water logging, salinity, oxygenation and biolurbation. The survival of mangroves and benthic fauna, their population maintenance and hence their zonalion between mean sea level and mean high water spring are dependant largely on prevailing marine conditions. Terrestrial modifications generally have little or no influence on those environments dominated by marine processes. The terrestrial processes on the higher portions of tidal flats include evaporation, desiccation, sedimentation, and freshwater seepage. The processes of evaporation and desiccation, together with infrequent tidal recharge, develop extremely hypersalinc salinity regimes which culminate in the development of salt flats at the expense of mangroves and other biota. However, freshwater seepage along the edge of the hinterland and, in particular at the exit points of terrestrial drainage (eg high-tidal alluvial fans), provide a specialised habitat where there is a local occurrence of mangroves and other biota because of a regime of lower salinity (Semeniuk, 1983). Thus an important factor in the maintenance of mangroves and fauna along the edge of the hinterland and on high tidal alluvial fans is the terrestrial process of freshwater seepage. Each of the medium scale to small scale geomorphic units (or mangrove habitats) if viewed from this perspective can be seen as distinct products with their own internal suite of processes and gradients that provide distinct habitats that support mangroves. Special mention should be made of the transgression of spits across a tidal flat surface because in effect there may be developed two intersecting gradient systems. As a result spits have a more complex array of mangrove species inhabiting them. Mangrove Vegetation Species Pool Only six species of mangrove occur in the littoral zone of the area (Semeniuk et aL 1978, Semeniuk 1983). In alphabetical order, these are: Species Family (1) Aegialitis annulaiaK.hv. PLUMBAGINACAE (2) Aegiceras corniculatum MYRSINACEAE (L.) Blanco 48 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. (3) Avicennia marina (Forsk.) Vierh. AVICENNIACEAE (4) Bruguicra exaristala Ding Hou RHIZOPHORACEAE (5) Ceriops tagal ( Perr.) C. B. Rob. RHIZOPHORACEAE (6) Rhizophora stylosa Griff. RHIZOPHORACEAE This low species richness of mangrove^ compared with that of the humid tropics (Saenger cl al. 1977, Tomlinson 1986). is representative of northwestern Australian coastline and is typical of mangrove assemblages that occur in tropical arid climates (Scmcniuk 1983). Notes on habit, relative abundance, and associations of these species in the Dampier Archipelago follow. TABLE 5 Mangrove Habitats; A Summary of Their Important Features Habitat Description Main Gradients Important for Mangrove Zonation Comments on Features that Maintain Habitats & Gradients Muddy tidal flat gently inclined surface between MSL and HAT, underlain mainly by mud frequency of inundation; salinity habitat surface is maintained by scdimcniaiion/erosion; salinity is maintained by marine water recharge/discharge, seasonal meteoric recharge and evaporation Sandy tidal flat gently inclined surface between MSL and HAT underlain mainly by sand and muddy sand frequency of inundation; salinity habitat surface is maintained by scdimeniaiion/erosion; salinity is maintained by marine water recharge/dischargc. seasonal meteoric recharge and evaporation Rocky-bouldery shore Sleep to moderately inclined surface underlain by bedrock or boulder/pebble sheet frequency of inundation; salinity habitat surface is maintained by crosion/sedimenlaiion; shallow soil or no soil; marked surface water runoff; salinity maintained by marine water recharge, efticicni discharge and evaporation Spits and cheniers Steep to moderately inclined surface underlain by sand, shelly sand and muddy sand frequency of inundation; salinity; substrate grainsizc; drainage habitat surface is maintained by sedimentation/erosion; salinity maintained by marine water recharge, efficient discharge, evaporation and periodic freshwater seepage Beach steep surface underlain by sand frequency of inundation; drainage habitat surface is maintained by scdimenlaiion/crosion. salinity is maintained by efficient recharge/dischargc of marine water Hinterland margin moderately sleep to gently inclined surface underlain by muddy sand/gravel salinity; substrate grainsize habitat is developed and maintained by extensive freshwater seepage, and sediment sheetwash off the hinterland Gravel tidal flat gently inclined surface between MSL and HAT underlain by gravel, sandy gravel and muddy gravel frequency of inundation; salinity habitat is maintained by crosion/sedimenlaiion; salinity regimes maintained by marine water recharge, discharge and evaporation High tidal alluvial fan gently inclined surface underlain by muddy sand/gravelly sand, and gravel salinity; substrate grainsize habitat is developed and maintained by extensive freshwater seepage, and sediment input along strcam/creek channel Tidal creek (bank) steep to moderately inclined muddy surface between MSL and HWN frequency of inundation; salinity; drainage habitat surface is maintained by erosion; steep banks and bio-cxcavalion/burrows ensure rapid drainage Mid-tidal alluvial fan gently inclined surface underlain by muddy sand/gravelly sand, and gravel frequency of inundation; salinity; drainage habitat is developed and maintained by extensive freshwater seepage, and sediment input along strcam/creek channel; marine water rechargc/discharge High tidal dune margin narrow gently inclined surface at margin of dunes salinity habitat and surface maintained by sheet wash off dunes onto salt flat, salinity regime maintained by seepage of seawatcr/freshwalcr from dune reservoir High-tidal lagoon linear depressions developed al c. HWS salinity; drainage habitat formed by impounding of drainage channel; seepage of freshwater or seawater from dunes and hinterland maintains the salinity regime Shoal hummocky muddy surface al c. MSL distinct gradients not evident habitat surface is maintained by sedimentation alternating with erosion Limestone pavement planed surface of limestone with local bouldcr/slab accumulations frequency of inundation habitat is maintained by constant sediment winnowing 49 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Notes on species Aegialitis anmilala, commonly known as the Club Mangrove, is a small slender shrub up to 1 m high. The species most commonly occurs scallcred as an undcrslorey component in mixed associations between MSL and HWS or sandy beach shores; it also occurs scattered as an undcrslorey component to Avicennia forests and scrub on gravel substrates at levels of HWS and occurs mixed with Avicennia and Ceriops at HWS on spit margins and beaches. While the species could not be considered regionally or locally abundant, it is nonetheless well distributed and most embayments have some individuals. Aegiceras eonucitlalum is known inlernalionally as the River Mangrove because of its tendency to line those rivers and creeks with a fresh water influx. It is a bushy shrub up to 2 m high. In the Dampicr Archipelago the species is not associated with freshwater influx but is most commonly developed at c. MSL on creek banks and shoals. In these habitats the species forms a closed heath structure or alternatively occurs in association with Avicennia and/or Aegialitis. Locally it also occurs in mixed association on mid tidal levels of sandy beach shores. The species is not regionally or locally abundant but can be considered to be widely but sporadically distributed. Avicennia marina the While Mangrove, is the most widespread, the most abundant, and physiognomically the most variable of s\)cc\es in the Dampier Archipelago. It occurs as a medium tree up to 6 m high forming closed low forests, it forms closed scrub 3-4 m high at HWN to HWS and. where salinity of groundwatcr/soilwater is excessive, it can form a low heath up to 0.5 m high at HWS. Physiognomically it varies from bushy shrubs, to mullistemmed shrubs, to columnar small trees, to recumbent and gnarled trees. Avicennia can occur virtually in any habitat encompassing the full mangrove tidal range from MSL to HAT: as such it can form pure stands at the limits of its tidal range (where other species arc eliminated), or can occur with other species in mixed stands. Bruguiera e.xaristata. commonly called the Ribbed- fruited Orange Mangrove overseas, is a slender shrub to small tree up to 5 ni high, buttressed at the base. Its most common occurrence is within mangrove scrub along sandy beach shores and the margin of spits at levels of HWS. where it occurs in association with Avicennia and Ceriops. It also may occur in the interior of mangrove forests at level of HWN in association with Rhizophora. The species, although widespread throughout the Dampicr Archipelago, is not particularly abundant at any given site. Ceriops tagal is commonly called the Yellow-leaved Spurred Mangrove. It occurs as a slender, columnar, buttressed shrub up to 2.5(-3.0)m high. The species occurs at le\els of HW'S along sandy beach shores, margins of spits, along gravelly hinterland margins and on high tidal alluvia! fans, the headwaters of tidal creeks and, locally, on the high tidal portion of muddy tidal flats. It occurs locally as pure stands of closed or open scrub but more commonly it is associated in scrub with Avicennia and Bruguiera. Although not particularly abundant at any given site it is nonetheless quite a widespread species. Rhizophora stylosa commonly termed the Red Mangrove, occurs as a shrub to tree up to 6-7 m high. Near levels of HWN on muddy tidal flats it forms dense closed forests of pure stands. Elsewhere such as on rocky shores, sandy tidal flats, or sandy beach shores it occurs at about HWN as a shrub subordinani to other species. The species is widespread and second in abundance only to Avicennia. although in some embayments it may exceed the abundance o\' . ivicennia. Floristic and structural classification In categorising the mangrove vegetation of the Dampier Archipelago an approach utilising floristics and structure was adopted. The structural categories follow that of Specht (1981) with the following modifications. Vegetation with heights of 0.25-1.5 m is termed low shrub and forms heath: those plants taller than 1 .5 m but less than 5 m are tall shrubs and form scrub or shrubland; and those with heights of 5-7 m arc termed trees and form forests. The term “overmature" used in this paper refers to large diameter trees that have hollowed trunks and that have become recumbent and gnarled. The tloristic categories are based on numerically and structurally dominant species within the assemblages. Since there are only 6 species of mangroves in the region, the categorisation of the vegetation only on floristics becomes relatively simple. Furthermore many of the categories are comprised of monospecific stands. A description of the lloristic/structural units is provided in Table 6. Typical distribution of the units within the medium scale habitats is shown in Table 7. The main lloristic and structural categories of mangroves recognised in the region are: 1 . .1 vicennia forest ( .A I ) 2. .•lv7cc/7/;/a scrub (A2) 3. . {vicennia heath (A4) 4. . {vicennia-Aegialitisscvwh {AAc2) 5. Avicennia-Ceriops scrub {NCI. CA2) 6. Ceriops- Bruguiera-Avicennia scrub (CBA2, ABC2, BCA2) 7. Rhizophora forest ( R 1 ) 8. Rhizophora scrub ( R2) 9. Rhizophora- Bruguiera scrub (RB2) 1 0. Avicennia shrubland {A3) 11. Mixed scrub (eg .ABRC2). The mangrove assemblages The mangrove flora of the Dampier Archipelago is treated as assemblages that inhabit discrete habitats. The approach involves the following procedure: 1 the identification of tracts of mangrove formations within a large scale geoniorphic setting (Figure 6). such as extensive occurrencies in sheltered embayments, or as strips along a rocky coast. 2. the delinalion of habitats within this system based on medium scale geomorphology (Figure 7). substrate, and salinity. 3. the assignment of assemblage nomenclature to vegetation, based on its occurrence on a given habitat unit. 4. the documentation of the structure and floristics of the vegetation units and zones within each assemblage. 50 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. TABLE 6 Description and Distribution of Vegetation Types in Mangals of the Dampier Archipelago Vegetation type* Structure and florislics Distribution within habitat and assemblage A1 Closed forest of mature, gnarled sometimes recumbent Avicennia marina trees, 5-6 m tall. Occurs most commonly as a narrow band 1 or 2 trees wide at the seaward edge (MSI.) of the mam mangrove belt of the sandy tidal flat assemblage. It may also occur in the same manner on muddy tidal flats. A2 Closed scrub of mature gnarled, recumbent or multi- stemmed shrubs of Avicennia marina, from 1 .5 m to less than 5 m tall; may become open in more saline situations such as MHWS. The most widespread vegetation unit within the archipelago mangroves. It forms the main mangrove bell on sandy tidal flats, gravelly tidal flats and mid tidal alluvial fans, comprises the high tidal lagoon assemblage, occurs on muddy tidal flats between Ml IWN and MHWS, and commonly occurs on sandy beaches. A3 Closed to open shrubland of multi-stemmed shrubs of Avicennia marina, from 1.5-2. 5 m tall. Quite widespread and occurs in the more saline extremes of, nutrition poor habitats such as; Ihe periphery of salt flats (i.e. the landward edge of the main sandy tidal flat assemblage); high tidal dune margin; hinterland margin, high tidal alluvial fan; the seaward most portions of spits which extend across the tidal gradient. This unit also most commonly comprises the gravelly tidal flat assemblage. A4 Open heath of mature multi-stemmed shrubs of Avicennia marina, less than 1.5 m tall. Not widespread but occurs most commonly at the landward edge of the main Avicennia belt on sandy tidal flats (i.e. adjacent to salt flats) and rarely on high tidal alluvial fans, and the landward most edge of gravelly and muddy tidal flat mangrove stands. AR2orRA2 Closed scrub of mature.4i7(rrj«/ti and Rhizophora shrubs, from 3 m to less than 5 m tall. The Avicennia occur as mature recumbent or multi-stemmed individuals. Rhizophora occurs as mature single stemmed or laterally spreading multi-stemmcd individuals. C omprises the tidal creek assemblage and also occurs at the seaward edge of the main mangal belt on muddy tidal flats, either seaward to, or instead of an A1 vegetation unit. Batches of RA2 or ,AR2 also consistently occur within the central portions of mid tidal alluvial fans and on larger mud shoals. The unit also occurs on rocky shores and on gravelly tidal flats. Ae2 Scrub of Acfiiceras to 2 m tall with an understorey of less than 1 m tall Occurs on the sandy shoal habitats. This habitat and vegetation unit are not widespread. AAc2 Scrub ofmulti-stemmed shrubs 1.5-2. 5 m tall with an understorey of Aegialitis, less than 1 m tall, or . k'.tf/crmv shrubs 1.5 m tall. Noted on rocky shores, gravelly shores and sandy tidal flats. The unit is not widespread. AAe3 Open shrubland of multi-siemmed Avicennia shrubs 1.5-2 m tall with an understorey of Aegialitis or Aegiceras. Occurs on rocky shores and sandy shoals. It is not widespread. AAe4 An open to closed heath of multi-stemmed Avicennia shrubs with an understorev of Acgialilis shrubs to 1 .5 m tall. May occur as a small patch on the middle of a sandy tidal flat to the landward and seaward surrounded by A2. This unit was noted once during the study. ABCAe2 & CBAAe2 Closed scrub of Avicennia. Hriiguiera and Ceriops shrubs 2.5-3 m tall, with an understorey of Aegia/iiis shrubs less than Im tall. Includes an unusually diverse array of species and was noted to occur on a rocky shore habitat and a beach. ACAe2 Closed scrub of Avicennia and Ceriops shrubs to 2 m tall with an understorey of .scattered Aegialitis shrubs less than 1 m tall. Occurs as a narrow bell (5 m wide) along a high tidal dune margin flanked by A3/A2 at the seaward edge. AC2 & CA2 C’loscd scrub of multi-stemmed Avicennia and columnar Ceriops shrubs 1 .5-2 m tall. Occurs on spits and cheniers. high tidal dune margin and rarely on rocky shores. AAcR2 Scrub of Avicennia and Rhizophora shrubs. 2.5 m tall, with an understorey of shrubs, less than 1 m tall. Not widespread; noted as a small shoal at the seaward front of the main mangrove belt in Scaripple Passage. Rl Closed forest of mature primarily single trunked, Rhizophora trees. 5-6 m tall. Not widespread and may rarely occur as a thin bell of one to two trees wide along the seaward edge of the muddy and sandy tidal flats. R2 f’losed scrub of mature, multi-stemmcd and in ten wining, or sometimes single trunked, Rhizophora shrubs generally 2-4 m tall. Rare Hruguiera shrubs may also occur in the unit. Consistently occurs as a seaward /one between MSI. and MHWN on muddy tidal flats. It also rarely occurs as a seaward /one on sandy tidal flai.s. An abrupt conlael consistently occurs at its landward edge with an A2 unit. An RA2 or R 1 unit sometimes occurs at its seaward edge. RB2 Closed .scrub of mature, mulli-slcmmcd and intertwining, or sometimes single trunked, Rhizophora shrubs, with common to occasional columnar liruguiera trees 2-4 tn tall. Occurs asa distinct /one between MSLand MHWN on muddv tidal flats, flanked to its seaward edge by a narrow RA2 belt and its landvvard edge by the A2 zone. Although scattered liruguiera trees (shrubs) freiiucnily occur, but not always, in the Rl vegetation unit they are not generally abundant enough to include in a map unit. 51396-4 51 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. TABLE 6 Description and Distribution of Vegetation Types in Mangals of the Dampier Archipelago — continued. Vegetation Type* Structure and floristics Distribution within Habitat and Assemblage B2 Closed scrub of columnar Bruguicra trees 2-3 m tall. Occurs occasionally as a landward zone from 1 to 1 0 trees wide on the spit or Chenier habitat and was also noted as a landward zone on a sandy beach. It may however be flanked by a C2 belt at its landward edge. B3 Shrubland of columnar Bruguicra shrubs 2-2.5 m tall. Occurs on a high tidal gravelly apron immediately landward of a mangrove vegetated sandy tidal flat; recorded once in the study areas. C2 Scrub of columnar Ceriops shrubs 1.5 m tall. May occur on the high tidal alluvial fan habitat, as a narrow (1-3 trees wide) belt at the landward edge of beach and chenicr habitats, and rarely on the hinterland fringe habitat. C3 Shrubland of columnar Ceriops shrubs 1.5 m tall. May occuron the high tidal alluvial fan habitat. It is not common. BAC2& BCA2& ACB2 Closed scrub of mixed columnar Bruguicra and Ceriops shrubs with multi-stemmed . Iv/ccwn/a shrubs 1.5-2. 5 m tall. Commonly occurs on the higher tidal zone (i.e. within the zone of greatest freshwater seepage) of spits and cheniers. ACB2 was also noted on the high tidal portion of a gravelly tidal flat in King Bay. BA2 Closed scrub of Bruguicra and .4viccnnia shrubs l.5-2.5mia!l. Noted to occur on the chenier habitat. ABRC2 Closed scrub of .1 vicennia. Bruguicra. Rhizophora and Ceriops shrubs to 2 m tall. Occurs on some sandy beaches, and spits, and was noted on a gravelly tidal flat. RCBA2/4 C'loscd to open scrub of Rhizophora, Ceriops. and - li’/cvn/imand Bruguicra 2 m tall. Occurs on a high tidal alluvial fan on Angel Island. The various species form zones several trees wide, with Rhizophora at the seaward edge and Avicennia at the landward most edge of the vegetated portion of the high tidal alluvial fan. RBA3 Closed shrubland of Rhizophora and Bruguicra columnar shrubs and multi-stemmed Avicennia shrubs 2-2.5 m tall. Not common; occurs on a spit. ABR2 Scrub of , lv7£wm/a. Bruguicra znA Rhizophora sMrnhs 2-2.5 m tall. Not widespread; noted on a rocky shore. CA3 Shrubland of mature columnar Ccriop.s and Avicennia recumbent and multi-stemmed shrubs 1 .5 m tall. Occurs on high tidal alluvial fan and rarely on rocky shores. * Avictmnia marina: B = Bru^^uiera c,xarista{a\ C = Ceriop.s tagal: Ac * AcgiahUs annulala or Acgiceras corniculatum: R = Rhizophora stvlosa. Order ol letters in the abbreviated nomenclature indicates relative abundance of species; thus ABC indicates Avicennia > Bruguicra > Ceriops. The numerical postscripts 1 , 2, 3. 4 refers to structure of the unit; I * forest; 2 = scrub (closed or open); 3 = shrubland (closed or open); 4 = heath (closed or open). Viewing florislic composition alone would provide little differentiation of the underlying patterns within assemblages because in reality only a few species contribute to the mangrove formations. Considering vegetation structure and florislics together also does not fully exemplify the various responses of vegetation to different habitats. Ideally, a classification of vegetation within the compositionally simple systems of the Dampier Archipelago should be based on the following features: • floristics • structure • physiognomy of individuals • population dynamics and maintenance processes. However, a comprehensive categorisation of mangrove vegetation using such features at this stage is premature, because the complete data for the population dynamics and maintenance are not available. The approach adopted here therefore is to recognise that a suite of species inhabits a discrete habitat and that consequently, physico-chemical and biotic forcing factors eliminate, regulate and maintain the various species resulting in (I) a differential distribution of species, (evident as compositional zonaiion), (2) a variety of physiognomic types (evident as physiognomic and structural zonalion) and {^) variation in population maintenance processes (evident as physiognomic and structural zonalion). Arbitrarily choosing cither compositional or structural zones and nominating them as assemblages or “communities”, or alternatively combining them as floristic/struclural zones, is merely identifying vegetation responses to a number of underlying environmental factors. In this paper therefore an assemblage is recognised as inhabiting a given habitat, and thereafter the various florislic, structural and physiognomic zones that occur in the assemblage are described within a framework of physico-chemical gradients within the habitat. 52 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. TABLE 7 Occurrence of Florislic/Structural Vegetation Categories in Relationship to Habitats* Habitat Type VEGETATION Vegetation type occurring on habitat Vegetation type less commonly occurring on habitat Vegetation type occurring at least once on habitat Muddy tidal flat R2, A2 A3 A4. AR2, RA2, RB2, Rl. AAc4, AAeR2 Sandy tidal flat A2. A3 > > RA2, R2. AAe2. AC2. AR2. R 1 Rocky/bouldery shore A3 A2, AR2. RA2, AAe2, ABCAc2, AC2, B3, ABR2 Spits and cheniers A3 A2 RBA3, AC2, B2, BA2, BAC2, BCA2. BRAC2. CA2 Beach A2. A3 B2, C2.CBAAc2. ABRC2 High tidal lagoon A2 A3 High tidal dune margin A3 RA2, A2. ACAe2, C2 Tidal creek AR2. R2 Mid tidal alluvial fan A2, A3 R2, CA3 RA2. Al, AR2. C3 High tidal alluvial fan A3 A2, C2, A4, C3. RCAB2 Hinterland margin A3 A2 C2, CA2 Limestone pavement A3 A2 Shoals AAc2. AAe3, A2, A3 Gravelly tidal flat A2 A3 R2, CA3. BRAC2, AR2, ACB2. AAc3. AAe2. A4 Note that the ranking of vegetation units on a given habitat is according to their presence or absence only and not on their distribution across that habitat. For example A! {Avicennia marina forest) occurs usually on sandy tidal flats although it occurs only as a narrow seaward band and is not areally significant. Although the mangrove assemblages arc named after their habitat, it should be stressed that terms such as “muddy tidal flat assemblage” is intended to convey a meaning of: “assemblages that inhabit muddy tidal flats”. In order of abundance the assemblages are: 1 . Muddy tidal flat assemblage 2. Sandy tidal flat assemblage 3. Rocky-bouldcry shore assemblage 4. Spit and chenicr fringe assemblage 5. Sandy beach assemblage 6. Hinterland fringe assemblage 7. Gravelly tidal Hat assemblage 8. High tidal alluvial fan assemblage 9. Mid-tidal alluvial fan assemblage 10. Tidal creek assemblage 1 1 . High tidal dune margin assemblage 12. High-tidal lagoon assemblage 13. Shoal assemblage 14. Limestone pavement assemblage. Description of assemblages The mangrove assemblages of Dampier Archipelago arc described below in the following format: • Distribution and occurrence • Main physico-chemical gradients within the habitat • Composition of assemblage • Physiognomy and structure of assemblage • Zonation in relation to gradients Data on the structure, composition and abundance of species within the assemblages arc described below and in the following figures, figures 1 1-22 present maps of habitats, assemblages and vegetation distribution. The maps are selected from key areas that illustrate the variability of mangrove habitats in the Dampier Archipelago region. These maps also illustrate the marked local heterogeneity within a given area. In addition. Figures 23-30 illustrate cross-sections along selected transects from the areas mentioned above. 53 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. These cross-sections present information on a range of geoniorphic, physico-chemical and vegetation features. These figures also present quantitative information on mangrove abundance. These quantitative data form the basis for the description of relative abundance and zonation as presented in the text following, for the identification of the floristic structural units as described in Tables 6 and 7, and for the information presented in summary form in Figure 31. A summary of the main vegetation features of each assemblage is presented in Table 8. I . Muddy tidal flat assemblage Distribufion: The muddy tidal Hat assemblage inhabits the inclined surface of the main tidal mud wedge (Figs 12, 14, 15, 20, 21 and 22). As such it is a common and extensive unit. It is generally a wide unit ranging in width from c. 100 m to 1 000 m depending on the size of the tidal flat and embayment in which it occurs. Main physiahchemical features and gradients within habitat: The muddy tidal flat is an inclined surface (1:1 000-1:5 000) underlain by substrates of bioturbated mud which grades from thixotropic, burrowed and root structured at t. MSL to firm, desiccaicd/plaslic homogenous/root-slructurcd and burrow-punctured at MHWS. Much of the tidal water recharge is into burrows, root conduits and ped boundaries, otherwise excess water sheet flows off the surface. Biolurbation is important and results in the recycling of soils, as well as aeration and groundwater recharge/mixing. Salinity of groundwater and soil water show gradients of c. 45%o at MSL to c. 90-I00°/oo at MHWS. Frequency of flooding varies from twice daily at MSL to fortnightly at levels above MHWS. Comp(fSition, zonation and structure of assemblage (Figs 24, 27 and 30): The muddy tidal flat assemblage is predominantly composed of three species. It is the most widely developed assemblage m this area and is zoned, with distinctive species occurring at various tidal levels across the tidal flat. The zonation and structure of the assemblage are: (1) The most seaward zone at c. MSL is a scrub (or forest) of mature recumbent and overmature Avicennia: the contact with the next zone is usually sharp. (2) closed scrub (or low forest) of Rhizophora and scattered Bruguiera where salinities are 45-55*^/00 (3) closed scrub of Rhizophora (4) closed scrub of Avicennia grading landward to closed heath or shrubland of/lv7a'/7/2/i3. (5) the salt flat. If the assemblage directly adjoins a rocky hinterland then the scrub (^Avicennia may contain an underslorcy of scattered Ceriops and Aegialitis. TABLE 8 Summary of Compositional and Physiognomic features and zonation within assemblages Assemblage Main Vegetation Zones Muddy tidal flat well zoned Avicennia and Rhizophora scrub Sandy tidal flat physiognomically zoned Avicennia forest, scrub or heath Rocky bouldcry shore generally unzoned Avicennia shrubland or scrub; a scrub of mixed species may also occur Spits/cheniers generally zoned Avicennia shrubland, Ceriops scrub. Bruguiera scrub and Avicennia scrub Sandy beach mainly Avicennia scrub mixed with Ceriops. Bruguiera. Aegialitis and Aegiceras Hinterland fringe generally Avicennia scrub or shrubland Ciravcl Hat generally ivicennia scrub High tidal alluvial fan Avicennia and Ceriops or Avicennia shrubland or scrub Mid tidal alluvial fan physiognomically zoned Avicennia forest and scrub with patches of Rhizophora scrub Tidal creek Avicennia and/or Rhizophora scrub High lidal dune margin generally Avicennia scrub or shrubland High tidal lagoon Avicennia scrub or shrubland Shoal Aegiceras scrub with underslorcy of Aegialitis Limestone pavement Avicennia shrubland 54 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. LEGEND Rocky terrestial hinterland Supratidal beach ridge, dune, sandplain Intertidal sandy beach Supratidal spit or chenier Intertidal spit or chenier High tidal dune / tidal flat contact Hinterland / tidal flat contact Gravel delta and high tidal alluvial fan Salt flat Mid to high tidal mud flat HABITATS Mid to high tidal sand flat Mid to high tidal gravelly flat High tidal lagoon Limestone pavement Limestone cliffline Rocky or bouldery shore Intertidal mud or sand shoal Tidal creek Undifferentiated mid to low tidal flat Study Transect location. Profile diagrams of numbered transects are shown in Figures 24 to 30 ASSEMBLAGES + + + 4- Terrestial o 0 » O □ , „ 0 0 « 0 ° - 0 0 o Supratidal sand plain and spit Sandy beach Spit and chenier fringe iii High tidal dune margin iXTt Hinterland fringe s s High tidal alluvial fan Mid-tidal alluvial fan Sandy tidal flat Muddy tidal flat Gravelly tidal flat High tidal lagoon Rocky bouldery shore Shoal Limestone pavement Tidal creek FLORISTICS Avicennia marina Rhizophora stylosa Ceriops tagal Bruguiera exaristata or Bruguiera and Avicennia Aegiceras corniculatum and Aegialitis annulata Avicennia and Rhizophora Avicennia and Ceriops Avicennia and Aegialitis and Aegiceras Avicennia, Bruguiera and Ceriops A R C B Ae AR AC AAe ABC STRUCTURAL TYPES 2/4 Forest Scrub Shrubland Heath Scrub/heath 2/3 Scrub/shrubland Figure 1 1. — Legend lo habitat, assemblage and vegetation maps shown in Figures 12-22. The location of maps is shown in Figure 2. Note that; (1) the order that plant name symbols are written on a given map unit (eg .AC'2 versus CA2) indicates the order of abundance of the component species: (2) the A3 and A4 vegetation units may be associated v.ith microscale development of salt flat. If a habitat pattern appears on the vegetation map it indicates that the habitat is bare of mangroves. 55 lournal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987 3 KING BAY KING BAY HABITATS A 200m ASSEMBLAGES B Figure 12.— King Bay. A. Habiiai map. B. Vcgciaiion assemblage map. 56 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 12. — King Bay. C. Vegetation tloristics and structure map. 57 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. Figure 13. — Wiihncll Bay. A. Overall habitai map of Withneil Bay. B. Closcup of nonheasiern Wiihnell Bay. C. Vegetation assemblage map. D. Vegetation floristics and structure map. 1000m I A HABITATS WITHNELL BAY B HABITATS y B 200 m D VEGETATION WITHNELL BAY 200m 58 Journal of the Royal Society of Western Australia, Vol. 69, Pari 2, 1 987. Figure 14. Conzinc Bay. A. Habitat map. B. Vegetation assemblage map. C. Vegetation lloristicsand structure map. 59 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. SEARIPPLE PASSAGE HABITATS A OYSTER CATCHER ISLAND SEARIPPLE PASSAGE 200 m Figure I 5. — Searipple Passage. A. Habiial map. B. Vegeiaiion assemblage map. 60 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. Figure 1 5.— Scaripple Passage. C. Vegetation florislics and structure map. 6 ! Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. ANGEL/GIDLEY ISLANDS HABITATS GIDLEY ISLAND ANGEL ISLAND 500m i Figure 16.— Angel Island &. Cddley Island: A. Habiiai map. B. Vegetation assemblage map. Limestone pavement ' assemblage ASSEMBLAGES High tidal alluvial fan assemblage Sandy tidal flat 500m _i 62 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. Figure 16. — Angel Island & Gidley Island: C. Vegetation lloristics and structure map. 63 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 17. — Lewis Islands. .A. Overview of habitats. B, East Lewis Island habitat map. C. East Lewis Island vegetation assemblage map, D. East Lewis Island vegetation Horistics and structure map. E. West Lewis Island habitat map. F. West Lewis Island vegetation assemblage map. G. West Lewis Island vegetation floristics and structure map. 64 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 18. — An overview of habitats on Endcrby Island. 2. Sandy tidal Hal assemblage Distribution: The sandy tidal flat assemblage inhabits the inclined surface of the main tidal sand sheet (Figs 12, 13, 14, 16, 17, 18 and 1 9). It also is a common and extensive unit. The formation is variable in width from c. 10 m up to 200-500 m, depending on the size of the embayment and the steepness of the shore where it occurs. Main physico-chemical features and gradients within habitat: The sandy tidal flat is an inclined surface {c. 1:1000) underlain by substrates of bioturbated root structured sand at c. MSI. through to homogcncous/root- structured and laminated sand at MHWS. Much of the tidal water recharge is into burrows, root conduits and interstitial pore space such that excess water drains quickly and internal drainage is rapid. Bioturbation is important and results in the recycling of soils, soil aeration and groundw'atcr recharge/mixing. Salinity of groundwater and soil water show gradients of c. 45*Voo at MSL to r. 90Woo at MHWS. Frequency of flooding varies from twice daily at MSL to fortnightly at levels above MHWS. Composition, zonation and structure of assemblage {¥\gs. 25, 26 and 28): The sandy tidal flat assemblage is a widely developed mangal in this area. It consists mainly of Avicennia, with very subordinant occurrence of Rhizophora and Ceriops. It is structurally zoned with distinctive structure types occurring at various levels across the tidal Hal. The zonation and structure of the assemblage are: (1) The most seaward zone at c. MSL is a forest (or closed scrub) of mature, recumbent and overmature Avicennia: the contact with the next zone is gradational; (2) closed scrub of Avicennia-, very scattered shrubs and saplings of Rhizophora may occur as an understorcy component: (3) closed Avicennia scrub grading landward to heath or shrubland: (4) the salt flat. 3. Rocky-bouldery shore assemblage Distribution: This assemblage inhabits the bouldcry or rocky environments that form the headlands of embayments and occurs elsewhere intermittently along the extensive rocky shores of the archipelago islands (Figs. 12. 14, 16, 17, 18 and 19). The assemblage is regionally common since rocky shores arc widespread. It is variable in width (less than Im ranging up to 30 m) depending on slope, lype of substrate and degree of exposure. Main physico-chemical features and gradients within habitat: Bouldcry, gravelly and fissured rock substrates are wave agitated and arc veneered by a thin and patchy sediment cover. Internal drainage and seepage arc rapid. Groundwater and soilwatcr where sampled show a gradient of salinity from c. 45W oo at MSL to c. 90°/oo at HWS. Composition, zonation and structure of assemblage (Figs. 24, 28 and 29): In most locations the rocky/bouldery shore assemblage is simply an open shrubland of Avicennia. However, in some rocky shore locations the assemblage contains a diverse aggregation of mangrove species. Where zonation is fully developed the assemblage is comprised of the following zones: — (1) a seaward shrubland mainly of mature and overmature Avicennia: (2) closed to open scrub of immature and mature Rhizophora and Avicennia: (3) a landward open heath/shrubland of mixed Avicennia Ceriops ?iw<\ rarely Brugiiiera. The density of mangroves is variable dependent on the exposure of the shore. 4. Spit and chenicr fringe assemblage Distribution: This assemblage inhabits the intertidal portions of sand spits and cheniers (Figs. 12, 13, 15, 20, 21 and 22). As such it is common but limited in area' The formation is usually only narrow c. 5-10 m wide. Its length is determined by the length of the spit on which it occurs; this is variable from 20 m to greater than 500 m. 65 Journal of Ihe Royal Society of Western Australia, Vol. 69, Part 2, 1987 Figure 19. — Endcrby Island. “Alpha Cove''. .A. Habitat map. B. Vegetation assemblage map. 66 Journal of Ihc Royal Society of Western Australia, Vol. 69, Part 2, 1987. Main physicn-chemica! features and gradients within habitats: The habitat where this assemblage occurs exhibits three types of gradients. The first is the inundation frequency. The sandy slopes of spits are inundated by tides twice daily at levels of HW'N, fortnightly at HWS levels and arc inundated by only storm water at supratidal levels. The second gradient involves substrates which vary from clean sands at landward parts of spit (and chenier) slopes, to mixed muddy sand/sand intercalations in transition /.ones, to sandy mud/muddy sand surfaces at seaward parts of slopes. Burrowing organisms mix the various layers of sediment at any given site. The third gradient is groundwalcr/soilwatcr salinity which varies from 40‘V()o at MSL to 60 or 70"/oo at MHWS; thereafter depending on whether there is a freshwater lens under the crest of the spit or chenier. salinity may trend towards freshwater or to increasing hypersalinity. Composition, conation and structure oj'assemh/aye {F\gs. 25, 26 and 30); The assemblage fringing a spit or chenier forms closed low forests and scrub at seaward parts, to open heath at high tide extremities. If fully developed on a relatively gently sloping shore, broadly four zones can be distinguishable: ( 1 ) a seaward zone oi' Avicennia scrub; (2) a zone between MHWN and MHWS of mixed species in which Rhizophora. Avicennia and Bruguiera arc dominant forming closed low forest or scrub, with subordinanl Ceriops and Aegialitis: (3) a middle zone at c. MHWS of Ceriops and Avicennia forming scrub to tall heath with subordinate Bruguiera, Aegialitis; (4) a landward zone above MHWS of Avicennia forming open to closed heath or shrubland. Where the spit or chenier shore is steep, a zone of mixed Ceriops. Bruguiera, Avicennia and Rhizophora may develop. 5. Sandy beach assemblage Distribution: The beach assemblage inhabits the steep surface of semi-exposed sandy beaches (Figs 12, 14, 18 and 19). As such it occurs in many locations throughout the region but is limited in area. The unit generally occurs as a narrow (c. 5-20 m wide) patch on the most sheltered portion of the beach. Main physico-chemical features and gradients within habitat: As with the spit environment this habitat exhibits three types of gradients: (1) inundation frequency between levels of MSL and HWS (2) substrates, and (3) groundwater/soilwatcr salinity which varies from 40"/oo at MSL to 60 or 70^/oo at MHWS. Internal drainage within the habitat is rapid. 67 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987 Figure 20. — tndcrby Island. “Zeta Bay”. ,A. Habitat map. B. Vegetation assemblage map. C. Vegetation floristics and structure map. 68 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. Figure 2] . — West Inicrcourse Island Complex channel system habitat map overview. WEST INTERCOURSE ISLAND COMPLEX 1000m I Solar salt ponds Composition, zonalion and structure of assemblage (Fig. 24): The beach assemblages form mixed scrub. Broadly, three zones are distinguishable: ( 1 ) a seaward zone between MSL and MHWN of mixed species in which Avicennia is dominant, with subordinant Rhizophora, Aegialitis^w^ Aegiceras: (2) a middle zone at c. MHWN of Rhizophora, Avicennia with subordinate Rruguiera, Ceriops. Aegialitis; (3) a landward zone above MHWS of Avicennia and Ceriops. 6. Hinterland fringe assemblage Distribution: The hinterland fringe assemblage forms a discontinuous belt of mangrove vegetation along the edge of the hinterland (Figs 1 2, I4, 1 5, 1 8, 1 9 and 20). The unit is very narrow (usually 2 m-5 m wide), but may be developed along a substantial length of the hinterland margin. Its continuity is disrupted by either alluvial fans, or spits, or patches of extreme hypersalinily of groundwater/soilwater. Main physico-chemical features and gradients within habitat: The hinterland fringe assemblage inhabits the tidal llat/hinterland contact. The habitat is distant from any prevailing marine processes such as frequent tidal inundation and there is no gradient of inundation- frequency that is relevant. The main two gradients within the habitat are salinity and substrates. The groundwater and soilwaicr show a gradient of salinity from hypcrsaline at landward (usually 70-90®/oo) to extreme hypersaline at seaward (in excess of I00%o). Additionally, the substrate grades from being predominantly gravel and muddy sand/gravcl to being more sand and mud dominated to seaward. Mangrove physiognomic/slructural zonation within the assemblage virtually parallels these gradients. Crab bioturbalion is a common process that results in soil turnover, aeration and groundwater/soilwater mixing. 69 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 22.— West Iniercoursc Island. A. Conneciive tidal land habitats, B. Connective tidal land vegetation llorisiics and structure. C'. C'hannel shore habitats. D. Channel shore vegetation structure and lloristics. 70 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. Composition, zonation and structure of assemblage (Figs. 24, 25 and 27): The mangrove of the hinterland fringe assemblage forms a shrubland to closed scrub to open heath oi Avicennia. 7. Gravelly tidal flat assemblage Distribution: The gravel flat assemblage inhabits the inclined surface of a tidal sheet of gravel/sandy gravel (Figs. 12, 14 and 17). It is an areally limited and intermittently distributed unit. Generally the forrnation is narrow, c. 50 m wide, but may be up to 200 m wide. Main physico-chemical features and gradients within habitat: The habitat is developed as a gently inclined surface (c. 1:1 000) of gravel/sandy gravel. Much of the substrate is composed of gravel with variable interstitial sand. Tidal flooding varies from twice daily at MSL to ibrtnightly at NHWS. Fauna is not abundant but bioturbation by crabs and other benthos effect soil turnover in interstitial spaces between gravel particles. Groundwaler/soilwalcr salinity is c. 40”/oo at MSL grading up to c. 70-90*^/00 at HWS. Much of the tidal water recharge is through the interstitial pore space such that excess water drains rapidly. Internal drainage is rapid and consequently there is efficient recharge and discharge of groundwater. Composition, zonation and structure of assemblage (Fig. 27): This assemblage has simple composition and structure with little tendency toward zonation. However, when developed, zonation is as follows: (1) to seaward there is a closed low forest to scrub of Avicennia, followed to landward by (2) closed scrub of Avicennia with a scattered understorey of Rhizophora shrubs and saplings, followed by (3) closed scrub of Avicennia with localised patches of salt flat. 8. High tidal alluvial fan assemblage Distribution: High-tidal alluvial fan assemblage inhabits the high-tidal to supralidal alluvial fans (Figs. 12, 13, 14, 15. 16, 18, 19 and 20). The assemblage is common but scattered in small patches corresponding to sites where terrestrial drainage intersects the tidal flats. The extent of the unit is determined by the size of the habitat; usually the assemblage covers an area some 1 00 m or less across. Main physico-chemical features and gradients within habitat: the habitat is generally fan or deltoid in shape and is underlain by gravelly and sandy substrates intcriaycred with muddy layers and cemented by calcrete sheets. Freshwater seeps into the habitat and maintains a gradient of groundwaler/soilwater salinity which vanes from bO^^/oo at extreme landward sites to (•. 90°/oo and greater at seaward sites. Composition, zonafton and structure of assemblage (Figs 25, 26, 28 and 29): The high tidal alluvial fan assemblage contains 2 species of mangrove: Avicennia and Ceriops. There is compositional and structural zonation in response to salinity gradients: (!) a landward zone of Avicennia forming closed to open heath where there is marked freshwater seepage; (2) a middle zone of Avicennia and Ceriops and rare Bruguiera forming closed to open heath to scrub; (3) a seaward zone mostly of closed to open heath of Avicennia which may grade into, or abruptly join, the salt flat. Where there is limited freshwater influx the assemblage may consist of Avicennia open shrubland. 9. Mid-tidal alluvial fan assemblage Distribution: The mid-tidal alluvial fan assemblage inhabits the gravelly fan-shaped alluvial deposits that debouch onto the mid-tidal fiat (Figs 13, 1 4, 2 1 and 22). As such it is an areally limited and intermittently distributed unit. Generally the formation is c. 300 m across the fan. Main physio-chemical features and gradients within habitat: The habitat is developed mainly at MSL-HWN and accordingly there is little development of gradients. The surface of the habitat is comprised of gravel. Groundwatcr/soilwalcr is c. 40*’/oo at MSL grading up to 55*Voo at HWN. Internal drainage is rapid. Bioturbation by crabs and other fauna effect soil turnover in interstitial spaces between gravel components. Composition, zonation and structure of assemblage (Fig. 30): This assemblage has simple composition and structure with little tendency toward zonation. However, three compositional/structural extremes arc evident in the assemblage: ( 1 ) closed low forest to scrub of Avicennia: (2) closed scrub of Avicennia with a scattered understorey of Rhizophora shrubs and saplings; (3) closed scrub of Avicennia and less common Rhizophora. 10. Tidal creek assemblage Distribution: This assemblage inhabits the banks of tidal creeks (Figs 12. 15 and 20). As such it is a relatively common, but generally narrow unit only c. 5-10 m wide; its length, as determined by the length of the creek habitat, may be up to 500 m long. Main physico-chemical features and gradients within habitat: The creek banks are underlain by a substrate of bioturbated mud which grades from thixotropic and root-structured at c. MSL, to plastic, root-structured and burrow-structured at r. HWN. The surface is sleep or cliffed at the creek bank edge, and adjoining slopes arc relatively more steeply inclined (1:100-1:50) than the main tidal flat. Much of the tidal water recharge is through burrows and root conduits. Internal drainage and run off are relatively more rapid than the main tidal fiat. Groundwater and soilwater salinities for wide, deep creeks, vary from c. 40*^/oo at MSL to c. 50®/oo at (landward) parts of the habitat where it grades upslopc and laterally into the main tidal fiat, and c. 80-90‘Voo at headwater extremities at levels of c. MHWS. Frequency of fiooding is twice daily for several hours at the creek- bank channel edge, and for several hours per week at the headwater extremes of the habitat. 71 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Composition, zonal ion and structure of assemblage (Fig. 24); The tidal crock assemblage is composed of Avicennia and Rhizophora and the assemblage is not zoned. The assemblage consists of either of these two components: (1) mainly Rhizophora with very scattered Avicennia: these inhabit and overhang steep banks and form closed low forests, or (2) Rhizophora and 'Uicenma in approximately equal proportions forming closed low forests. Frequently, the only difference between the creek bank assemblage and the adjacent tidal Fat assemblage is an increase in the height and luxuriance Avicennia and Rhizophora. As such the creek bank assemblage is generally structurally distinct from the adjacent tidal flat. 1 1 . High tidal dune margin assemblage Distribution: This assemblage inhabits the narrow zone of contact where a muddy tidal salt flat adjoins a dune terrain (Figs 13, 14, 16, 19 and 20). The assemblage Itself is narrow. 5-10 m wide, and its length is determined by the extent of the interface between the geomorphic units. Main physico-chemical features and gradients within habitat: There is a minor slope gradient since the habitat is characterised by a slightly steeper slope than the adjoining tidal flat. In addition there may be a gradient in substrate properties with more sandy substrates occurring to landward. Within the habitat there also is a narrow gradient in groundwater and soilwatcr salinity since the hypcrsalinily of the adjoining sail flat is diluted by seepage of fresh water or oceanic water from the toe of the dunes. The salinity is c. 50-70"/i>o within the habitat, and grades abruptly to 90-100*^/00 at the contact with the adjoining salt flat. Crabs and other burrowing biota are important benthic faunal components of this habitat. Composition, zonation and structure of assemblage (Figs 26 and 27); The dune margin assemblage generally consists of a simple Avicennia shrubland or scrub. However in some localities where there is marked freshwater or seawater seepage there is a zonation of mangrove from landward to seaward; ( 1 ) A vtcennia low open heath; (2) mixed assemblage oi Avicennia and Ceriops closed heath to scrub with less common Bruguiera and Aegialitis: (3) Avicennia low closed heath to scrub. 12. High-tidal lagoon assemblage Distribution: This assemblage occurs where a beach ridge or dune has impounded a depression or a tidal creek which has cut through a dune terrain (Figs 14 and 18). The assemblage occurs along the lagoon margins. As such the assemblage is restricted in distribution. The unit is very narrow fringe usually 1-4 shrubs wide along the edge of lagoons, and may be developed along some lagoons for up to 300 m. Main physico-chemical gradients with habitat: This habitat is situated mainly at HWS and receives tidal recharge fortnightly: its substrates are relatively homogeneous and sandy. Accordingly, the main gradient within the habitat is salinity and depth of waterlogging. The soilwater/groundwater salinity varies from hypersaline (c. 50-90®/oo) along lagoon margins, where seepage from adjacent sand bodies occurs, to excessively hypersaline along lagoon centres. The depth of the waterlogging varies from less than Im depth along lagoon banks to greater than I m depth in the centre. Composition, zonation and structure of assemblage (Fig. 27); The assemblage is composed only of one species, Avicennia manna, which forms an open scrub or shrubland. 13. Shoal assemblage Distribution: The shoal assemblage occurs on localised shoal build-ups within creek channels and creek mouths (Figs. 12, 15 and 20). However, since not all creeks have appropriate shoals, the distribution of this assemblage is limited and patchy. The unit is usually limited in area extent and covers shoals c. 1 0-20 m across the w idth. Main physico-chemical features and gradients within habitat: The shoal habitat is fairly consistent in its internal physico-chemical environment with little lenciency for differentiation into internal gradients. The habitat is underlain by thixotropic mud, or muddy sand or sand with salinity of c. 40®/oo. The only gradient is a relatively minor one of frequency of inundation, but even so the bulk of the shoals arc inundated daily for several hours with each tidal flooding. The slopes of shoals arc variable ranging from hummocky and flat- topped types where they occur in tidal creek areas, to steeply-inclined surfaces where they are plastered on the low parts of the cliftbd creek banks. Composition, zonation and structure (Fig. 24): This assemblage has simple structure with little tendency toward zonation. However, three structural/ compositional extremes arc evident in the assemblage: ( 1 ) closed heath to scrub dominated by Aegiceras; (2) open heaths oCAcgialitis: (3) closed/open heath Aegiceras with an understorey Aegialitis. Avicennia seedlings, saplings and small shrubs form a local minor part of this assemblage. 1 4. Limestone pavement assemblage Distribution: This assemblage inhabits limestone pavements and associated limestone slab/boulder sheets (Figs. 13, 15, 16. 19 and 20) but it is not a widespread unit. Generally the formation is limited in both its area extent and width (less than 1 5 m wide). Main physico-chemical features and gradients within habitat: The habitat is wave agitated and generally swept clean of sand and muddy sediment. Large slabs and boulders may be strewn on the surface and locally there may be n.ssured and cliffed limestone. The main gradient evident is frequency of inundation; tidal waters flood the habitat twice daily at MSL and fortnightly at MHWS. Boring and burrowing fauna (such as crabs, gobioid fish, octopus) inhabit cavities and excavations in the limestone but these appear to have little intluence on mangrove populations. Groundwater was loo difficult to sample in limestone so that information on this factor is lacking, and soilwatcr was not sampled because of the paucity of soil. Compositon. zonation and structure of assemblage (Fig. 26); In most locations the limestone pavement assemblage is an open shrubland of Avicennia developed in small groves or patches. 72 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. 73 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 24. — Profiles showing the information obtained from field surveys. .A, Transect 1. King Bay, B. Transect 2. King Bay. C. Transect 3. King Bay. D, Transect 4, King Bay. 74 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 25. — Profiles showing the information obtained from field surveys. A, Transect 5. King Bay. B. Transect 6, King Bay. C, Transect 7, King Bay.D, Transect 8. King Bay. 75 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. 76 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 27. — Profiles showing the information obtained from field surveys. A, T ransect 1 4. C on/'inc Bay. B. Transect 1 5. Conzinc Bay. C , Transect 1 6. Zela Bay. Endcrby Island. D, Transect 17. Searipple Passage. 77 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 28. — Profiles showing the information obtained from field surveys. A. Transect 18. West Lewis Island. B, Transect 19. East Lewis Island. C, Transect 20. Angel Island. D, Transect 2 1 . Angel Island. 78 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 29. — Profiles showing the information obtained from field surveys. A, Transect 22, "Alpha Cove", Enderby Island. B, Transect 23, "Alpha Cover”, Enderby Island. C. Transect 24. "Alpha Cove”, Enderby Island, D. Transect 25. "Alpha Cove”. Enderby Island. 79 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Figure 30.— Profiles showing the information obtained from tlcld surveys. A. Transect 26. West Intercourse Island Complex B Transect 27 West Intercourse Island C omplex. C, Transect 28. West Intercourse Island Complex. 80 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. Description of selected areas This section is a description of the coastal and mangrove environments of several areas which cover the variability of habitat type and coastal setting in the Dampicr Archipelago. The areas chosen for description are (Fig. 2): 1. King Bay 2. Withnell Bay 3. Conzinc Bay 4. Searipple Passage 5. Angel Island/Gidley Island 6. Malus Islands 7. Lewis Islands 8. Enderby island 9. West Intercourse Island area. Each of these areas are described in a similar format of: (1) coastal setting. (2) large scale and medium scale geomorphology and mangrove habitats and (3) mangrove vegetation. This approach thus provides information on the mangrove and their environments within one of three categories: on the large important mangroves areas in the Dampier Archipelago (eg. King Bay and Western Intercourse Island area), on other typical areas (such as Lewis Islands) and on unique or atypical areas (such as Searipple Passage). Figure 32 schematically illustrates some of the key features occurring in selected areas. While the descriptions of the selected areas indicate a degree of similarity between various parts of the archipelago in that there is a recurrence of habitat types, they also highlight the heterogeneity that occurs at all scales and throughout the region. This aspect of heterogeneity thus emphasises the peculiarities of each selected area. I . King Bay Coastal selling. King Bay is an E-W oriented embayment system formed by tidal sedimentation and dune backfill of a narrow strait between two former islands. The embayment is backed by rocky hinterland to the north and south, and is backed by an extensive supratidal flat followed by dunes of Hcarson Cove to the east. The main King Bay embayment thus forms a V-shaped embayment system. However, its north and south margins are markedly scalloped and indented such that a number of smaller subsidiary embayments arc developed, some of which arc tidal mud filled and some beach/dunc backfillcd. (jeomorpholo^y and habitats: The King Bay embayment is a typical example of a large narrow embayment system (Fig. 12A). At flic exposed entrance to the embayment there are local rocky and bouldcry shores and dune- backed beaches. The southeast shore of King Bay is more exposed to wave action (from westerly, northwest, and refracted rionherly sectors) so that it is dominated by spits and sandy tidal flats. Generally, the King Bay system is dominated by typical medium scale geomorphic units of embayment settings, namely muddy tidal flat, sandy tidal flats, spits, alluvial fans, hinterland margin, rocky shores and tidal creeks. In addition, at the mouths of tidal creeks there is local development of shoals. Mangrove vegetation: Density, floristic diversity and width of the mangrove belt are well developed in King Bay. The vegetation types typical of the various medium scale geomorphic units/habitats in embayment settings arc well represented in the area (Fig. I2B). At the medium scale Avicennia and Rhizophora forests and scrub arc dominant (Fig. 12G). At the smaller scale, various other flori.slic and structural units are developed in response to small scale coastal heterogeneity. For instance: there are additional species and marked zonalion around small and localised high-tidal alluvial fans where there is freshwater seepage; there is structural zonation and a greater diversity of species adjoining the sandy spits; and there is an assemblage of Avicennia- Aegialitis-Aegiceras ow local shoals. 2. Withnell Bay Coastal setting: Withnell Bay is a NE-SW oriented broad rectangular embayment some 2 km long and 0.8 km wide formed by marine flooding of a valley system eroded from a scries of dyke rocks (Fig. 13A). For most of its margins the embayment is bordered by rocky shores, however sedimentary fill at the northeast extremity has developed tidal flats. In contrast to King Bay and Gon/inc Bay, Withnell Bay has relatively straight margins with little development of smaller subsidiary embayments. (ieomorphology and habitats: Withnell Bay is an example of a large open embayment system where wave action has dominated the development of tidal flat features. There is little or no sediment along most of the shores such that rocky shores dominate. Where sediment has accumulated there is development of wave- dominated features, such as sandy tidal flats, spits and dune-backed beaches (Fig. I3B). Other important and well-developed mangrove habitat features arc: a mid tidal alluvial fan: a high tidal alluvial fan; high tidal dune margin habitat: rocky channel or chasm (=a narrow embayment). Mangrove vegetation: Mangroves occur as a relatively thin belt in Withnell Bay and are best developed on the sandy tidal flat, the mid-tidal alluvial fan, along spits, along the rocky channel/chasm to the southwest, and bordering the dune terrain. Avicennia scrub is dominant (Fig. 13C). Elsewhere mangroves arc generally only developed in patches on rocky shores and pocket sandy beaches. Habitat heterogeneity occurs at the small scale and as a result there is marked local floristic and structural variability in mangrove assemblages. For example in the northeast corner of the bay. there are assemblages of spit fringe, hinterland fringe, high tidal alluvial fan, dune fringe and sandy tidal flat within an area 200 m x 200 m (Figs. 1 3B.C). 3. Conzinc Bay Coastal Setting: Conzinc Bay is an open but elongate embayment, oriented SW-NE. It measures some 1 km across and is approximately 3 km long. It has formed by marine flooding and sedimentary backfill of a valley narrowing to the NE. (Ieomorphology and habitats: Conzinc Bay contains three main categories of large scale geomorphic settings for mangroves; these are; straight coast of dune-backed beaches; straight coast of rocky bouidery shores; and an embayment. The beach shores occur along the southwest shore of the bay. They arc open with steep slopes and are backed by dunes. Locally there is developed a high tidal lagoon system. Rocky shores occur dominantly along the northern headland and southwest shores of the bay. The embayment setting occurs in the extreme northeast of Conzinc Bay and there are developed spits, muddy tidal 81 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. Figure 31. — Summary diagram showing key habitat features, such as slope, salinity gradients, substrates and physico-chemical processes, and the resulting mangrove composition, structure and physiognomy for the main assemblages in the Dampier .Archipelago. 82 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1 987. flat, sandy tidal flat, gravel tidal flat, high tidal alluvial fans and hinterland margin habitats. The southeast coast of the embayment is fairly exposed to refracted swell and also is exposed to westerly to southwesterly waves. A gravel tidal flat and mid tidal alluvial fan mark the junction between the more sheltered northern sector of the embayment and the exposed southern coast comprised of rocky shores and sandy beaches (Fig. I4A). Mangrove vegetation: Mangrove vegetation is best developed in terms of width and biomass on the muddy tidal flat, where there is a zoned scrub o{ Avicennia and Rhizophora, and on the mid tidal alluvial fan, which supports Avicennia scrub (Fig. I4B. C). A Rruguicra scrub fringes the spit which occurs immediately behind the north shore headland. The other habitats support more spatially narrow or less dense mangrove stands such as Avicennia shrubland on the rocky shores and tracts of the gravelly tidal flat. 4. Scarippic Passage Coastal setting: Scarippic Passage is a narrow strait, 6 km long and I km wide, separating Burrup “Island" from Dolphin Island. The strait has formed by erosion of E-W oriented fractures and dykes. Presently the strait is underlain by Pleistocene limestone overlying Precambrian bedrock. The limestone forms a pavement at c LWN. and is undercut and eroding at its cast and west extrcmctics to form wide deep-water channels. The north and south shores of Searipple Passage are markedly indented or scalloped such that small embaymenls several hundred metres long and less than 1 00 m across arc developed. . The strait tends to be dominated by waves translating only from east and west, and by turbid tidal currents from the adjoining Nickol Bay. Nonetheless the deeply indented shorelines are protected from these wave directions and consequently are accumulating mud. Geomorphology and hahitats: The large scale gcomorphic components of Searipple Passage are: the passage itself: embayments dominating the north and south shores; small islands. Mangroves occur on the small islands and the embayments (Fig. 15A). Within the embayments there is sediment accumulation and hence there is the typical range of medium scale geomorphic and habitat units: rocky shore headlands, hinterland margin, high- tidal alluvial fans, muddy tidal Hat and tidal creeks. Locally sand-filled embayments have dune-backed beach shores (Fig 15A.B). A few' islands and reefs are scattered through the central Searipple Passage system. These may be developed merely to MSL as bedrock prominences and therefore not as mangrove habitats, or may be larger bedrock islands now flanked by extensive limestone (beachrock and Pleistocene limestone) and sediment deposits. Mangrove vegetation: Well developed and zoned scrub of Rhizophora and Avicennia dominate the small, mud- filled embayments along the shores of the strait (Fig. !5C). Brugutera shrubs also are a common component of the Rhizophora zone: this contrasts with areas elsewhere where the zone of Rhizophora tends to develop pure stands or lends to be associated with uncommon Bruguiera. Avicennia and Rhizophora mixed scrub occurs on the muddy accumulations developed peripheral to the large island in Searipple Passage. Bruguiera and Ceriops occur in association with the spit on this island. Rocky shores and dune-backed beach shores are vegetated by Avicennia scrub and shrubland. 5. Angel Island and (iidlcy Island Coastal selting:T\\Q Angel Island/Gidley Island system is a series of elongate bedrock islands separated by channels formed from inundated valleys developed by eroded dyke rock and fracture systems. For most of the length of the coast in the island complex straight rocky shores dominate. The west shores of Angel and Gidley however are comprised of alternating headlands and embaymenls. but since these shores are exposed to wave action the embaymenls are back-filled with bcach/dune sediments. The most sheltered shores occur along the channel or strait that separates Angel and Gidley Islands. Geomorphology and habitats: Rocky shores and dune- backed beach shores dominate the Angel Island/Gidley Island system. In the more protected passage between the two islands, rocky and bouldcry shores, fronted by a narrow strip of sandy tidal flats, are dominant, but there is locally developed a diversity of small habitat units of connective tidal-lands, limestone pavement, pockets of dune plain and small embayments with the units of rocky heaoland, sandy tidal flats, high tidal alluvial fan, and hinterland margin (Fig. 16A). Mangrove vegetation: Scrub and shrubland, mainly of Avicennia. inhabit local areas along the extensive rocky shores and dune-backed beach shores of this system. Within the relatively protected strait there is more variability in the composition and structure of mangrove vegetation with the occurrence of Ceriops, Rhizophora, and Bruguiera (Fig. 16B, C) in association with the greater habitat diversity. Mixed Avicennia and Rhizophora .scrub dominates the rocky shores with Bruguiera scrub and Ceriops scrub also locally present. These floristic combinations are uncommon for rocky shore assemblages in the Dampicr Archipelago region. Mixed Avicennia and Aegialitis scrub also occur, and this combination is more typical of rocky shores in the region. The sandy tidal Hats of the small embayments support zoned scrub oi Avicennia and Rhizophora. There also is an unusual combination of Bruguiera. Ceriops, Rhizophora and Avicennia mixed scrub on the high tidal alluvial fan of this embayment. 6. Malus Islands Coastal setting: The Malus Islands are a series of small bedrock islands connected by tidal and emergent sand spits. On the southern and relatively more protected side of the system there is local development of mangrove. Geomorphology and hahitats: The Malus Island system represents an example of a locally developed habitat where partial protection from prevailing waves has developed a relatively small area of sandy coast amenable to mangrove. The mangrove habitat is comprised of a steep sandy beach and adjoining bouldcry shore. Mangrove vegetation: Scrub of mixed Avicennia, Rhizophora, Bruguiera, Ceriops and Aegialitis occur in this local small development of mangrove. 83 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. 7. Lewis Islands Coastal setting: The Lewis Islands are composed of East Lewis Island and West Lewis Island. The terrain of the large, irregular, bedrock islands is well dissected and accordingly the coastal topography is dominated by rocky shores alternating with small scale embayments. back filled either by beach/dune sediment on exposed shores, or by tidal flat sediment in protected environments. The two islands are connected by tidal spits which are an extension of sandy cusps developed on the shores; these spits span the inter-island channel. Geomorphology and hahiiais: Four coastal geomorphic settings arc present on the islands: straight coasts of rocky or gravelly shores; straight coasts of sandy beaches; straight coasts of narrow sandy tidal flats; and embayments. Rocky and bouldery shores arc the main type of coastline because of the exposed nature of the islands. Sandy beaches and sandy tidal flats, backed by dunes, are developed along some portions of the straight coast, along the cuspate segment of coast (Fig. 17). and within small exposed embayments. The embayment settings, although small and not common on these islands, exhibit the largest variability of habitat, with the development of tidal flat, hinterland margin, high-tidal alluvial fans and rocky headlands. Mangrove vegetation: Mangroves along the rocky shore are dominated by Avicennia shrubland and scrub with less common subsidiary occurrences of mixed Avicennia and Rhizophora scrub. Those along dune-backed beach or sandy tidal flat shores are composed of Avicennia scrub and shrubland (Fig. 17). The vegetation formations on rocky shores and dune backed beach shores are the most common types along the Lewis Island system. In the localised embayment setting there is development of more diverse mangrove assemblages in response to development of diverse habitats, but Avicennia and Rhizophora scrub are most common. 8. Endcrby Island Coastal setting: Endcrby Island is a markedly indented/scalloped island system which has formed by the post glacial transgression isolating a scries of bedrock hills (Fig. 18). Earlier in the Holocene the island system was comprised of at least 5 separate, closely spaced islands. However, sedimentary 1111 by tidal mud deposits, sand spits and dunes has formed connective land bridges such that the main island today is a complex amalgamation of these once isolated smaller islands. This .sedimentary modification and addition has also resulted in development of various shore types additional to the simple distribution of rocky shores. Geomorphology and habitats: Endcrby Island contains the three major categories of large scale geomorphic settings for mangroves, i.e.. embayments, straight coast of dune backed beach shores, and straight coast of rocky bouldery shores. The embayment settings range in size from small systems some 100 m across, to large systems some I 000 m broad and may be muddy or sandy. They occur predominantly on the south to east facing sectors of the island, protected from prevailing swell. The south facing embayments also are protected from prevailing wind waves. The embayments have the typical range of medium scale geomorphic units and habitats, ie, muddy tidal flat, sandy tidal flat, spits, rocky shore, hinterland margin and high tidal alluvial fans. The dune-backed beach shores are widespread but discontinuous along Endcrby Island. The best developed beach-dune shores occur on the north facing coasts that receive prevailing swell and refracted wind waves. Generally beach-dune shores are comprised of a steep beach with a backing dune. Locally there is developed the high tidal lagoon system wherein a tidal creek draining a dune terrain has been partly blocked by shoreline spits and dunes. The rocky/bouldcry shores arc widespread units. Generally, they are either fissured rock shores or bouldery shores with local small pocket beaches and sand sheets. Vegetation and habitat maps of the 2 largest embayments on the island “Alpha Cove" and “Zeta Bay” are presented in Figs. 19 and 20. "Alpha Cove" is a broad embayment, opening to the north cast and is consequently exposed to wind waves. As a result the inner portion of the cove consists of a sandy tidal flat. Rocky shore habitats extend into the mouth of the cove, and tracts of tidal flats arc flanked at their landward edge by sand accumulations. “Zeta Bay" has been formed by dune backfill of an inter- island strait. Interestingly, this dunc/tidal flat contact is marked by freshwater seepage and a fringe of mangrove has established. This bay has developed in the lower energy conditions behind the rocky shores and a sand spit constriction at its narrow mouth, and consequently consists predominantly of a muddy tidal flat. Mangrove vegetation: Mangrove vegetation is limited in extent along the shores of Enderby Island and occurs mainly in the two large embayments, the most extensive occurring in “Zeta Bay". In these embayments the mangroves form forests, scrub and heath typical of embayment habitats. The rocky and gravelly shore assemblages of “Alpha Cove” are exceptionally well developed and regionally unusual, consisting of dense stands of floristically mixed scrub. Avicennia scrub dominates the main mangrove belt, although zones of both Rhizophora scrub and forest occur. Ceriops scrub occurs on sandy beaches. The narrowness of the tidal flat along much of the shores of this cove has meant that salt flat generally is not widespread, although some patches of heath and shrubland grading into incipient salt flat occur in the middle of the zones Avicennia scrub. Zoned Avicennia and Rhizophora scrub dominate the main mangrove flat of “Zeta Bay". Avicennia scrub and shrubland comprises the high tidal dune margin, high tidal alluvial fan, spit margin and limestone pavement assemblages. Bruguiera was noted to occur in scrub with Rhizophora and Avicennia as a spit fringe assemblage at the mouth of the embayment. Overall, however, the mangroves of this embayment are not structurally or floristically diverse. Elsewhere on Enderby Island mangrove cover along exposed rocky and bouldery shores and dune-backed beach shores is discontinuous, patchy and narrow'. 9. West Intercourse Island Area Coastal setting: The area south of West Intercourse Island has developed by marine flooding and sedimentary fill of low-lands between a series of bedrock hills (now islands). This area adjoins the Maitland Delta complex to the west and the alluvium/sand plains of the mainland hinterland to the south. The West Intercourse Island area is significantly protected from wave action. 84 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. and consequently large areas of connective tidal-lands have developed. The area is however subject to tidal current flow through the abundant channels in the system. The best developed large scale geomorphic units in the area are the connective tidal-lands. These units are composed of mud and sand accumulations forming tidal flat surfaces transversed by tidal drainage channels and creeks. The connective tidal-lands bridge the alluvial sand plain of the mainland hinterland to Burrup "Island" and West Intercourse Island, and also bridge a series of smaller islands that arc amalgamated into an island complex that comprises the West Intercourse Island system. Geomorphology and habitats: The West Intercourse Island area is a complex coastal system comprised of: rocky and gravelly shores; connective tidal-lands; and major tidal creeks and straits. Rocky shores occur along the channel defined by the south shore of West Intercourse Island and the north shore of the southwest part of Burrup "Island". These shores arc comprised of alternating stretches of fissured rock shores, bouldery to gravelly shores, local bedrock prominences (islets), pocket beaches, discrete alluvial fans and locally developed small embayments which are small scale sediment pockets (Fig. 21). Within the connective tidal-lands system there arc muddy tidal flats, spits and cheniers. tidal creeks, shoals, rocky shores, and, where there is nearby bedrock, hinterland margin units and high-tidal alluvial fans. The junction of this tidal flat terrain with the sandy mainland hinterland is a system of low red sand hummocks dissected by small creeks around their margins. Mangrove vegetation: Mangroves are best developed, in terms of abundance and diversity, on the connective tidal-lands. These systems are dominated by muddy tidal flats which support a zoned assemblage of Avicennia forest. Rhizophora scrub, Avicennia scrub and locally Ceriops-Avicennia heath, grading to landward into a salt flat. Ceriops-Avicennia scrub and shrubland occur along the margins of spits and cheniers in the area (Fig. 22). Tidal creek banks support Hhizitplutra- Avicennia low forest or scrub and. on accretionary points, Aegiceras-Aegialitis heath. Mid-channcl shoals within creeks, depending on geomorphic stage of development, support heaths of Aegiceras-Aegialitis. as well as scrub and low forest of Rhizophora and Avicennia. Where locally developed, hinterland margins and high-tidal alluvial fans support Avicennia heath with subordinani Ceriops. The rocky shore mangrove formations consist of a narrow scrub (up to 10 m wide) dominated by Avicennia with subordinate Rhizophora. The local areas where alluvium has penetrated onto the shore to develop high- tidal to mid-tidal alluvial fans support a well developed, zoned mangrove assemblage where Avicennia. Rhizophora, Ceriops. and Aegialitis contribute to heath, scrub and low forests (Fig. 22). Discussion The various areas described above provide several insights into mangroves and their habitats in the Dampier Archipelago. Firstly, it is obvious that mangrove formations inhabit a wide range of small scale habitats that are developed by regional processes and small scale processes interacting with the local environmcnl. Thus a terrain comprised of dissected bedrock will develop, depending on aspect and coastal processes, a range of habitats such as mud-filled embayments, sand-filled embayments, spit-lined headlands, bouldery or rocky headlands, high-tidal alluvial fans, tidal creeks, etc. Because of the heterogeneity in landform and its relationship to metcrologic and oceanographic aspect (waves and wind), there is a concommitant heterogeneous distribution of mangrove habitat. The embayment setting provides the most internally heterogeneous aggregate of habitats in the Dampier Archipelago. The heterogeneity is a function of ancestral landform interacting with waves and wind. Superimposed on this is the further pattern of discordant incursions of alluvial fans shed onto the tidal zone by upland streams. The interaction of marine w'ater and fresh water along localised, or specific, or ubiquitous aquifers also provides small scale habitat heterogeneities. As a result, it may be considered that mangroves have opportunistically inhabited the variable habitats, resulting in the complex mosaic of assemblages and populations evident today. However, while small scale heterogeneity is emphasised here it should be noted that many of the habitats arc recurring. For instance, rocky shores, with or without continuous mangrove cover, are essentially widespread units and may be considered to be the most common mangrove habitat (per unit length of coast). Other units such as spits, or alluvial fans, while discrete in occurrence and limited in area, also arc quite widespread as units. They occur virtually in every setting and in most of the described areas. Secondly, the best developed mangrove formations, as reflected in lu.xuriance. abundance, and diversity, occur in embayment settings and connective tidal-land settings, where there is an accumulation of tidal mud. Here the habitats are protected from wave action and the substrates support a rich benthic fauna. The areas of West Intercourse Island and King Bay exemplify this. Thirdly, the zonation involving the greatest number of species occurs in 2 main settings: (1) peripheral to those alluvial fans that intrude onto the tidal flat across the full mangrove tidal range; and (2) peripheral to a sand terrain such as spits and beaches. In both cases the substrates are coarse, well drained and arc subject to fresh water seepage or at least oceanic water seepage. Finally, since there is only a small species pool in the region, the environmental forcing factors can only influence three aspects of mangrove population; these arc: (1) composition of populations, by regulating and eliminating species, (2) structure, and (3) maintenance of population. Environmental gradients and differences between habitats therefore are reflected in a variety of structural/physiognomic categories within a limited floristic assemblage. Thus Avicennia will form forests of 85 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. ENDERBY ISLAND i N Sandy beach shores | Sandy tidal flat Rocky shores | Muddy tidal flat | CONZINC BAY WITHNELL BAY High tidal alluvial fan Dunes High tidal dune margin) Mid-high tidal alluvial fatTj KING BAY Muddy tidal flat Sandy tidal flat Muddy tidal flat, ^4^^ A N .High tidal alluvial fans 4- + + + 4- + 4- + + 1 i- + + + + + + + 0 200 m 1 I Figure 32. — Summar\' diagram showing main features or unique features of the various selected areas in Dampier Archipelago. At this scale the heterogeneity of the coast is evident but the recurrence of some specific geomorphic units, such as alluvial fans and spits, is also evident. 86 Journal of the Royal Society of Western Australia, Vol. 69, Part 2, 1987. recumbent mature and overmature trees in one habitat, it will develop scrub of multi-stemmed shrubs in another, and perhaps it will form a heath of columnar shrubs in a third. This aspect of differential response of mangrove composition, physiognomy, structure and maintenance to variable habitats has to be addressed and understood if environmentally meaningful classifications of floristics and structure arc to be developed for mangrove vegetation. The approach adopted in this study has emphasised the physico-chemical factors that lead to habitat variability and coastal heterogeneity at various scales of reference. Mangroves then are viewed simply as opportunistic inhabitants of this variably developed coast. Acknowledgements. — Portions of the MS relevant to their fields of expertise were reviewed by S. Buchan and CJ. Marston; their help and discussions arc gratefully acknowledged. Contributions to some of the costs of publication of this paper were made by VCSRG Educational &. Research Consultants, LeProvost Semcniuk & Chalmcr. Environmental Consultants, and Woodside Offshore Petroleum Ply Ltd. This support also is gratefully acknowledged. 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To avoid unnecessary handling of the original illustrations, which arc best prepared between I '/z and 2 limes the required size, authors are advised lo supply extra prints already reduced. Additional printing costs, such as those for folding maps or colour blocks, will be charged lo authors. Supplementary Publications. Extensive sets of data, such as large tables or long appendices, may be classed as Supplementary Publications and not printed with the paper. Supplementary Publications will be lodged with the Society’s Library (C7- Western Australian Museum, Perth, W.A. 6000) and with the National Library of Australia (Manuscript Section. Parkes Place, Barton, A.C.T. 2600) and photocopies may be obtained from either institution upon payment of a fee. JOURNAL OF THE ROYAL SOCIETY OF WESTERN AUSTRALIA CONTENTS VOLUME 69 PART 2 1987 Page Mangroves oflhe Dampicr Archipelago. Wcslern Australia by V. Semeniuk and P. A. S. Wurm 29 Editor: B. Dell Regislcred by Australia Post — Publication No. WBG 0351. No claim for non-rcccipt of the Journal will be entertained unless it is received within 12 months after publication of Pan 4 of each Volume. The Royal Society of Western Australia, Western Australian Museum, Perth. i 51396/11/86- L/4223 WILLIAM C. BROWN. Government Printer, Western Australia ISSN 00-35-922X VOLUME 69 PART 2