EPA-905/3-79-001 PHYTOPLANKTON ASSEMBLAGES OF THE NEARSHORE ZONE OF SOUTHERN LAKE MICHIGAN by E. F. Stoermer and Marc L. Tuchman Great Lakes Research Division University of Michigan Ann Arbor, Michigan 48109 Grant R005337 01 Project Officer David C. Rockwell Great Lakes National Program Office 536 South Clark Street Chicago, Illinois 60605 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION V CHICAGO, ILLINOIS 60609 DISCLAIMER This report has been reviewed by the Great Lakes National Program Office, U. S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the U. S. Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. 11 ABSTRACT Phytoplankton samples from nearshore stations along the Indiana coast of Lake Michigan were analyzed to determine the composition and seasonal abundance of phytoplankton populations in this region. Occurrence patterns of major populations and population groups were inspected to detect unusual patterns which might be indicative of particular inputs to this region of Lake Michigan • As might be expected in a local inshore region where physical mixing and advection processes are relatively intense, phytoplankton distribution is highly variable. The largest general effect noted is a continuing increase in groups other than diatoms, apparently as a result of silica depletion resulting from phosphorus enrichment. The singular exception to this trend is the abundant occurrence of Cvclotella comensis . a diatom which has only recently become abundant in Lake Michigan and which can apparently tolerate very low silica levels. An effect more specific to the region is the atypically high abundance of members of the diatom genus Nitzschia during some sampling periods. High abundance of these organisms is often associated with organic nitrogen and ammonia inputs, and this appears to be the case in the Indiana nearshore region of Lake Michigan. Occasional occurrences of populations such as Th^l,a?949?Jrra sp. and Skeletonema spp. were also noted in the samples. These appear to be associated with isolated water masses and may be indicative of local areas of high conservative ion input. It should be noted that the thermal bar period, when the effects of conservative ion loadings might be expected to be most intense, was not represented in the samples examined. Another characteristic of the phytoplankton assemblages in the Indiana nearshore region is the high abundance of microflagellates, especially iii t^i-^s^rxiains which apparently belong to the Haptophyceae or Prasinophyceae. Although these organisms are known to be abundant in areas of the Great Lakes which are substantially perturbed, and are apparently generally increasing in Lake Michigan, little is known about their specific ecology due to methodological difficulties in identification. Further research should be devoted to this topic. iv CONTENTS Abstract ill List of Figures ......... vi List of Tables vi Acknowledgments .vii 1. Introduction 1 2. Materials and Methods 4 3. Results Overall Abundances of Major Algal Groups 7 Regional and Seasonal Trends in Abundance of Selected Taxa. . . 10 Bacillariophyta 10 Chlorophyta 28 Cryptophyta 28 Cyanophyta 32 Microflagellates. . 36 4. Discussion 38 References. 45 Appendix Figures . 4? Appendix Table 1 80 FIGURES NWBt>gr Page 1 Sampling station locations; southern Lake Michigan, 1977 5 2 Seasonal Distribution and Abundance Trends of the Total Phytoplankton Assemblage 9 3 Distribution of A?tgr4agna liaa-aaaas. 33 19 Distribution of AnagYsti? 4ng?rta 34 20 Distribution of 0g(?Ulat9r4a bornetii 35 21 Distribution of Haptophyte sp. #1 37 TABLE Table 1 Statistical summary of major algal groups in southern Lake Michigan, 1977 8 vi ACKNOWLEDGMENTS We would like to thank Ted B. Ladewski for his assistance on the computer analysis used in this study. We would also like to acknowledge the U.S. Environmental Protection Agency for collecting the samples and performing the physico-chemical analyses . Vll INTRODUCTION Compared to most areas of the Great Lakes, the southern shoreline region of Lake Michigan has enjoyed a relatively long period of study. Qualitative records of phytoplankton and benthic algal occurrence extend back into at least the 1870 's. The history of these investigations has been reviewed by several authors, including Stoermer and Yang (1969). One of the more interesting, aspects of this historical perspective is that the majority of the studies undertaken, including some of the very earliest, were in response to some perceived pollution problem of the day. This long succession of practically oriented studies has provided a record of extensive population replacement in phytoplankton communities, and some indication of change in absolute abundance of populations and modification of seasonal population dynamics. Although there is little doubt that the major factor driving algal succession in Lake Michigan is phosphorus pollution due to both its primary and secondary effects, there are undoubtedly other effects which are partially masked by this overriding factor. Perhaps the least understood of these is the effect or effects of increasing conservative ion abundance. It has long been recognized that there are striking differences in the algal floras in waters of different salinities. It is also very apparent that many of the algal populations which have invaded the Great Lakes during the past few decades are characteristically found in high salinity environments. This particular modification is, of course, true not only of algal populations but of consumer organisms as well. The actual physiological and/or ecological mechanisms operating have not been satisfactorily determined. It is clear, however, that further modification of the indigenous biota is highly undesirable, and that if conservative ion contamination is a major contributory factor, it poses a very serious problem due to the very long residence times of the Great Lakes and the difficulty in controlling sources. At the present time the most pronounced and complicated effects of multiple loadings occur in the nearshore waters. This zone is also the region of most intensive physical effects. Local and transient advection regimes may cause gross variations in the dispersion of pollutants entering the lake and, as would be expected, the influence of these materials on the algal flora may be highly variable. These regions may thus show a long term statistical trend in population modification but, at any given time, these trends may be submerged by local effects of transient intensity. The present project deals with a limited area of Lake Michigan along the Indiana coastline. This region lies within the area of Lake Michigan which has been extensively modified by factors which affect phytoplankton occurrence and abundance. Several qualitatively different local sources are present, and the effect of these sources on phytoplankton composition and abundance are of special interest because adjacent regions of the lake are intensively utilized for both recreational purposes and as a source of potable water. The primary objectives of the project, which is part of a more comprehensive investigation, are the following: 1 . To determine the composition and abundance of the phytoplankton flora in comparison with past conditions to the extent that they are known, and provide firm documentation for comparison with future studies; 2. To determine if there are occurrence patterns of specific phytoplankton physiological group populations which may reflect the effects of specific sources; 3. To determine if distribution patterns in the phytoplankton are correlated with particular chemical or other biotic parameters which may indicate a cause-effect relationship. MATERIALS AND METHODS The sampling array utilized in this study is shown in Figure 1 . Sampling was conducted on 11 June, 20 August, and 24 September, 1977. Four transects were analyzed with stations located 1/4, 1/2, 1, and 2 miles offshore. The eastern-most transect did not have a 2 mile station. For each station a 2 m and bottom sample were analyzed. All samples were collected by the U.S. Environmental Protection Agency. All samples for phytoplankton population analysis were taken as 250 ml splits of the original 5-liter Niskin Bottle cast. These subsamples were fixed with a Lugols solution and stored. For subsequent analysis, the sample bottles were agitated and a 50 ml aliquot was removed. Material was concentrated by filtration onto 25 mm "AA" Millipore filters, partially dehydrated through an ethanol series, and embedded in clove oil. Prepared filters were mounted on a 50 X 75 mm glass slide and covered with a 43 x 50 mm #1 cover glass. Preparations were kept in a horizontal position and allowed to dry for approximately two to four weeks, during which time embedding medium lost by volatization was periodically replaced. When the filter was completely cleared, the edges of the cover glasses were sealed with paraffin. Slides were analyzed by visual counts of phytoplankton cells present using Leitz Ortholux microscopes fitted with fluorite oil immersion objectives with a nominal Numerical Aperature of 1.32. Magnification used for identification and enumeration was approximately 1200 X. Population estimates given are the average of two 10 mm radial strips counted. Effective filtration diameter in the filtration apparatus used is 20 mm. Raw counts were transformed to computer format on punched cards. Computer ^ ^ ro D Q 2 o o GO o _j CO TO Q ,0 GO Q 2 CVJ O o CVJ 00 o 2 C CO bO •H x: o 0) ;^ (0 c x: O (0 00 c o •H 4-> CO o o c o •H +3 CO 4-> GO CO o M 1X4 data summaries are available for all samples counted (see Stoermer and Kreis, in press). Summaries include estimates of absolute frequency and associated error, and an estimate of relative abundances for individual taxa as well as major algal divisions. Assemblage parameters calculated included an estimate of total phytoplankton abundance and a measure of error associated with the estimate, an estimate of assemblage diversity (H), and an estimate of the evenness component of the calculated diversity. Summary information is stored on magnetic tape and is available for further data anlysis. Various physico-chemical analyses were conducted by the EPA at the time the samples were collected, and they provided this information to us. Contour plots for these data are presented in Appendix Figures 1-33. RESULTS OVERALL ABUNDANCES OF MAJOR ALGAL GROUPS Relative and absolute average abundances of major algal groups for a given cruise are presented in Table 1 . Highest overall densities were attained by blue-green algae and diatoms, with greens, chrysophytes, and cryptomonads , secondarily important. The undetermined category consists almost entirely of micro flagellates which presumably belong to the Haptophyceae (Stoermer and Sicko-Goad, 1977; Sicko-Goad, Stoermer, and Ladewski, 1977). Total absolute abundances for the three cruises are presented in Figure 2. A total of 288 taxa were identified and recorded. The average total density for all samples was 4420 cells/ml, ranging from 844 to 12,078 cells/ml. In June, depending on the station, either blue-green algae, diatoms, or haptophytes were the major group present. At Gary Harbor, haptophytes were dominant at the two offshore stations, with diatoms dominant nearshore. At Burns Harbor, diatoms were the dominant group at all stations except the 1/2 mile station, where blue-green algae dominated the assemblage. At the Indiana Dunes transect, diatoms were the most numerous at the near- and offshore stations, with haptophytes dominant at the 1/2 mile and 1 mile stations, composing 57 and 50$ of the total community respectively. Along the eastern-most transect near Michigan City, diatoms were dominant at the two nearshore stations, averaging 55$ of the population, with blue-green algae dominant at the 1 mile station, composing about 38$ of the community. Over all 15 stations in June, diatoms averaged 35.8$ and blue-green algae averaged 18.6$ of the total phytoplankton. 2: < o M a: o W a: o CO rz: M CO o O -J < o < o o CO < o M CO M H < CO rH < rH L. Q) 0) O Xi B 0) 4-> 0) a > Vt 0) •H CO 4J O OJ o rH C Q) ai K -O C • D 0) J3 > < < rH < rH 0) o 4^ 0} D bO D Q) < > b^ •H 4^ 0) (0 O rH C < < rH < rH 0) o Q) C 3 ^ b^ •H 4^ Q) OJ O rH C (0 PC -o C • D 0) X> > < < in o • on o VO CO o^ on CVJ in on 00 ON in as in o 00 in OVJ cx> CO o in CVJ CO ;:r 00 ,. ::d- o> VO VO • • • • • cn r— VO o OJ CO CO in cn oo o> o t*- 00 *"■ in t^ VO t*- « « • • • « • 00 c^ o OJ VO ::r cn cn cr* 00 in on t— o CO VO VO c^ in in cn cn cn co cn .=r VO on 00 cn in t— t- -- on o in C7> t- o in CO cn 00 VO CO VO CO on oo o CVJ VO CO :;r T- c^ ^ CO on r- OJ OJ r- 00 CO T- iTk o f- CO CO ON O^ o> VO o 00 in ON VO in • « « • • • « CO CO in CO :^ o CO *" cn CO 03 0) 4^ 03 03 Cd •o 03 a> •o r-i a> c 4-> cd r-^ c Q) >> c Q) •H Q) 0) 4^ >» CU o 0) D 0) CO S-. >> c "O rH S^ •H X5 u •H c PQ O Q o o Q o / '1 II JUNE 77 20 AUG. 77 24 SEPT. 77 FIG. 2. Seasonal Distribution and Abundance Trends of the Total Phytoplankton Assemblage . In the August sampling period, blue-green algae were the dominants at all stations along all four transects. They ranged from a low of 33$ of the total algal assemblage at station 33 to 67$ at station 37. The diatom component ranged from 9 to 28$ of the total assemblage. The green algae composed approximately the same proportion of the community as did the diatoms, ranging from 12 to 28$. In September, blue-green algae maintained their dominance. Only at the 1 mile station off Burns Harbor and Gary Harbor did the diatoms dominate. Over all 15 stations, blue-green algae averaged 52$ (Table 1) of the total community and reached a maximum of 58$ at the 1 mile station at Indiana Dunes. Diatoms composed 23$ of the total and were more numerous at the western-most two transects, possibly as a response to the higher silica values. REGIONAL AND SEASONAL TRENDS IN ABUNDANCE OF SELECTED TAXA A?t?r4.Qn?].X?i formo?^ Hass. (Fig. 3) This species occurs in all regions of the Great Lakes, and appears to be eurytopic. In southern Lake Michigan, it seemed to be fairly sensitive to silica levels. In June, it was found in higher numbers offshore and along the Gary Harbor transect, where all silica values were above 0.30 mg/1 (Appendix Fig. 22). In August, it was present in low densities, not over 30 cells/ml. It was found at its maximum at the 1/2 mile station off Burns Harbor, with a silica value of 0.58 mg/1 (Appendix Fig. 23). This large silica value was probably the result of an isolated "slug" of river water derived from Burns 10 / y / / ■'/ II JUNE 77 i '/ / J 'J 20 AUG. 77 FIG. 3. Distribution of Aaterionella formosa. 11 Harbor and entrained in a local circulation gyre* The effect of this "slug" was also noted in various other species examined. Highest abundances of A. formosa were present in September, probably related to the higher silica values present in this month (Appendix Fig. 24). CVQlot^^;;^ QorQ^^gj.^ (Grun.) V.H. (Fig. 4) This species is a relatively recent introduction into Lake Michigan, and its ecological affinities are not well known. In Saginaw Bay in 1974, it appeared in bloom quantities in August, and was still abundant into October (Stoermer and Kreis, in press). They also noted that it was able to tolerate very low levels of silicon. In southern Lake Michigan, a similar seasonal effect was present. It was found in very low numbers in June, but increased substantially in the August and September sampling periods. Abundances of this species appeared to be highest near shore and along the Gary Harbor and Burns Harbor transects, possibly displaying a high tolerance for more perturbed areas. Analysis of variance determined that, in August, densities were significantly lower (.05 level) at the offshore stations on each transect than at the three nearshore stations. Additionally, significant positive correlations at the .01 level were found with NO^, NH^, and conductivity in both August and September. However no such significant correlations were found with silica. Cvclotella crvotica Reimann. Lewin, and Guillard (Fig. 5) This species was originally described from a brackish-water habitat (Reimann jgi jlI- > 1963). Most of the records of its occurrence in Lake Michigan come from harbors and inshore areas subject to high chloride levels (Stoermer 12 II JUNE 77 20 AUG, 77 FIG. 4. n-iat-rihiition of Cvclotella comensis. 13 JUNE 77 20 AUG. 77 'J 4 /200 i 4 /l50 24 SEPT. 77 /lOO '50 FIG. 5. Distribution of CvclotPn;:. crvptica . 14 and Yang, 1969). In southern Lake Michigan, it was found in low numbers in June and September. However, in August, it was found in fairly high densities near shore along the Gary and Burns transects. A maximum density of 180 cells/ml was found at the 1 2 mile station at Burns Harbor, probably due to the presence of river water derived from the harbor. In August, ^. cryptica was significantly correlated with conductivity, N0<., SiO^, and aerobic heterotrophs. Cyclotella pseudostelligera Hust. (Fig. 6) Populations of this species are usually found in eutrophied areas. Its range in the Great Lakes appears to be restricted to harbors and river mouths (Stoermer and Ladewski, 1976). It was present in southern Lake Michigan in low numbers in June and September. However, in August, it had a distribution similar to Cyclotella cryptica . A maximum density was found at the 1/2 mile station at Burns Harbor once again, where it appears that the river water is having a great effect on the makeup of the algal community. High numbers were also found at the 1 mile station at Burns Harbor, and the 1/4 mile station at Gary- Similar to C. cryptica, it also was found to be significantly correlated with conductivity, NO^, SiO^, and aerobic heterotrophs in August. Cyclotella stelligera (CI. & Grun.) V.H. (Fig. 7) This species has been reported to be intolerant of high levels of eutrophication, and is usually removed from regions of the Great Lakes which have undergone extensive perturbation. Hohn (1969) reported that its abundance in Lake Erie has declined since the 1930' s. In southern Lake Michigan, it was present in highest numbers in the August and September cruises. In August, the 15 II JUNE 77 20 AUG. 77 24 SEPT. 77 FIG. 6. Distribution of Cvclotella pseudoatelligera . 16 I J J n I I I I JUNE 77 20 AUG. 77 no. 7. Distribution of Cvglot.«11» stelHgni-a . 17 analysis of variance procedure demonstrated that, at the 5% level of significance, higher numbers were found at the less disturbed eastern-most two transects than at Gary or Burns Harbor, In June and September, no trends were apparent. No significant correlations were found for jC. stelligera versus any physico-chemical parameter monitored for the three sampling dates. Fragilaria crotonensis Kitton (Fig. 8) This species is one of the most common plankton diatoms. It is present in all the Great Lakes, and can tolerate a wide range of ecological conditions. As in Saginaw Bay in 1974 (Stoermer and Kreis, in press) densities were lowest in August, with only isolated low level populations found. Highest densities were recorded in September, with a decreasing offshore trend apparent. In this month it exhibited a significant positive correlation at the .01 level with conductivity, NO^, NH«, and SiO^. In June, densities were intermediate between August and September values, with no trends evident. Nitzschia acicularis Wm. Sm. (Fig. 9) This species is widely distributed in the Great Lakes. In southern Lake Michigan, in June, its distribution was erratic, although it was found in slightly higher numbers at the Burns Harbor transect. Populations declined to very low numbers in August, similar to the seasonal pattern observed in southern Lake Huron (Stoermer and Kreis, in press). Densities increased in September, attaining their highest values, with greatest abundances tending to be concentrated along the nearshore areas and the Gary and Burns Harbor transects. In September, significant positive correlations at the .01 level were found with conductivity, NO^, NH^, SiO^, and aerobic heterotrophs. 18 'i I '/ ' i II JUNE 77 / / 20 AUG. 77 I J y J 'It 24 SEPT. 77 FIG. 8. Distribution of Fragilaria crotonensis. 19 / 'I 1 I J 'L I II JUNE 77 'l 20 AUG. 77 FIG. 9. Distribution of Nitzsohia aoiGularis. 20 Nltzschia fontlcola Grun. (Fig. 10) This species has predominantly been recorded from nearshore localities in the Great Lakes (Stoermer and Yang, 1969). In June, its distribution was erratic, with no trends evident. Its numbers declined in August, and it was not present at all at the offshore stations. By September, it attained its highest densities. Like N_. acicularis it demonstrated a preference for the nearshore regions and for the Gary and Burns Harbor transects. In September, it was found significantly correlated at the .01 level with N0«, NH«, and aerobic heterotrophs. Overall, its distribution appears to be fairly similar to _N. acicularis , and it too displays a tolerance for polluted conditions. Nitzschia palea (Kutz.) Wm. Sm. (Fig. 11) Stoermer and Yang (1969) noted that most of the records for this species in Lake Michigan come from polluted harbors and river mouths. In this study, it reached its greatest abundance in June, and displayed definite affinities for the Gary and Burns Harbor regions. On this first cruise, it was found to be significantly correlated at the .05 level with NO^ and NH^. Its numbers decreased in August and September. Larger densities were found at the nearshore stations than at the furthest offshore station for all transects in the latter two months. Stephanodiscus minutus Grun. (Fig. 12) This species has been reported to be a winter dominant in mesotrophic to eutrophic lakes (Huber-Pestalozzi , 1942). In Lake Michigan, Stoermer and Yang (1970) noted that it becomes abundant in early spring collections from certain near and offshore localities. They also noted that it was more abundant in 21 II JUNE 77 20 AUG. 77 FIG. 10. Distribution of I'itzschia fonticola . 22 I JUNE 77 ? J i u 7 20 AUG. 77 FIG. 11. niat.rihnt.ion of Nitzschia palea. 23 II JUNE 77 20 AUG. 77 FIG. 12. Distribution of SteDhanorii..^nn^ 'nlmitlP 24 1967 than 1964. In June, in southern Lake Michigan, this species was present in fairly high numbers, and displayed a preference for the more eutrophied Gary and Burns Harbor regions. By August and September, its numbers declined drastically and it was present in very low numbers at a few stations. Svnedra filiformis Grun. (Fig. 13) This species has been recorded mainly from offshore regions in Lake Michigan, and Stoermer and Kreis (in press), regard it as one of the characteristic species of the offshore phytoplankton in the upper Great Lakes. However, Cleve-Euler (1953) noted that it can tolerate brackish water. In September, it was found in greatest abundance in the polluted Gary Harbor region, and it decreased moving east to Michigan City. Its abundance was also found to correlate highly with conductivity, NO^, and NH^ at the .01 level. Its numbers were very low in June and August at all stations. T^l?^;i,^ri^ flocQi^losa var. linearis Koppen (Fig. 14) This species was present at scattered stations in June. By August, its numbers declined, similar to the occurrence pattern noted in southern Lake Huron (Stoermer and Kreis, in press). In September, large populations occurred at the two nearshore stations at Gary Harbor (Fig. 14). Generally, in September, offshore densities were lower than they were near shore. It also exhibited significant positive correlations with NO^ and SiOp at the .01 level in September. 25 JUNE 77 20 AUG. 77 '250 * / /200 /lOO 24 SEPT. 11 h'' FIG. 13. Distribution of Svnedra fillformis. 26 I JUNE 77 'i / // '-/ 20 AUG. 77 /> / 24 SEPT. 77 FIG. 14. n^o^^r>^h„^^inr. nf Tabellaria flocculosa var. lin^arAg - 27 Chl.amY^?n^ f;og-$igy^^ (LyngbJ Breb- (Fig, 18) Low levels of this species are found consistently in the Great Lakes; however, high densities are found only in the more eutrophic regions within the system. Stoermer and Kopczynska (1967) found the maximum abundance of A- flos-aouae in southern Lake Michigan to be on the same order as in eastern Lake Ontario. In this study in June, it was found at only three of the stations sampled. By August, it was found scattered in about half of the stations, with a maximum occurrence at the nearshore station at Indiana Dunes. In September, it was found at 10 stations, with no trends apparent. An^QY^t^^s 4nQ?rta Dr. & Daily (Fig. 19) This species is common in the summer and fall phytoplankton assemblages in the Great Lakes. Stoermer ^ jlI* (1975) noted that this species is most successful under conditions of silica depletion. In the June cruise, this species was not present to any significant extent. By August, it increased greatly, and was dominant at every station. It maintained these high numbers into September, and remained as one of the dominants. No trends could be detected within the sampling scheme, and this may be due to its pattern of indeterminate colonial growth, which causes varying degrees of error in abundance estimates. 09Q41].^1^Qr4^ l;)orn^tJ,j. Zukal (Fig. 20) This species has previously been reported from Lake Michigan mainly at the thermocline depth during summer stratification, but rarely in the surface 32 7 JUNE 77 20 AUG. 77 / H FIG. 18. Distribution of ijiaiiasaa llaa-^fluaa. 33 II JUNE 77 20 AUG. 77 FIG. 19. Distribution of Anaovstla 1nrf?rM 34 JUNE 77 20 AUG. 77 FIG. 20. Distribution of OsolllahnnT^ ^"m2ill 35 waters. It has only occasionally been found in abundance in the upper Great Lakes. In southern Lake Michigan, in June, it was one of the dominants. It tended to be in higher densities both nearshore and at the Gary and Burns Harbor regions. In August, it decreased in numbers, and only the 1/2 mile station off Michigan City had an extensive population. It increased slightly in September, with highest abundances along the Gary Harbor transect. M49rof],^g^;i^t;^? He^PtPPhYl^^ sp. #1 (Fig. 21) One group of flagellates which may be playing an increasingly larger role, especially in the Lake Michigan phytoplankton assemblages, are the haptophytes. Little is known about the freshwater ecology of this primarily marine group . Chrvsochromulina oarva has been reported from both Lake Erie and Lake Ontario. Munawar and Munawar (1975) regard it as one of the most numerically abundant species in the St. Lawrence Great Lakes. In Lake Ontario, Stoermer jst ill. (1975) found Chrvsochromulina oarva to be abundant, with its largest populations occurring in June and July. In southern Lake Michigan, Haptophyte sp. #1 was a dominant component of the system in June. Lowest densities were recorded at the 1/4 mile stations at each transect, and it appeared as if some type of nearshore inhibition effect was occurring. Highest abundances were found along the Gary Harbor and Indiana Dunes transects. Numbers decreased in August, with abundances lowest at the offshore stations. Densities further decreased into September, where this species played only a minor role in the system. 36 'J ii II JUNE 77 I V \ I I. 20 AUG. 77 I t FIG. 21. Distribution of Haptophyte sp. #1 37 DISCUSSION There is no doubt that diatoms as a group have decreased in dominance in south Lake Michigan since the early 1960's, when Stoermer and Kopczynska (196?) found them dominant at all stations and all months. The diatoms that are now of consequence are typically those species associated with varying degrees of eutrophication. Green and blue-green algae are composing larger portions of the phytoplankton assemblages. They can outcompete diatoms, especially in the summer, under conditions of low silica and high temperatures. In this study, Anacvstis incerta was the dominant taxon in both August and September. Additionally, phytoflagellates are now a major part of the Great Lakes system (Munawar and Munawar, 1975). Within the past 50 years, chloride concentrations have been increasing in the Great Lakes. Beeton (1965) noted that chloride levels, along with other conservative elements, have been increasing in Lake Erie, Lake Ontario, and Lake Michigan since the early 1900's. A number of marine and halophilic algal species have been recorded in the Great Lakes, with most of these reported within the last twenty years. Stoermer (1978) noted that most of the phytoplankton species which have invaded the Great Lakes are halophilic in nature. In 1977, the brackish-water species Biddulohia sp. and Terosinoe musica were recorded from southern Lake Michigan for the first time (Wujek, pers. coimn.). In this study, a variety of salt tolerant forms were present in fairly high numbers, including CyQ],ot^^j,j.^ prVPtiQ^, gyc^ot^^i;^ Dseudostelligera. Diatoma tenue var. ^j^ong^tun^. and §Yn^<;^ra^ f j,;4forffl4$. Also, two species of Skeletonema . a brackish-water genus, were found. Haptophytes were found as a dominant in June, and this group appears to be increasing in 38 abundance in the Great Lakes. Aside from the halophilic forms, other species typical of eutrophic waters were present. Many of the diatoms found in abundance were species described by Stoermer and Yang (1969) as tolerating moderately disturbed portions of the Great Lakes. Included in this group are Asterionella formosa , Fragilaria crotonensis . Stephanodiscus alpinus , Stephanodiscus minutus , and Synedra ostenfeldii. Other species found in southern Lake Michigan that are common in disturbed areas include Nitzschia spp- , Stephanodiscus hantzschii and Stephanodiscus subtilis. In the green algae, the genus Scenedesmus is common in eutrophied areas. It was also noted that, in many instances, those species which were found to tolerate disturbed waters were found in higher densities at the Gary Harbor and Burns Harbor regions than at the eastern-most two transects. One of the more striking aspects of phytoplankton distribution in the area of study is the atypically large abundance of species of the genus Nitzschia, particularly species such as N. palea and other forms which are often found in areas with extremely degraded water quality conditions. Cholnoky (1968) and others have noted that many members of this genus are often associated with high levels of organic or reduced nitrogen compounds. Species such as N. palea are, in fact, often considered to be indicative of this type of pollution. Although NH, levels are not extremely elevated at the stations studied, our data suggest that this type of perturbation may be an important factor in the area of study. This hypothesis is further enhanced by the fact that the occurrence of Nitzschia species reported to be tolerant of organic loadings is positively correlated with estimates of aerobic heterotroph abundance developed and furnished to us by the EPA Region V laboratories. In the August cruise, with low silicon levels and increased temperatures, 39 diatoms were found in low densities. Only Cvclotella comensis and Cvclotella stelligera were able to thrive under these conditions to any significant extent. Both of these species have been reported to be extremely tolerant of low silica levels, and it appears they can outcompete other species when such conditions exist. It is interesting to note that in August, j£. comensis was found in higher numbers along the more polluted Gary and Burns transects, while £.• stelligera was recorded in greater abundances at the less disturbed eastern-most two transects. The sudden dominance of Cvclotella comensis . a diatom which, so far as we have been able to ascertain, was exceedingly rare in Lake Michigan prior to 1975 is difficult to interpret. This species has become an important element of the flora throughout the lake (GLRD, unpublished data) during the summer and fall. Similar blooms have been noted in Lake Huron (Lowe, 1974; Stoermer and Kreis, in press). Although the ecological tolerance of this species is very poorly known, it is usually reported from oligotrophic systems which are under some nutrient stress. Based on data from Lake Huron, the species is very tolerant of low levels of dissolved silica but does not tolerate nitrate depletion (Stoermer and Kreis, in press). In Lake Huron it forms large late summer and fall blooms in the interface waters of Saginaw Bay and Thunder Bay. It is interesting to note that JQ.. comensis has apparently effectively replaced £. michiganiana . an indigenous species which was previously a consistent summer dominant in Lake Michigan. Although there may be several plausible explanations for this, it would appear that the superior competetive ability of JC- comensis may be related to its tolerance of low silica levels, and thus related to continued phosphorus stress on the system. It should be recognized that, due to the timing of collections in the 40 present study, maximum levels of phytoplankton standing crop were probably not encountered. Previous studies have shown that maximum phytoplankton density and maximum regional differentiation of the flora occur during the spring thermal bar period. In most instances, phytoplankton abundance in the nearshore zone, except in the immediate vicinity of strong sources, is on average lower, but extremely variable following stratification. The samples reported here thus represent a "best case" situation so far as illustrating the effects of eutrophication and salinification on the phytoplankton flora is concerned. It would be reasonable to expect much larger and better defined trends within the area of study during the spring thermal bar period when the dispersion of inputs tends to be constrained by the thermal bar. Another interesting result of the study is the documentation of the increasing importance of microflagellates in the Lake Michigan phytoplankton. In the Great Lakes system, extreme abundance of these organisms is generally associated with regions which have undergone extensive modification. Unfortunately the taxonomy of microflagellates in the Great Lakes is very poorly known since many of them can be identified with certainty only with the aid of the electron microscope. Because of their increasing ecological importance more research should be devoted to determining what entities are present and to developing techniques whereby reliable population estimates can be made. One of the characteristics which appears to distinguish the phytoplankton flora of the study area from that of previously studied nearshore localities is the abundance of filamentous blue-green algae. Although the Cyanophyta have become more abundant in the offshore waters of Lake Michigan in the past decade, the most numerically important forms are generally coccoid species. 41 Although these forms are also a dominant element of the phytoplankton flora in the study area, filamentous species are relatively more abundant than expected. Running concurrently with this study was an investigation into the phytoplankton assemblages of Green Bay (Stoermer and Stevenson, in press). The phytoplankton components of both systems are highly comparable. The dominant taxa of Green Bay for May, August, and October 1977 included Anacvstis incerta . CVQlol^ell,^ coiq^pgi,?, QXo^oqys%X^ p],^qct^opj,9^, and Rhodomonas maut^* These species were also of major importance in southern Lake Michigan in 1977. Total densities were higher in Green Bay, averaging 5400 cells/ml, while southern Lake Michigan averaged only 4400 cells/ml. The August sampling period in Green Bay was also dominated by the Cyanophyte Anacvstis incerta . with Cvclotella comensis the dominant diatom. Cvclotella stelligera was not present in appreciable numbers in Green Bay, possibly due to the more perturbed conditions found there than at the eastern-most two transects of southern Lake Michigan where it was found in fairly high abundances. Phytoflagellates were also of importance in Green Bay in August. In October in Green Bay, similar to the September sampling period in this study, Anacvstis incerta was still the dominant species at a majority of stations with diatoms and phytoflagellates secondarily important. It is interesting to note that, for all of the sampling dates, the flagellate composition was very similar. Species of consequence common to both systems include: Chroomonas spp., Crvptomonas marssonii. Crvptomonas ovata. Qchroffon^s spp., Ochromonas vallesiaca. R]:^o(aop|on^s iqi^nut^, and undetermined haptophytes. In 1971 and 1972, GLRD collected and analyzed samples along the Burns Harbor transect. This allowed for the comparison of the phytoplankton 42 assemblages on a temporal scale. In June 1971, at the 1/4 mile station, a plume emerging from Burns Ditch was intersected and a bloom condition of 31,000 cells/ml was recorded. Diatoms, most of which were centrics, composed 98$ of the assemblage. The plume did not reach the three stations further offshore, and total numbers averaged only 2500 cells/ml, with Rhizosolenia gracilis the dominant taxon. Diatoms as a group were also dominant, averaging 70% of the phytoplankton component. In June 1972, diatoms were once again most numerous, averaging 93% of the assemblage. The major taxa included Steohanodiscus tenuis and ^. minutus . In June 1977, the assemblage was different than five and six years previous. The blue-green algal filament Oscillatoria bornetii and undetermined haptophyte species were most numerous. Diatoms as a group were still dominant, but only composed 38$ of the assemblage. Blue-green algae increased to 23$ of the phytoplankton component, whereas in 1971 and 1972 they averaged only about 2.5$. Phytoflagellates as a group increased dramatically in importance, averaging 31$ of the assemblage in June 1977, while in 1971 and 1972 they averaged only M.2 and 1.5$ respectively. The August phytoplankton assemblage in 1977 appeared to have changed from the earlier studies. In 1977, the blue-green algae, mainly due to Anacvstis incerta . were dominant, while diatoms as a group were most abundant in 1971 and 1972. In 1971 and 1972, blue-green algae averaged about 23$ (300 cells/ml) of the assemblage, while in 1977 they averaged about 52$ (2030 cells/ml) off Burns Harbor. However, the blue-green algal species present over the three years were similar, mainly belonging to the genera Anacvstis and Gomohosphaeria , As in June, flagellates were found in much higher abundances in 1977 than in the previous years. In 1971 and 1972 they averaged 7$ (180 cells/ml) and 1$ (14 cells/ml) respectively, and increased to 22$ (890 cells/ml) of the assemblage 43 in 1977. In September, blue-green algae were dominant over all three years, averaging 66$ (1200 cells/ml), 6^% (3500 cells/ml), and 49$ (1660 cells/ml) for 1971, 1972, and 1977 respectively. Interestingly, Anapvstis incerta was the dominant taxon for all three years. Diatoms composed a larger portion of the assemblage in 1977 due to the appearance of Cvclotella comensi;? ^ a species not found in the earlier two years. Once again, flagellates were found in greater abundances in 1977 (14$), than in 1971 (1$) or 1972 (3$). From the analysis of the phytoplankton component over the three years, some definite assemblage shifts can be noted. These include: 1. An increase in filamentous blue-green algae in 1977; 2. An earlier seasonal dominance of blue-green algae in 1977, to the point that they are found in higher abundances in June, and are dominant in August ; 3. Extremely large increases in the flagellate component; 4. A decrease in the relative abundance of the diatom component in May and June, with it now being dominated by Cvclotella comensis and £.. st^^^rJliR^r^ in the summer months under silica-limited conditions. It still appears that the cultural eutrophication of southern Lake Michigan is continuing, albeit at a slower rate than five to ten years ago. Under silica-depleted summer conditions, blue-green algae, phytoflagellates, and low-silica-tolerant diatoms are now the dominant phytoplankters in the system. However, those diatom species characterized by Stoermer and Yang (1969) as thriving only in disturbed habitats are not present in large abundances in 1977 in southern Lake Michigan, although it is possible that they may have been present in April and May under thermal bar conditions. 44 REFERENCES Beeton, A. M. 1965. Eutrophication of the St, Lawrence Great Lakes. Limnol . Oceanogr . , 10: 240-254 . Cholnoky, B, J. 1968. Die Okologie der diatomeen in Binnengewassern. Verlag von J. Cramer, Lehre. 699 pp. Cleve-Euler, A. 1953. Die Diatomeen von Schweden und Finnland. Teil II. Arraphidae, Brachyraphidae. Kungl. Svenska Vet. - Akad. Handl,, Fjarde Serien, 4(1):1-158. Hohn, M. H. 1969. Qualitative and quantitative analyses of plankton diatoms, Bass Island area, Lake Erie, 1938-1965, including synoptic surveys of 1960-1963. Ohio Biol. Surv., N. S., Vol. 3i No.1. 211 pp. Huber-Pestalozzi, G. 1942. Das Phytoplankton des Susswassers, Teil 2, 2 Halfte. Diatomeen. Jjj.: Thienemann, A. (Ed.), Die Binnengewasser, Band 16, pp. 366-549. Lowe, R. L. 1974. Environmental requirements and pollution tolerance of freshwater diatoms. EPA-670/4-74-005, 333 PP. Munawar, M. , and I. F. Munawar. 1975. The abundance and significance of phytoflagellates and nannoplankton in the St. Lawrence Great Lakes. 1. Phytoflagellates. Verh. Int. Verein. Limnol., 19:705-723. Reimann, B. E. F. , J. M. Lewin, and R. R. L. Guillard. 1963. Cvclotella crvDtica . a new brackish water diatom species. Phycologia, 3(2):76-84. Sicko-Goad, L., E. F. Stoermer, and B. G. Ladewski. 1977. A morphometric method for correcting phytoplankton cell volume estimates. Protoplasma, 93:147-163. Stoermer, E. F. 1978. Phytoplankton assemblages as indicators of water quality in the Laurentian Great Lakes. Trans. Amer. Micros. Soc, 97:1-16. Stoermer, E. F., and E. E. Kopczynska. 1967. Phytoplankton populations in the extreme southern basin of Lake Michigan, 1962-1963, pp. 19-46. In : J. C. Ayers and D. C. Chandler, Studies on the environment and eutrophication of Lake Michigan. Univ. Michigan, Great Lakes Res. Div., Spec. Rep. No. 30. Stoermer, E. F., and R. G. Kreis. In press. Phytoplankton composition and abundance in southern Lake Huron. Univ. Michigan. Great Lakes Res Div. Special Report No. 66. 382 pp. Stoermer, E. F., and T. B. Ladewski. 1976. Apparent optimal temperatures for the occurrence of some common phytoplankton species in southern Lake Michigan. Univ. Michigan, Great Lakes Res. Div. Publ. No. 18. 48 pp. 45 Stoermer, E. F., and L. Sicko-Goad. 1977. A new distribution record for Hvmenomonas roseola Stein (Prymnesiophyceae, Coccolithophoraceae) and §P4nif?rQffl9nag t^r49ra3i4g Takahashi (Chrysophyceae, Synuraceae) in the Laurentian Great Lakes. Phycologia, 16(4) : 355-358 • Stoermer, E. F,, and R. J. Stevenson. In press. Green Bay phytoplankton composition, abundance, and distribution. Stoermer, E. F., and J. J. Yang. 1969. Plankton diatom assemblages in Lake Michigan. Great Lakes Res. Div., Univ. Michigan, Spec. Rep. No. M7. 268 pp. Stoermer, E. F. , and J. J. Yang. 1970. Distribution and relative abundance of dominant plankton diatoms in Lake Michigan. Univ. Michigan, Great Lakes Res. Div. Publ. No. 16. 64 pp. Stoermer, E. F., M. M. Bowman, J. C. Kingston, and A. L. Schaedel. 1975. Phytoplankton composition and abundance in Lake Ontario during IFYGL. Univ. Michigan, Great Lakes Res. Div., Spec. Rep. No. 53. 373 PP. 46 o o .o GO o 00 O CVJ .o 00 0) c c •H o c u o 0} o 4-> c o o Q) U 4-> cd 0) a a 0) CD •H c o a a 47 o o 00 o 00 o CVJ .o 00 4^ 00 bO < O (\J c CO bO •H x: o (0 c 0) x: 4^ O CO 00 u o 4J c o o 0) 4J u Q) OU B 0) H (\i tc c 0) a a 48 o o -O 00 o >- 00 O CVJ .o 00 Ox 0) JQ s a CO CO c CO •H x: o 0) CO C O &4 D O 4-> c o o 4-> CO £h Q) a s - 00 o CM -o f^ 00 cr> c C bO •H O c s: O CO o c o o >> 4-> 4-> o D c o o c 50 4^ O c •H O 0) c u 0) 3 O 00 GO i* D O 4-> c o o >> 4-> 4-> o D c o o in T3 C 0) a 51 o o .o 00 .0 >- GO O CVJ -o 00 52 o o .0 CD 00 o .o 00 ON c c 03 •H o z: c CD o 00 00 :3 o c o o o CO x: o o 0) CO CD c 0) a a 53 o o .0 00 ,0 00 o CVJ GO 4^ 00 < O c to •H o C 0) x: O 00 00 D o 4^ c o o o 00 x: o o 0) CO C30 0) D •H X •H TJ C 0) a a < 54 o o .o >- 00 o CM .o 00 0) B a> a 0) CO CO c CO bO o 0) CO c 0) XI o 00 00 L. o c o a a 00 o o 0) CO C7N C>4 c a> a a < 55 o o .o 00 .o GO O CM .o GO 0) c C CO •H s: o CO C Cm 0) jC O 00 a: a .2 00 u D O c o o c a 56 o o .0 In 05 o CO o CO a: < X CO CD ON -P 00 o C\J C bO •H o C 0) D o 00 a L. o OQ o c o o \ o CM 00 00 C 0) a \ 57 o o o 00 o 00 O CM .o 00 0^ 0) S Q) a CO c to o 0) c 0) XI 43 D o (0 o. .2 GO O 43 c o o C\J Q) T3 C 0) a a 58 o o .o 00 o 00 O CM -o 00 0) c c bO O cd c Li 0) o (0 D O c o o 4J cd 0) L. c 0) a. 59 o o .0 00 .o >- 00 O CVJ .o 00 60 o o .0 00 o 00 o CVJ -o 00 ON U Q) X^ B CD a 0) CO C\i c CO to •H o CO C 0) jC ^-> O 00 CO u o c o o >> •H C •H cd LO 0) bO C 61 o o ?^ 00 o 00 0) c 3 C CO to •H x: o CO c s: D O 00 Of} o c o o o CM -o r^ 00 # • § VO D U) c a a < 62 o o 00 .0 >- 00 o CM -o h- 00 ON 4-> O CVJ c •H o 0) c Q) D o OQ 00 D O 4^ c o o I on o z 0) to c 0) a a < 63 o o o 00 >- GO O CVJ -o 00 64 o o .0 00 ,0 >- GO o CM -o 00 0) C 13 C to •H O C 0) D O 00 CO ;^ o 4-> c o o c o ON 0) Cm :3 to X •H c a a 65 66 o o .o 00 o 00 o CM .o 00 0) JQ B Q) a 0) CO C\J c bO •H x: o c 0) o 00 00 u o -p c o o c o C\J 0) u D bO •H CX4 c a a 67 o o .0 00 ,0 00 o CM 00 68 o o 00 o 00 o CM -O 00 4-> O OsJ C •H a CD C (D x: O 00 00 o 4^ c o o (d o CO m a CO CM C x: o 0) c a> s: O OQ 00 O -P c o o CO o CO CNJ c o o x: a o u ^ o 0) x: o •H JOt o (^ 0) CO c in OsJ 0) •H Ci4 C • 71 o o 00 CO o CVi • o 00 72 o o 00 .0 00 o CM 00 CO c (0 x: o -a c 0) x: 4-5 O 00 CO o -p c o o s: a o o L. 0) 4-> < CO 73 o o o 00 o 00 O CM r<- 00 ON C •-0 C (0 to •H s: o Q) C > J3 Eh 00 CvJ 0) c 0) a 74 o o 00 .0 00 o CM o 00 75 o o o 00 o >- 00 o CM K 00 B Q) 4-> a a> CO C\J c CO bO •H o 0) c O 00 CO o -p c o o >» -p O on a a 76 o o 00 .0 00 o CM — o 00 ON a> c c to •H o CO C u 0) x: o O 4^ c o o B u o O o o 0) 1X4 0) 1X4 c 0) a a 77 o o 00 p 00 o CM .o 00 78 9 O O o .0 00 .0 00 o CM -o 00 ON U B 0) a