- l« ■ ',*■ vr r5HT • . . DISCOVERY REPORTS VOLUME XIX CAMBRIDGE UNIVERSITY PRESS LONDON: BENTLET HOUSE NEW TORK, TORONTO, BOMBAT CALCUTTA, MADRAS: MACMILLAN TOKYO: MARUZEN COMPANY LTD All rights reserved DISCOVERY REPORTS Issued by the Discovery Committee Colonial Office, London on behalf of the Government of the Dependencies of the Falkland Islands " ^ VOLUME XIX CAMBRIDGE AT THE UNIVERSITY PRESS 1940 PRINTED IN GREAT BRITAIN BY WALTER LEWIS, M.A., AT THE UNIVERSITY PRESS, CAMBRIDGE CONTENTS PHOSPHATE AND SILICATE IN THE SOUTHERN OCEAN (published 23rd September, 1938) By A. J. Clowes, M.Sc, A.R.C.S. Introduction page 3 The phosphate and silicate cycles in the Southern Ocean 9 Antarctic surface water 16 Sub- Ant arctic surface water 55 Subtropical surface water 60 Antarctic intermediate water 63 Warm deep water 77 Antarctic bottom water 96 The regeneration of phosphate and silicate 105 Phosphate and silicate concentrations as possible factors limiting plankton production i i i Summary 115 List of literature 117 Notes on the plates 119 Plates I-XXV following page 120 A SECOND REPORT ON THE SOUTHERN SEA LION, OTARIA BYRONIA (DE BLAINVILLE) (published 1 6th December, 1939) By J. E. Hamilton, M.Sc. Introduction Material and methods Osteology Reproduction Census of the herd in the Falkland Islands Notes References Plates XXVI-XXXIII page 123 124 126 135 155 163 164 following page 164 MACROBERTSON LAND AND KEMP LAND, 1936 (published 15th May, 1940) By George W. Rayner Historical Narrative Scullin Monolith Bertha Island . Summary . List of Literature Chronological Table Rocks from MacRobertson Land and Kemp Ph.D., F.R.S. Plates XXXIV-XXXVIII Land, Antarctica By C. E page 167 171 176 177 178 178 179 Tilley, B.Sc, . 180 following page 1 84 **Tj vi CONTENTS ON THE ANATOMY OF GIGANTOCYPRIS MVLLERI (published 30th July, 1940) By H. Graham Cannon, Sc.D., F.R.S. Introduction page 187 187 Methods Systematics General comparison with a typical Cypridinid Swimming and feeding Constitution and origin of the dorsal body wall Endoskeleton and articulated sclerite system Blood system . Nervous system Excretory organs Summary . Literature cited Plates XXXIX-XLII 189 190 193 195 200 212 223 235 240 243 following page 244 WHALE MARKING. PROGRESS AND RESULTS TO DECEMBER 1939 (published 30th July, 1940) By George W. Rayner Introduction Evolution of the mark . Practical methods at sea Scope of the marking accomplished Return of marks .... Movements of whales Percentage return of marks . Summary References Notes on the tables Notes on the charts Plates XLIII-LXVIII . page 247 248 249 250 254 256 273 274 276 277 283 following page 284 DISTRIBUTION OF THE PACK-ICE IN THE SOUTHERN OCEAN (published 30th July, 1940) By N. A. Mackintosh, D.Sc, and H. F. P. Herdman, M.Sc. Introduction . Construction of the charts The Antarctic ice-edge . Seasonal distribution of the References Notes on the charts Plates LXIX-XCV . ice-edge page 287 288 290 291 295 296 following page 296 [Discovery Reports. Vol. XIX, pp. 1-120, Plates I-XXV, October 1938.] PHOSPHATE AND SILICATE IN THE SOUTHERN OCEAN By A. J. CLOWES, M.Sc, A.R.C.S. CONTENTS Introduction page 3 The phosphate and silicate cycles in the Southern Ocean 9 Antarctic surface water 16 Phosphate content at the surface 16 Phosphate content of the 0-100 m. layer 23 Silicate content at the surface 27 Silicate content of the 0-100 m. layer 35 Seasonal variation of phosphate at the surface 39 Seasonal variation of silicate at the surface 40 Seasonal variation of phosphate in the surface layer around South Georgia 44 Seasonal variation of silicate in the surface layer around South Georgia . 47 Seasonal variation of phosphate in the surface layer in the Scotia Sea . 48 Seasonal variation of silicate in the surface layer in the Scotia Sea . . 51 Seasonal variation of phosphate in the surface layer in 80° W . . . . 53 Seasonal variation of silicate in the surface layer in 80° W 54 Sub-Antarctic surface water zc Phosphate and silicate contents at the surface 55 Subtropical surface water 60 Phosphate and silicate contents at the surface 60 Antarctic intermediate water 63 Phosphate and silicate content 63 Warm deep water 77 Phosphate and silicate content 77 Antarctic bottom water 96 Phosphate and silicate content 96 The regeneration of phosphate and silicate 105 Phosphate and silicate concentration as possible factors limiting plankton production m Summary nr List of literature ny Note on the plates 119 Plates I-XXV following page 120 PHOSPHATE AND SILICATE IN THE SOUTHERN OCEAN By A. J. Clowes, m.Sc, a.r.c.s. (Plates I-XXV; Text-figs. 1-29) INTRODUCTION FOR some years past the Discovery Committee has been collecting hydrological data in southern waters. Amongst these data is a large number of estimations of the phosphate and silicate contents of the various waters encountered in the Southern Ocean. The importance of a knowledge of the distribution and fluctuations of phosphate and silicate in this ocean may clearly be seen when it is realized that these nutrient salts are essential to the life economy of the huge concentrations of plankton found in southern waters. The plankton forms the basic food supply of the whales, and in the production of this food supply enormous quantities of these two nutrient salts are withdrawn from the sea water. This report describes our present knowledge of the distribution, fluctua- tions and cycles of these two important salts. Consideration has only been given to those estimations which were obtained by analysis as soon as the samples of sea water had come to laboratory temperature ; only the results of analyses carried out on board R.R.S. 'Discovery II' are included here. It is felt that by excluding all material which had to wait more than about 12 hours before analysis, a more accurate series of results is obtained, since changes in the nutrient salt content of the samples due to phytoplankton and bacteria is minimized. So far only part of the data has been published {Discovery Reports, iv, pp. 1-232), but reference to the plates at the end of this report will show that most of the data have been inserted in the vertical sections which have been so carefully drawn by Miss E. C. Humphreys. The data not shown in the vertical sections are usually those between o and 200 m. ; these have been omitted only to prevent confusion in the plates which, however, were constructed with the aid of all the data. At most stations observations were made at o, 10, 20, 30, 40, 50, 60, 80, 100, 150, 200, 300, 400, 600, 800, 1000, 1500, 2000, 2500, 3000 m. and at intervals of 500 m. to the bottom. Unprotected thermometers were used with the reversing bottles so that the true depth of the sample could be calculated. No attempt has been made to correct the data for salt error. It was considered that the present state of our knowledge on this question was insufficient to warrant the making of such corrections which indeed might have to be recalculated later on. All values of phosphate and silicate contents are given as milligrammes of P205 and Si02 respectively per cubic metre of sea water. The coeruleomolybdic method of Deniges as adapted by Atkins (1923 a), was used for the analysis of phosphate, whilst the method of Dienert and Wandenbulcke, introduced into sea-water analysis by Atkins (19236), was used for DISCOVERY REPORTS silicate analysis. The silicate results are given according to the standards of King and Lucas (1928). I should like to express my indebtedness to my colleagues, Mr H. F. P. Herdman and Dr G. E. R. Deacon, who have assisted me in collecting all the samples and, together with Mr Saunders, have helped to make the estimations on which this report is founded. In the actual writing of the report I have been very fortunate in having access to a proof- copy of Dr Deacon's "Hydrology of the Southern Ocean", Discovery Reports, xv, 120° BOW / 156 w [^ ISO w 1 vis 1 "f^ ^^T1£RRaV OEL FuEGO N. C Horn DRAKE STRAIT 60 '■■■■ 'v, 0^ I iJJiatl1 1 M«i OiB^U3 1 i47Sf ; '• iiss* 1 M73l \\*P* N 1 *I3* 14*4 3 •' *iE« " Lift' I \i 'l2M I— " \~^ GRAHAM L #/ / n1 35"S Fig. 1. Positions of stations where observations for phosphate or silicate were made in the South Pacific sector of the Southern Ocean. pp. 1-124, which has been of great help. Dr T. J. Hart and I have discussed many of the problems which have arisen, and I am indebted to him for many suggestions and for all my facts about phytoplankton. Finally, but by no means least, the samples could only have been obtained by the full co-operation and skill of the ship's officers and by the loyal and painstaking service of the seamen, often in the worst of weathers. Figs. 1-5 show the position of the majority of the stations at which observations were made. In some of the routine lines of repeated observations and in particular in the Scotia Sea and in 8o° W, all the stations are not shown. Before proceeding to the main portion of this report a preliminary and very brief account of the chief water layers of the Southern Ocean and their movements will be INTRODUCTION 5 given. For a more complete account the reader is advised to consult Deacon (loc. cit.). Also, since we are principally dealing with the Antarctic zone, some remarks on the close connexion between nutrient salts and plankton in this important zone will be made in this introduction. All round the Antarctic Continent, Antarctic surface water spreads northwards in a shallow layer until it reaches a position which has been termed the Antarctic convergence ; here it sinks below the surface layer of sub-Antarctic water. In the area of mixing, 1233 386 • / ,|4S4 385" .1234 .1325 .,_,. OBV KG.*1*25 '** 'M40 1139 1434. ,337 '2'4.635 «2i»" •"•38 1429. •6371333. •530 1332*1492 35°S Fig. 2. Positions of stations where observations for phosphate or silicate were made in the South Atlantic sector of the Southern Ocean. 100-200 miles north of the convergence, the Antarctic surface water becomes mixed with warmer water from the subsurface and surface waters of the sub-Antarctic zone and eventually forms part of the Antarctic intermediate current. The layer immediately below the Antarctic surface water is the warm deep water, whose movement has a southerly component. This layer lies at a great depth north of the Antarctic convergence and flows almost horizontally until it approaches the Antarctic zone. It then climbs steeply over a third layer of water, the Antarctic bottom water which itself sinks towards the north, and south of the Antarctic convergence continues its flow towards the south at a much lesser depth. North of the Antarctic convergence and south of the subtropical convergence the DISCOVERY REPORTS upper part of the water column consists of sub-Antarctic water, a much thicker layer than the Antarctic surface water and including a subsurface current and the Antarctic intermediate current. Below this there is the warm deep water which in turn overlies the Antarctic bottom water which flows to the north. 90E Fig. 3. Positions of stations where observations for phosphate or silicate were made in the sector of the Southern Ocean south of South Africa. In general the north and south circulation of the various waters of the Southern Ocean are very similar to the South Atlantic circulation which is shown in Fig. 6.1 Amongst the nutrient salts utilized in the life-economy of the plankton, more par- ticularly by the phytoplankton, are those of phosphate and silicate. Of the form in which these salts exist in the ocean very little is known, and at present we shall deal only with the amount of nutrient salt available for the metabolism of the plankton. It seemed to me that a very close relationship must exist between the phytoplankton of the Southern Ocean and the amount of phosphate and silicate present, and that any 1 This figure is taken from Deacon (1937), Discovery Reports, xv, p. 4. INTRODUCTION 7 discussion of the content of these salts in the sea must necessarily involve some concep- tions of the distribution of phytoplankton and a brief resume of the waxing and waning of the enormous concentrations found in spring and summer in the Antarctic. Dr T. J. Hart informs me that within the Antarctic zone it is possible to distinguish three areas characterized by the type of phytoplankton communities they support, and, broadly Fig. 4. Positions of stations where observations for phosphate or silicate were made in the sector of the Southern Ocean south of Australia and New Zealand. speaking, by their geographical relation to the Antarctic convergence and to the ice- edge. This distinction is necessarily less clearly defined where geographical features lead to any marked difference from the average latitude of the convergence, as for instance in the eastern South Pacific Ocean. The three areas are: a northern region extending some 300 miles south of the Antarctic convergence, a southern region extending some 200 miles north of the ice-edge, and a rather less definite intermediate zone within which our observations are not yet sufficiently numerous to permit any average extent being determined. These limits are to some extent arbitrary, as the latitude of the Antarctic 8 DISCOVERY REPORTS convergence and more particularly of the ice-edge vary considerably both with the season and with the year ; but the arbitrary regions have been found to agree so well with the types of phytoplankton communities encountered that there is little doubt that they have a real biological significance. There is a very strong possibility that the southern region comprises the area of the East Wind Drift of the Southern Ocean, unless the ice is very far north. 4?2° 4-0° 38' $&° 3**° •49i •300 • 490 „„„ *»1402 l062* •"n.soi •469 «302 •493 •494 S3 .-V88 303.'40' 53' • 320 "2fl.« 1*1502 •48T7 304 #L92 •495 •357 ■ v •475 •477 ••478 •479 1066 4 1067 • 32" •322 • 480 • 481 I06S •I497* •«• 12*0 l404 12*03 • 323 • 319 • 318 482 «3i • 483 1064 • •1070 . ^ 1208 • .326 *330 •305 «496 1060 »I400 486* • ••201 -. 123 V>6 .500 1 1 *3"3 -ifit;. »307 7 3I5*.|,30 <9f .3" .3,6 . :W '310 •358 • 50i •502 •503 * • • 1-406 121! M' 23 •525 -^?° *3oa "505 %$a 506 »356 W^ '^07 *H * -354 *355 ^%^X, 3S, * >^3 .l|03fl ' 340\ ; ^Ji '39G >s" . 333 . 325 324 *334 32 •524 34i. .si-*, ^ 339* *| 33a. *S|e fe* .348 •513 .512 .511 •523 337. ,34'"J ♦5J9 • )46 ■509 • 506 • 52i 336 • •516 •345 56 •521 520 a 3 35 • 34* •515 • 514 • 344 • 343 • 34 S 4-2° 4.0° 36° 36° 3+°W Fig. 5. Positions of stations where observations for phosphate or silicate were made around South Georgia. Dr Hart further informs me that in the northern region the phytoplankton does not begin to increase very rapidly until the first week in November, when the gradient becomes extremely sharp. By the middle of December the numbers are falling off and the downward gradient of the concentration becomes as sharp as the upward gradient of early November. At the beginning of April the numbers are comparable with the concentration of early November before the large increase ; a slight increase was recorded in May. In the southern region, on the other hand, the increase in phytoplankton con- centration is never rapid and the maximum is far below that of the northern region. Moreover, the increase starts in the first week of November and carries on until a maximum is reached in the third week of February. A slow decline then sets in to the third week of April, followed by a small autumnal increase in May, after which it declines again. All the dates mentioned above are subject to an annual variation of ± 2 weeks. 30 I 1000m 2000m 3000m- 4000m PHOSPHATE AND SILICATE CYCLES 40" 50° I I sub-tropical antarctic convergence: convergence sub-antarctic zone ^ G- 160 mg. at 800-1000 m. between 2\° and 70 N) causes the south-going warm deep water to have a greater content than the north-going Antarctic intermediate water. The course of the Antarctic intermediate current may be traced in the closely separated iso-lines in the upper part of the section. Thus in the western part of the South Indian Ocean the phosphate cycle is more complicated than in the South Atlantic Ocean. This is because in the former the North Indian deep water has a very high phosphate content, particularly in its upper part, whilst in the western part of the South Atlantic Ocean for example, the North Atlantic deep water has a much lower phosphate content north of the Rio Grande ridge than has the Antarctic intermediate water. In the western part of the South Indian Ocean there appear to be two localities where decomposition of plankton must be very extensive, since apart from the influence of the North Indian 12 DISCOVERY REPORTS deep water at lower latitudes and a high content in the Antarctic bottom water south of 560 S, maximum phosphate occurs in these two localities ; the first lies between the Antarctic convergence and the Atlantic-Indian cross-ridge, and the second at about 3 8° S, although the significance of the latter position may be affected by the fact that the section is composed of two sets of observations over a year apart and the division lies close to 380 S, i.e. annual variation is possibly the cause of the higher values at St. 1567 at 1500-2000 m. Also it is not yet certain how far south the last traces of North Indian deep water extend in the upper layers of the warm deep water which is mainly of North Atlantic origin south of 200 S (Clowes and Deacon, 1935). The south-going warm deep water rises steeply towards the surface on the northern side of the Atlantic- Indian cross-ridge and continues to rise to the Antarctic convergence. Its upper layers are in contact with the descending Antarctic surface water, and on the southern side of the cross-ridge considerable mortality and decomposition of plankton must occur to account for the high values of phosphate content found there. The regenerated phosphate is then swept to the far south on the upper layers of the warm deep water, so that at the ice-edge in 64I0 S the content immediately below the discontinuity between the surface and warm deep currents is 156 mg. The source of the high phosphate values in the upper layers of the North Indian deep current is at present unknown, and in any event these high values should be checked by other observations. In the South Pacific Ocean we have only data from the western part between New Zealand and the ice-edge north of the Ross Sea. One section (12, Plate XVII) was made in late winter, September 1932, and the other section (13, Plate XIX) in midsummer, January 1934. Phosphate leaves the Antarctic zone in the South Pacific in two currents : (1) in the Antarctic surface water which sinks at the Antarctic convergence and spreads northwards as part of the Antarctic intermediate current, and (2) in the highly saline bottom water which flows northwards from the far south. The return of phosphate to the south is effected in the mixing processes which take place between the Antarctic intermediate current and the bottom current with the warm deep water. The largest concentrations of phosphate are found in the bottom layers of the intermediate water and the upper layers of the warm deep water. In the South Pacific sector the Antarctic intermediate water is a very strong current, and the boundary between this current and the warm deep water is a thick layer in which maximum decomposition of the plankton is occurring, thus releasing the phosphate which is carried back to the south in the upper layers of the warm deep water. However, as is stated on p. 15 of this report, not all the water in the upper part of the warm deep water reaches the Antarctic zone. The silicate cycle in the eastern part of the South Atlantic Ocean can be followed by reference to Plate VI. This shows a section from the ice-edge in 690 21' S, 90 33-8' E northwards to 30 46-2' N, 120 55-1' W, although north of i2h° S the observations only extend to 1400 m., i.e. just below the intermediate water. Silicate may be seen to leave the Antarctic zone in two ways ; the Antarctic surface water sinks at the Antarctic con- vergence carrying with it silicate up to about 2500 mg. in a free form plus an unknown SILICATE CYCLE 13 amount held by the plankton ; these amounts of silicate travel northwards in the Antarctic intermediate water. As the Antarctic intermediate current progresses northwards it steadily loses silicate by mixing with the upper layers of the warm deep water, which in this section have a lower silicate content, north of the subtropical convergence, than the intermediate water above them. The course of the northward flow of the Antarctic intermediate water may be traced from salinity measurements, and even as far north as 40 N there still remained about 1000 mg. of silicate at the salinity minimum of this water. Farther north than our section extends more and more silicate must be lost to the inter- mediate water and transferred to the southward-flowing warm deep layer. The second loss of silicate from the Antarctic zone occurs in the Antarctic bottom water, which as Plate VI shows had a content greater than 5000 mg. in March 1933 south of the eastern part of the South Atlantic Ocean. In our section some of this loss is prevented by the rise of the sea-bottom from an ocean depth greater than 4000 m. to one of 2633 m. at St. 11 60, which is situated on the Atlantic-Indian cross-ridge in 520 41-5' S, 140 30-4' E. North of this cross-ridge the lower layers of the warm deep water are seen to be returning silicate southwards, and this silicate is carried towards the surface in this current which rises rapidly near the Antarctic convergence. The majority of the silicate present in the Antarctic surface water is undoubtedly returned in the lower levels of the warm deep water which itself does not have a large silicate content but to which silicate is added by mixing at the boundary surface with the Antarctic bottom water. Between Cape Town and the ice-edge north of Enderby Land in 66|° S, 42J0 E, section 4, Plate VIII, passes through a possible gap in the Atlantic-Indian cross-ridge in about 500 S, 300 E and the sea-bottom does not rise above 4000 m. anywhere in our section. Consequently a greater amount of silicate is seen in the bottom observations in this section than was evident in the previous section (3, Plate VI), when the sea-bottom rose to a depth of 2633 m. in 520 41-5' S, 140 30-4' E. Silicate may be seen to leave the Antarctic zone in the usual manner, i.e. in the sinking Antarctic surface water, and in larger quantities, in the Antarctic bottom water. Mixing at the upper and lower boundary surfaces of the warm deep water transfers silicate to the southward-flowing warm deep water which returns it to the Antarctic zone. Some of the silicate in the Antarctic intermediate current, which at 380 S contains as much as 2750 mg. at the position of minimum salinity in this current, must be utilized by the plankton in the water which upwells on the continental shelf of South Africa and is thus lost to the silicate cycle in the Antarctic zone. Plate XI shows a section from the ice-edge in 640 37-6' S, 440 16-3' E to n° 32-3' N, 52-03' E, which is composed of observations made in May 1934 and April-May 1935 ; a break in the iso-lines at about 390 S indicates the time interval between the two halves of the section. This plate shows the distribution of silicate in the western part of the South Indian Ocean and the Southern Ocean to the south of it (section 6). At the Antarctic convergence the Antarctic surface water sinks below the surface carrying with it between about 850 and 2000 mg. of silicate in its upper and lower surfaces ; an un- i4 DISCOVERY REPORTS known amount of silicate, as represented by the plankton, is also contained in the sinking water which progresses northward as part of the Antarctic intermediate current. Even at 70 N there remain over iooo mg. of silicate at the salinity minimum of this layer. The lower boundary of the intermediate current is in contact with the upper layers of the warm deep current which north of 200 S in this section is composed of North Indian deep water. South of this latitude the warm deep water is mainly composed of water of North Atlantic origin with traces of North Indian deep water appearing in the upper layers. The silicate content of North Indian deep water is much greater than that of North Atlantic deep water, as a comparison of sections 3 and 6 in Plates VI and XI shows. Consequently in our section up the East African coast the warm deep water has always a greater content than the Antarctic intermediate water, in contrast to the section up the eastern side of the South Atlantic where the intermediate water has a greater silicate content north of the subtropical convergence than have the upper layers of the warm deep water. The sinking Antarctic surface water at the Antarctic convergence flows northwards with an initial content of between 850 and 2500 mg. of silicate. By the time this water has reached 70 N there are about 1000 mg. of silicate remaining at the depth of minimum salinity of the layer. The difference between these quantities has been lost to the northward flow of the intermediate water by mixing with the south- ward flow of the upper layers of the warm deep water. The circulation of the bottom water between the Weddell Sea and 300 E is such that the tendency of the cold water is to keep to the northern side of the Atlantic- Antarctic basin, whilst a stream of warmer bottom water exists along the edge of the Antarctic continent. A cyclonic circulation has been suggested for the bottom water in this region with temperature evidence for a southward movement in the bottom water in the eastern part of the basin. Sections across the Southern Ocean south of the Indian and Pacific Oceans suggest that the eastward current of water from the Weddell Sea spreads across these oceans without further additions of cold, poorly saline and highly oxygenated water sinking from the continental shelf of Antarctica. East of Enderby Land (500 E) the coldest bottom water is found near the continental shelf, and the temperatures suggest that the westward movement has ceased. Our section (section 6, Plate XI) begins in 44I0 E, and it will be noticed that south of 560 S the deepest observations show that the Antarctic bottom water has a silicate content greater than 7500 mg. at 4000 m. It is uncertain, however, how far south of 61 f° S this huge amount of silicate is found owing to the absence of deep observations at St. 1361 (640 37-6' S, 440 16-3' E). In addition to an eastward movement in the bottom water in our section there must also be a northerly component which carries away a large amount of silicate from the Antarctic zone. This amount is, however, very much less in our section than it would be farther towards the east, where a gap exists between the Atlantic- Indian cross-ridge and the Kerguelen-Gaussberg ridge. At St. 1368, in 440 54-6' S, 420 30-1' E, the sea-bottom has risen from depths greater than 4000 m. to a depth of 955 m., thus effectively cutting off any possible northward flow of bottom water in this longitude. Actually at the station on the southward side of the ridge, St. 1367 in 470 41-2' S, 440 427' E, the deep PHOSPHATE AND SILICATE CYCLES 15 observations show that at 3000 m. the water is mainly warm deep water which is already carrying back silicate to the Antarctic zone. South-east of New Zealand two sections (12 and 13, Plates XVIII and XX) are available. The section in Plate XVIII was made in September 1932 and the other, in Plate XX, in January 1934. In these sections the greatest concentration of silicate is always found at the deepest observations. As we have seen in the Southern Ocean south of the Atlantic and Indian Oceans the major portion of the silicate which leaves the Antarctic is returned in the lower layers of the warm deep water. In the Pacific sector the position is more complicated owing to the more complex nature of the deep currents in this part of the ocean, where the intermediate, warm deep and bottom currents are not separated by any well-marked boundaries. Extensive mixing no doubt takes place between these currents, and silicate exchanges undoubtedly occur, but it is quite plain that east of New Zealand as much as 5300 mg. of silicate are found at 3500 m. in the highly saline layer which is travelling towards the north. Deacon (1937, p. 103) has stated that a large part of the current which flows southward between 1500 and 2500 m. north of 500 S in the more central part of the ocean, sinks between 50° and 550 S to mix with the bottom water ; this means that a large part of the southward movement does not make the climb to the higher level of the current in the Antarctic zone but returns directly towards the north with the bottom current, carrying with it large amounts of silicate. On the other hand, the observations made across the sub-Antarctic zone in the South Pacific Ocean suggest that the volume of Antarctic intermediate water carried back to the south by the warm deep current is exceptionally large, although Deacon (loc. cit. p. 106) states : " It is not safe to assume that all the water at the level of maximum temperature of the deep water north of the Antarctic convergence finds its way into the upper part of the warm deep layer in the Antarctic zone because it usually has too low a salinity ; it appears to be part of a southward eddy which is returned to the north with the surface water which sinks at the convergence." In view of the absence of data from lower latitudes in the South Pacific Ocean any discussion of the return of silicate to the Antarctic zone south of the South Pacific Ocean must be deferred. We have seen therefore that phosphate leaves the Antarctic in the surface and bottom currents either in the free state or, in the surface current, in the form of plankton. North of the Antarctic convergence, decomposition of the Antarctic plankton occurs chiefly in the Antarctic intermediate current, and the phosphate so released is transferred by mixing to the upper layers of the southward-going warm deep water. Silicate is chiefly lost to the Antarctic zone in the Antarctic bottom water, although large quantities also travel northwards in the surface current. Unlike phosphate, silicate is always at a maximum in the Antarctic zone, where the greatest mortality of phyto- plankton occurs. The return of silicate to the Antarctic is effected by means of the warm deep water, to which silicate is transferred by mixing at both the upper and lower boundary surfaces with the intermediate and bottom currents respectively. Since the greater amount is carried out of the Antarctic in the bottom water the major quantity of silicate is returned in the lower portion of the warm deep water. i6 DISCOVERY REPORTS ANTARCTIC SURFACE WATER PHOSPHATE CONTENT AT THE SURFACE In the introduction to this report we saw that as far as the phytoplankton is concerned two very different regions exist in the Antarctic zone. One, the northern region, where the phytoplankton production is enormous and is confined to a relatively brief period of growth and decline, and the second in the southern region, where the production is far less in actual concentration but is spread over a longer period. With these facts in mind we will review the phosphate and silicate contents of the surface water in both the southern and northern regions of the Antarctic zone. The data are presented as surface values and later as mean integral averages of the o-ioo m. layer to satisfy both the biological and hydrological requirements. The photosynthetic layer of the Antarctic surface water is usually a very shallow layer ; under ideal conditions it consists of a well-illuminated, shallow layer separated from the rest of the surface layer by a strong discontinuity. Consequently surface values of nutrient salt content are of more importance to the planktologist than o-ioo m. averages. On the other hand, the o-ioo m. averages offer a good picture of the nutrient salt content of the surface layer in the Antarctic zone. During a cruise across the Southern Ocean south of the South Atlantic Ocean and as far east as 440 E, mainly in the second half of February 1935, the surface phosphate values were as shown in Table I. Table I Station Position Surface P2Os in mg./m 3 1510 590 36-9' S, 260 57-7' W 86 I5H 6i° 34-3' S, 220 46-3' W 94 I5J3 63° 54-2' S, i8°2i-2'W 94 1515 66° 147' S, 130 50-2' W 79 1517* 68° 447' S, 090 20-3' W 84 i5J9 66° 31-0' S, 03° 40-0' W 82 »"*" 1521 64° 34-5' S, oo° 45-8' E 83 i523 66° 39-2' S, 050 28-2' E 83 1525* 68° 41-8' S, io° 34-8' E 93 1527 66° 57-2' S, 15° 10-3' E 99 1529 64° 547' S, 20° oo-6' E 68 I531 62° 50-0' S, 24° 28:2' E 82 1533 65° 24-3' S, 27° 07-8' E 82 > 1535* 67° 18-0' S, 30° 34-1' E 98 1537 64° 33-6' S, 33° 09-4' E 102 , '539 62° 02-1' S, 35° 54-5' E 90 1541 64° 26-6' S, 39° 177' E 106 1543* 66° 297' S, 42° 26-0' E 72 1545 64° n-o'S, 44°05-i'E 9i !547 6i° 53-4' S, 40° 29-8' E 83 Denotes ice-edge station. ANTARCTIC SURFACE WATER ,7 All the above stations were made south of 6o° S except St. 15 10, and several were at the edge of the pack-ice north of the continent. The phytoplankton catches during this cruise showed that production had begun earlier in the east than in the west, even allowing for the time interval involved in a west to east cruise lasting 20 days. In general terms it may be stated that in the west (Sts. 1510 to 1523) the phytoplankton was evidently not yet at its maximum, in the centre (Sts. 1525 to 1533) the phytoplankton was in much greater quantity and was probably at its maximum, while from St. 1535 eastwards we entered a "grazed down" area. This area was one in which the diatom nets contained large numbers of euphausian faecal pellets, while the actual catch of undamaged phytoplankton organisms was small. The faeces contained numerous diatom fragments, indicating a previous heavy production of phytoplankton which had been consumed by zooplankton. A striking result of an inspection of the surface values is the general lowness of the phosphate having regard to the average high latitude of the stations. Although production of phytoplankton in the southern region does not come to a maximum until the third week in February ( + 2 weeks) there must have been a steady drain on the available phosphate from November onwards. It must be remembered that in the southern region of the Antarctic zone the phytoplankton production is a long steady process of small concentration, as opposed to the rapid rise and fall of the huge concentration in the northern region. From these same stations in Table I some idea can be gained of the amount of phos- phate that is withdrawn from the surface layer by the phytoplankton. Thus at St. 1543, an ice-edge station in 42I0 E, the phosphate content between 40 and 400 m. is uniformly 128 mg., which figure may be taken as a typical pre-production value for the surface layer at the ice-edge. On an average this value is reduced to about 87 mg. at the sur- face in February, i.e. a consumption of 41 mg. out of 128 mg. or about 32 per cent at the surface at the ice-edge. Exceptionally the consumption at the ice-edge may be greater, at the above station (1543), for example, a consumption of 56 mg. has taken place or a withdrawal of the order of 44 per cent of available phosphate. North of the ice-edge in 200 E the phosphate was even more reduced as the following figures from St. 1529 in 640 547' S, 200 oo-6' E show: Depth m. o 10 20 30 40 50 60 80 100 P205 mg./m.3 68 64 64 65 112 120 124 124 124 Thus out of an available 124 mg. throughout the surface layer, as much as 60 mg. or over 48 per cent of available phosphate has been withdrawn at 10 m. through the agency of phytoplankton. This reduction in phosphate content was accompanied by a very large concentration of phytoplankton, and it is of great importance to note that the reduction was confined to a shallow layer from the surface to a depth of 30 m., below which a considerable discontinuity existed. At this station the 0-30 m. layer was un- doubtedly the extent of the photosynthetic layer in this high latitude, and it is interesting to note that the value at 10 m. was lower than the surface value. Had a sample been taken at 5 m. it might have shown the seat of maximum utilization of the phosphate. i8 DISCOVERY REPORTS Some idea of the later autumn phosphate values of the surface water in the southern region near Enderby Land may be had from the results of section 6 in 45 ° E towards the Crozets in the first half of May 1934 (Sts. 1361-1366, Plate X), and section 7 in late April 1932 (Sts. 855-864, Plate XII). The observations in the Antarctic zone are given in Table II. The two ice-edge stations were made in comparable positions about 18 days apart though in different years. The phytoplankton conditions were very different; at St. 1361 a very small catch was recorded, whilst at St. 855 there was a rich phytoplankton concentration for the time of the year and the latitude. From the above Table II Station Position Surface P205 mg./m.3 Surface Si02 mg./m.3 1361* 1362 »363 1364 I365 1366 64° 37-6' S, 44° 16-3' E 61° 45-5' S, 44° 15-9' E 59° 14-0' S, 44° 27-9' E 56° 38-6' S, 44° 45-4' E 55° 12-8' S, 44° 52-5' E 50° 42-3' S, 44° 54-1' E 130 129 120 120 115 122 2200 2300 1600 1500 1500 850 855* 856 857 858 859 860 861 862 863 864 65° 15-0' S, 48° 437' E 6i° 06-6' S, 53° 39-8' E 60° 40-1' S, 59° 237' E 60° io- 1 'S, 63° 54-8' E 59° 19-1' S, 68° 51-8' E 57° 564' S, 73° 58-8' E 56° 28-9' S, 79° 18-2' E 55° 33-8' S, 83° 00-4' E 54° 15-3' S, 88° 22-4' E 53° 117' S, 93° io-6' E 84 87 96 96 95 108 117 121 121 100 3400 * Ice-edge station. table it is seen that at St. 1361, a station where a poor diatom catch was found, the surface phosphate was as high probably as it is at any time of the year, and the silicate was reduced, whilst St. 855, with a rich phytoplankton, had a low phosphate content in the surface and a silicate content equal probably to the maximum at any season at this latitude. The observed nutrient salt values do not agree either with one another or with the amount of phytoplankton found at each station ; it would have been expected that St. 855 would show low values for both phosphate and silicate contents, and St. 1 361 high values for each nutrient salt if there were an immediate connexion between nutrient salt concentration and the density of the phytoplankton. However, we can assume that the rich phytoplankton found at St. 855 was due primarily to a localized concentration which was now, i.e. late April, a fraction of its former concentration. This follows because in late April the concentration was large, and therefore it can be argued that at the end of February it must have been greater still unless the April concentration were due to an autumnal outburst ; this, however, is unlikely in view of the nature and volume of the catch. The ice-edge phytoplankton is characterized by a long steady growth period from November to the end of March, and, exceptionally, the ANTARCTIC SURFACE WATER 19 ice-edge is the seat of very large concentrations. Now if at St. 855 a large concentration had been existing for some months previously, it is obvious that a large amount of nutrient salt must have been removed from the surface water during this period. The phosphate value of 84 mg. is sufficiently low to agree with this large withdrawal, pro- vided a pre-production value of about 130 mg. is allowed at the ice-edge. The silicate value of 3400 mg. requires an explanation. Let us consider the utilization of phosphate and silicate by the phytoplankton. An immediate result is the withdrawal of a high proportion of these salts from the photo- synthetic zone of the surface layer. Then follows a consumption of the phytoplankton by the zooplankton, which latter as far as we know does not require much silicate for its nutrition. The zooplankton then excretes the undigested matter including the silica skeletons of the diatoms. We have observed the results of this last process in the area close to the position of St. 855. It can be argued that the zooplankton excretes the greater amount of the silicate which comes to it in the form of its phytoplankton food supply, whereas it utilizes a proportion of its phosphate intake in the formation of phosphoproteins, etc. Thus, temporarily, part of the phosphate originally consumed by the phytoplankton is withheld by the zooplankton, whereas the silicate, or a large proportion of it, is returned to the sea in the form of broken diatom skeletons. How rapid is the re-solution of these skeletons is not known exactly, but a rapid regeneration of silicate must be assumed to explain some of the high silicate values recorded just after phytoplankton maxima. By analogy with the English Channel silicate must be utilized by the plankton more than once in one season (Cooper, 1933). Thus it is probable that the high silicate value in the surface at St. 855 is due partly to regeneration in situ. As will be explained later in this report it is also due to the return to the ice-edge of water of high silicate content at this time of the year (see p. 36). Thus the incidence of a moderately rich phytoplankton at St. 855 with a high silicate content in the surface water is not anomalous. The value of 130 mg. as the phosphate content at the surface at St. 1 361 is quite compatible with the negligible amount of phytoplankton present; the silicate value is on the low side, however, for an ice-edge station in late April. Individual figures of phosphate and silicate contents for any region in different years may vary very considerably, and we have an example of this at Sts. 855 and 1361. The concentrations of nutrient salts preceding the main outburst of phytoplankton must vary from year to year. This follows because the amount of these salts returned to the ice-edge or anywhere in the Antarctic is dependent on the amount of nutrient salts that is lost to the Antarctic zone in previous years. The whole conception of the amounts of phosphate and silicate in the Antarctic zone is qualified by three considerations : the speed of regeneration in situ of these nutrient salts is different, the times of the maximum return to the south must be different, and the concentration of these salts anywhere in the Antarctic must show an annual variation. I have discussed the first consideration earlier in this section. The second consideration follows from the different methods of return of phosphate and silicate, which makes coincidence of the times of return ex- tremely unlikely. Around South Georgia, an area in which we have made more observa- 3-2 20 DISCOVERY REPORTS tions than anywhere else, we know there is an annual variation of temperature, salinity and phosphate, and it can be assumed that there are corresponding variations in the open ocean. In March 1933 the surface phosphate content at the ice-edge south of the South Atlantic Ocean in c.-|-° E (St. 1154) was 104 mg., whilst about 250 miles north of this position the surface value was 74 mg. ; this latter value was, however, accompanied by a much heavier phytoplankton. We have no actual ice-edge figures south of Australia, but at St. 889 in 6i° 44-6' S, 13 1° 38-4' E at the end of May 1932 the surface value was 135 mg., a high winter value. The position of this station was north of the ice-edge, which was in the extremely low latitude of 630 41-4' S at this time of the year. The value of 135 mg. is very high, and together with a content of 141 recorded at the ice-edge in 1530 57-2' W in September 1932 probably represents the extreme value for surface phosphate at the ice-edge in winter; a more usual figure is 130 mg. South of the Tasman Sea in mid-June 1932 the ice-edge surface value at St. 906 (6i° 247' S, 1540 26-2' E) was 130 mg., but a short distance east at St. 912 (6i° 05' S, 1580 24-5' E) the corresponding content was only 92 mg. St. 912 was situated in very much shallower water than was St. 906, the respective depths being 991 and 3041 m. The chemical and physical data for the water columns at these stations were very different, and in particular the warm deep water at St. 912 was markedly less saline and had a lower phosphate content and yet was much warmer than at St. 906. In September 1932 the ice-edge surface value north of the Ross Sea was 127 mg. at St. 956 in 620 12-8' S, 1580 n-o' W. In mid-January 1934 the corresponding station was situated at a much higher latitude owing to the difference of season. The surface phosphate at this station, 1267, in 690 49-4' S, 1590 12-6' W was 82 mg. at a date when phytoplankton production was in full activity and responsible for a large withdrawal of nutrient salt. In January, February, March and November a large number of observations were made along the Pacific ice-edge from the Ross Sea to 8o° W in which longitude there are also data for September and October. In the first half of January 1934 from 770 to i59°W the average ice-edge value of phosphate in the surface was 87 mg., which was also the average of some eighteen stations within 200-250 miles of the ice-edge. In late February to mid-March on the return voyage from west to east at an average latitude slightly higher than the westward cruise, six ice-edge stations gave an average surface phosphate of 88-5 mg., whilst all the stations within 200-250 miles from the ice gave an average value of 88 mg. In the interval between the westward and eastward cruises it is probable that the phyto-plankton of the southern region reached its maximum concen- tration, but it is doubtful whether the average value for the surface phosphate dropped much below 87 mg., which may represent an average minimum surface value for this region. A comparison of this value with figures for the lower part of the Antarctic surface layer shows that about 30 per cent of the available phosphate is withdrawn by the phytoplankton at the ice-edge of the South Pacific sector of the Southern Ocean. ANTARCTIC SURFACE WATER 21 In the eastern part of the south Pacific in early November 1934 the average ice-edge value for the surface phosphate was about 112 mg., whilst in an area between 66f° and 59!° S and between 8o° and 109J0 W an average surface value for fourteen stations was 105 mg. This was at a time when the phytoplankton outburst had begun and the phos- phate results showed this by the reduction of the ice-edge value to 112 mg. In 8o° W we have ice-edge results from the months of March, September, October and November 1934, but in considering these the effect of the differing latitude of the ice-edge in the various months must be taken into account. Observations were also made in December 1933, but there are good reasons for considering these unreliable and they are not included in the following table which consists of the phosphate data at the ice-edge stations together with the latitude of the stations. Station Date Latitude Surface P205 mg./m.3 1312 HI5 1450 1472 10. iii. 1934 12. ix. 1934 30. x. 1934 14. xi. 1934 68° 18' S 63° 40-6' S 66° 03-1' S 66° 31-6' S no i°5 120 112 In March the ice-edge was farthest south and the surface value was still some 23 per cent less than in the bottom of the surface layer. Regeneration has raised the content from a probable minimal value of 85-90 mg. The effect of seasonal variation in the latitude of the ice-edge is best seen in the September and late October figures that for October being greater than the value in September. There was practically no phyto- plankton activity in September and not much more in October. The September value was less of course because the ice-edge was far north at this time, but even so the value is considered to be low. The fall from late October to mid-November, during which time the ice-edge retreated a short distance to the south, can be accounted for by the increase of phytoplankton activity. In January 193 1 on a line of stations north and west of Adelaide Island, which is situated in the south-east of the Bellingshausen Sea, the surface phosphate increased from 73 mg. inshore to 104 at the outermost station. The low value near the coast was accompanied by a heavy concentration of phytoplankton. In late December 1930 off the north-west coast of Graham Land between Anvers Island and Adelaide Island the surface phosphate at three stations was 103, 102 and 93 mg., at a time when the phytoplankton had by no means come to a maximum. In general terms it may be stated that in the Antarctic zone, when allowance is made for the differing time of the maximum concentration of the phytoplankton across the zone, the phosphate and silicate values at the surface fall from south to north. In the above remarks the surface phosphate content at the southern end of the Antarctic zone has been described as far as our present knowledge allows, and the distribution at the northern end of the zone will now be considered. In April 1930 (in section 1, Plate III) at St. 384, a position just south of the Antarctic convergence in the Drake Passage, the surface water had a phosphate content of 1 15 mg., 22 DISCOVERY REPORTS as opposed to a content of 75-82 mg. at the end of December 1933 at Sts. 1234 and 1235. It is obvious that the April figure is showing the effect of regeneration, whilst the December results were obtained after the maximum concentration of phytoplankton in the northern region, and are correspondingly low. In the Scotia Sea between South Georgia and the Falklands a surface value of 99 mg. was observed in the second week of October 1934; the main outburst of phytoplankton had not yet occurred in this position. Just south of the convergence between the Falklands and Elephant Island the surface had a phosphate content of 98 mg. in late September 1934 (St. 1425), whereas earlier in March 1934 a surface value of no mg. was recorded at St. 1326 in the same position. Very poor catches of phytoplankton were taken at both stations ; the value of 98 mg. in September would appear to be low. As evidence of an annual variation we have a surface value of only 85 mg. in early March 193 1 at St. 634 (in a position very close to that of St. 1425) as opposed to 1 10 mg. in March 1934. Immediately south of the Antarctic convergence to the north of the South Orkneys we have data for late January, early April and early October. The surface contents are respectively 76, 113 and 113 mg. ; these figures probably approach the limits of the annual range of surface phosphate throughout the year in this position. If this is correct the withdrawal from the surface during the phytoplankton season at the Antarctic con- vergence is of the order of 37 mg. out of an available 113 mg., about 33 per cent. In the Scotia Sea, excluding South Georgia which is dealt with later in this report, we have no surface values for the winter months of May, June, July and August. Phosphate is probably maximal in these months unless a southward movement of sub- Antarctic water across the convergence considerably reduces the phosphate content. Consequently in the absence of winter surface values the seasonal range of phosphate just south of the convergence in the Scotia Sea must be given with some reserve. However, it seems clear that a midsummer value of about 75-80 mg. is usual and an autumnal value of 1 10-1 15 mg. is to be expected. The value for the winter months may be in the neighbourhood of 1 15-120 mg. In the South Atlantic Ocean in 300 W a value of 105 mg. was recorded in early April 193 1 for the surface phosphate just south of the Antarctic convergence, whereas in 220 W in mid- August 1934 only 94 mg. were found in the surface. In mid-October 1930 at St. 452 in 490 50' S, 080 32^' E, which position lies to the north of the easterly drift out of the Weddell Sea, only 79 mg. were found at a time when the main outburst of phytoplankton had not begun. At the Antarctic convergence north of Enderby Land the surface phosphate was of the order of 85-95 mg. in February and April, whilst in early May 1934 it had risen to 122 mg. On a diagonal course between Enderby Land and Fremantle, Australia, in late April 1932 the surface value was 100 mg. just south of the convergence. In late May 1932 the value farther east, and south of Australia, was 116 mg., whilst south of the Tasman Sea a month later 1 06-1 15 mg. were recorded. At the convergence north of the Ross Sea we have values for the surface phosphate ANTARCTIC SURFACE WATER 23 in early September 1932 and the third week in January 1934; these may be regarded as winter and summer values and are 126 and 89 mg. respectively. It may be remarked here that an exceptionally low value of 59 mg. was found south of the position where 89 mg. was recorded in January 1934 ; this represents a withdrawal of more than 54 per cent of the available phosphate. North-east of the Ross Sea the convergence intersects the ridge which runs between Cape Adare and Easter Island, and a very high value of 143 mg. was found just south of the convergence in mid-September 1932; upwelling is probably the cause of this high value. Farther east during the same cruise values of 1 10-120 mg. were established just south of the convergence. In 8o° W a routine section was made in 1934 and there are four sets of observations just south of the convergence. These are 112 mg. in mid-March, 107 mg. in mid- September, 103 mg. in late October and 96 mg. in mid-November. In March the 0-60 m. layer was largely composed of sub-Antarctic water which was overlying Antarctic surface water and regeneration in situ had been in full operation for some time past owing to the earlier date of the phytoplankton maximum in the sub-Antarctic zone. The Sep- tember, October and November figures show that in 8o° W in 1934 not much phosphate had been utilized by the phytoplankton up to the date of the November station, and that in this year at least the main outburst did not begin in this longitude until after 16 November. It has been our misfortune to be elsewhere at the time of the main out- burst of phytoplankton in 8o° W, and thus no estimate can be given of the amount of phosphate withdrawn from the surface. Thus as a short summary we can say that at the ice-edge in the South Atlantic sector of the Southern Ocean the amount of phosphate withdrawn by the phytoplankton is of the order of 41 mg. out of an available 128 mg. or approximately 32 per cent. Excep- tionally as much as 44 per cent has been withdrawn, whilst north of the ice-edge as much as 48 per cent has been utilized. In the South Pacific sector the withdrawal at the ice-edge amounted to about 30 per cent of the available phosphate, with again greater amounts on occasions north of the ice-edge. The data for the northern part of the Antarctic zone are far from complete, but the evidence at present suggests that a withdrawal of 33 per cent of available phosphate occurs during the phytoplankton season. It is suggested that when phytoplankton is consumed by zooplankton the latter incorporates a proportion of the phosphorus content of the plants and excretes most of the silicate content. The excreted silicate is considered to redissolve quickly and thus regenerate in situ whilst there is a distinct lag in the phosphate regeneration. PHOSPHATE CONTENT OF THE o-IOO M. LAYER From the foregoing remarks on the phosphate content at the surface in the Antarctic zone it will have been realized how the individual values obtained vary with the season of the year. As a means to a better understanding of the phosphate content of the Antarctic water the mean integral values for the 0-100 m. layer have been calculated. 24 DISCOVERY REPORTS The mean value between o-ioo m. has been selected because this layer with very few exceptions consists of true Antarctic surface water unmixed with the upper layers of the warm deep water. The mean values so obtained were averaged over two-monthly periods and plotted against the various months of the year. Having regard to the considerable differences in the actual phosphate content across such a large area as the Antarctic zone which stretches from the ice-edge to the Antarctic convergence, together with the differences in the concentrations of phytoplankton throughout the zone, it was decided to draw two seasonal curves : one, the curve for the northern region constructed from the results of stations within a distance of 300 miles south of the Antarctic convergence; the second, a curve for the southern region made from values at stations within 200 miles of the ice-edge. These curves are shown in Fig. 7 ; the numbers opposite the points on the curves give the numbers of observations from which each two-monthly average is calculated. No curve has been constructed for the area between the southern and northern regions of the Antarctic zone because at present our observations in this area are not sufficiently numerous. 2..IA 120 \4l HO o 2 2 |.T - -^ n« _n ^ o ^" o*J-