Monsoon-Related Changes in Zooplankton Biomass in the Eastern Banda Sea and Aru Basin

Biological Oceanography ISSN: 0196-5581 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/tboc20 Monsoon-Related Changes in Zooplankton Biomass in the Eastern Banda Sea and Aru Basin P.H. Schalk To cite this article: P.H. Schalk (1987) Monsoon-Related Changes in Zooplankton Biomass in the Eastern Banda Sea and Aru Basin, Biological Oceanography, 5:1, 1-12 To link to this article: https://doi.org/10.1080/01965581.1987.10749502 Published online: 01 Oct 2013. Submit your article to this journal Article views: 109 View related articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tboc20 Download by: [37.44.193.64] Date: 04 December 2017, At: 05:37

Biological Oceanography, Vol. 5, pp. 1-12, 1987 Printed in the UK. All rights reserved. 0196-5581/87 $3.00 +.00 Copyright 1987 Taylor & Francis Monsoon-Related Changes in Zooplankton Biomass in the Eastern Banda Sea and Aru Basin P. H. SCHALK Netherlands Institute for Sea Research Texel, The Netherlands Introduction Abstract Zooplankton biomass, depth distribution, and die/ migration have been studied in the eastern Banda Sea and Aru basin during the south-east monsoon (August 1984), with upwelling, and the north-west monsoon (February 1985), with downwelling conditions. Sixteen stations were sampled with a Rectangular Midwater Trawl (RMT 1 + 8) during both seasons. The average zooplankton biomass in August is twice that off ebruary, and spatial variation in zooplankton distribution was larger in August. Monsoon effects on zooplankton biomass were stronger in the southwestern and eastern parts of the area than in the central part, where a Pacific influence is assumed for the south-east monsoon period. The investigated area and adjacent waters can be considered rich in zooplankton for a tropical region but only during a limited period of the year. Upwelling conditions, with low vertical stability in the water column, are related to high zooplankton biomass values, deep depth distribution at night, and reduced die/ migration. Downwelling conditions, with a high vertical stability, are related to low zooplankton biomass, a relatively shallow depth distribution at night, and increased die/ migration. The Indonesian Archipelago forms a barrier as well as a passage for pelagic animals between the Indian and Pacific Oceans (Brinton 1979). Various expeditions covered the waters of the Archipelago (e.g., Challenger, Dana, Snellius, Siboga, Galathea, Rumphius, 110E, Monsoon), but information on the area is still incomplete. It is an area with important biogeographic zones (Vander Spoel and Heyman 1983), the faunal assemblages of which are only partly understood. The Indonesian waters are governed by a monsoon-type climate which affects the hydrographic conditions, so changes in plankton abundance and distribution can be expected. During the joint Indonesian-Dutch multidisciplinary oceanographic SNEL LIUS II Expedition (July 1984-July 1985), a study was made of the pelagic ecosystem in the eastern Banda Sea and Aru Basin and the changes related to the monsoon wind system. Wyrtki (1957, 1961) hypothesized that the south-east monsoon (June-September) induces upwelling conditions in the area concerned, while the north-west monsoon (December-March) would induce downwelling conditions, which is confirmed by recently obtained hydrographical data (pers. comm. Baars and Zijlstra). Due to the changing of the monsoon, the surface current pattern also changes (Wyrtki 1961); during the south-east monsoon, the direction of the currents is mainly westwards, while, during the north-west monsoon, it is mainly eastwards (Figure 1). 1

2 P. H. Schalk A: SE MONSOON B: NW MONSOON,,.... Figure 1. Surface current patterns in the south-east (a) and north-west (b) monsoons (after Wyrtki 1961) and the location of the area investigated. During the south-east monsoon, upwelling conditions occur (indicated by " x ") in large parts of the area; during the northwest monsoon, downwelling conditions occur (indicated by "0"). Tropical upwelling systems off north-west Africa and Peru show high zooplankton standing crops compared to much lower values in adjacent permanently stratified waters. In the area under discussion, up and downwelling conditions alternate twice a year. Such variations in hydrography must make high demands on the plankton community and can be expected to increase chlorophyll concentration (Montji 1975) and plankton biomass (Rochford 1962; Birowo and Ilahude 1977) during the upwelling period. There are indications also that depth distribution and diurnal vertical migration of the zooplankton are related to the vertical stability of the water column (Vinogradov 1968; Longhurst 1976b; Ortner et al. 1980). However, there are no data concerning the magnitude of the biological changes in this area. The present paper reports the abundance and spatial distribution of zooplankton biomass in relation to hydrographical changes. Methods Two cruises were made, one from 26 July-2 September 1984 (south-east monsoon) and one from 8 February-11 March 1985 north-west monsoon). Each cruise consisted of a survey programme of 12 days, in which 12 short stations were made covering the eastern Banda Sea and Aru Basin, followed by a second part of 14 days, when a more detailed study took place at four long stations near a drogue (Figure 2). The survey stations of both cruises were sampled between 9 and 12 p.m. local time in discrete layers from 0 to 100 and from 100 to 300 metres. Occasionally, this last stratum was sampled from 100 to 200 m and 200 to 300 m. At the drogue stations of both cruises, samples were taken in strata of 100 m down to a depth of 500 min a day and a night series, but sometimes, due to a lack of time, thicker strata had to be sampled to cover the 500 m depth range. At station D of the February cruise, no day series was made. During sampling, the ship speed was maintained between 1.8 and 2.2 knots. The 12 survey stations gave an impression of the horizontal distribution of the zooplankton in the area

Zooplankton Changes in Eastern Banda Sea and Aru Basin 3 128 129 130 131 132 134 4. 5. 0 15 c c~z BANDA SEA 129 130 9 21 131 -A-e57 ARU 63 45 BASIN 132 133 Figure 2. The eastern Banda Sea and Aru Basin with the locations of the 12 survey stations (dots) and the four drogue stations (lines; arrow point indicates drift direction of drogue), A, B, C, and D for the August cruise and A 2, B 2, C 2, and D 2 for the February cruise. concerned, while the drogue stations provided information concerning the vertical distribution during day and night and diel vertical migration. The sampler used for observations on zooplankton was a Rectangular Midwater Trawl, RMT 1 + 8, (Baker et al. 1973), which is an opening and closing net system with two nets in one frame; the RMTl with a mesh-size of 320 1-Lm for zooplankton and the RMT8 with a mesh-size of 5 mm for micronekton. Via an acoustic link, this system continuously monitors the depth of sampling, water temperature, the speed of the net through the water, and the net state (opened or closed). The opening and closing of the net is controlled from the deck by the acoustic system. The speed indicator provides a measure of the distance sampled, from which the average speed of the net can be calculated. Roe et al. (1980) described the relation between net speed and mouth angle so that the volume of water filtered by the nets can be estimated. This paper concerns RMT1 catches only. Biomass of the RMT1 catches was measured as displacement volumes, after removal by hand of occasionally caught organisms larger than 5 mm (mainly jellies). The samples were split afterwards and preserved in alcohol 70% and in 4% neutralized formaline (Heyman 1981) for further zoogeographic and taxonomic studies. For biomass distribution, only night catches were taken into account, because at night the major part of the zooplankton is concentrated in the upper layers, as indicated by the drogue station data. The variable factor of daylight, which causes an equally variable vertical distribution of the zooplankton (e.g., Longhurst 1976a) is absent, and during the night the factor of net avoidance is probably smaller 5. 6.

4 P. H. Schalk (Omori and Hamner 1982); thus, night catches can be expected to provide the most reliable biomass estimates. Results The temperature profiles (Figure 3) from the net monitoring system demonstrate that hydrographical differences were present between the two seasons. In August, at all stations, the surface temperature was about 25 C, and at most stations a pronounced mixed layer was present. In February, surface temperatures had risen to about 25 C, while the waters were more stratified in the western stations and mixed layers were present in the eastern stations. In February, all stations showed an enlarged temperature gradient in relation to depth compared with August. The drift of the drogues (Figure 2) in the southern and western direction in August and in the eastern direction in February confirmed the surface current patterns given by Wyrtki (1957). In August during the south-east monsoon, the average biomass for the area was estimated at 20.0 cc.m- - 2 (Table 1). Large differences in zooplankton biomass, varying from 7.2 to 39.8 cc.m- 2, were observed between the 16 stations. Highest values were found in the eastern part of the area, just west of the Aru Islands, and in the southwestern part of the area studied; lowest values occurred in the centre (Figure 4a). In February during the north-west monsoon, the average biomass was about half that of August, with an average of9.5 cc.m- 2 for the whole area, and differences between the stations were less variable, ranging from 5.8 AUGUST 100 E: t433 SOUTHERN SECTION 200 ~00 ~00 Figure 3. Temperature profiles as detected by the net monitor system for the northern and southern sections of the Banda Sea area. E represents a measure of vertical stability according toe= 1 /px 8 ""1az oo-s.m- 1 ). A: August cruise; B: February cruise.

Zooplankton Changes in Eastern Banda Sea and Aru Basin 5 Table 1 Zooplankton biomass (displacement in cc.m- 2 ) and its vertical distribution (biomass 0-100m layer as percentage of biomass 0-300m layer) and their mean and standard deviations for the August and February cruises, respectively Biomass cc/m 2 Vert. Distr. % ST August February August February 03 13.1 8.0 50 09 17.2 6.2 55 39 15 29.8 5.8 56 58 21 39.8 7.0 55 80 27 16.1 11.2 48 68 33 16.9 8.2 59 80 39 18.3 6.6 48 73 45 7.2 8.5 61 72 51 10.0 15.9 40 51 57 18.8 12.3 57 67 63 31.6 7.5 55 63 69 19.5 12.5 59 58 A 21.0 11.5 50 63 B 27.5 11.1 52 68 c 19.4 11.7 71 64 D 14.1 8.7 52 68 20.0 ± 8.4 9.5 ± 2.8 54.2 ± 6.9 64.7 ± 10.6 to 15.9 cc.m- 2 (Table 1). Values gradually decreased from the northwest to the southeast, with highest recordings south of Irian Jaya (Figure 4b). The largest differences in biomass between the two seasons occurred in the southwestern part of the area (stations 15 and 21) and west of the Aru Islands (station 63) and amounted to more than a factor of five. Except for stations 45 and 51, higher biomass values were found in August. The vertical distributions of zooplankton biomasses at both survey and drogue Figure 4. Distribution of the zooplankton biomass, measured as displacement volumes in cc.m- 2 of the night catches in the upper 300m. Arrows represent the suggested current patterns (Wyrtki, 1957). A: August cruise; B: February cruise.

6 P. H. Schalk 30 N" 20 E! 10 W+------+E 30 '-rn AUGUST 84 N-SECTION 20 10 FEBRUARY 85 N-SECTION 09 27 51 57 B 30 FEBRUARY 85 S-SECTION 10 ST 15 21 20 10 33 45 69 63 ST 15 21 33 45 69 63 Figure 5. Histograms of the zooplankton biomass (displacement in cc.m - 2 ) at the survey stations in a northern and a southern section. Dotted parts of bars represent the contribution of the 0-100 m layer. A: August cruise; B: February cruise. stations in the upper 300m were compared by expressing the biomass in the upper 100 m as a percentage of the total biomass in the upper 300 m (Table 1). The zooplankton showed a more shallow distribution in February, with a mean of 65% of the biomass in the upper 100 m, against 54% in August. The biomass in the upper 500 m, studied at four "complete" drogue stations (B and D in August, B and C in February), showed no consistent differences between day and night, variance being less than 6% from the mean of day and night values (Figure 6a and b). Based on this, it is presumed that no significant migrations from this stratum to depth ranges below 500 m occurred and that there was no enhanced net avoidance during daytime. The biomass values of the missing hauls of stations A and B of the August cruise and station A of the February cruise could thus be estimated with some confidence. At the five drogue stations (Band Din August, and A, B, and C in February), at which the 400-500 m layer was sampled during both day and night, there were no consistent day/night differences for the catches in this layer, suggesting that the diel vertical overall migration of the zooplankton did not stretch below the 400 m level. The arithmetic means (Table 2) for the depth of the biomasses for the day and night series have been calculated (as in Rudjakov 1971). In August, the centres of gravity for the biomass were shallower for the day catches than for the night catches in three out of four stations, indicating a reversed diel migration resulting in a deeper distribution at night time. In February, the normal pattern was found: daytime occurrences deeper than during the night. The average vertical distance covered by diel migration for the four drogue stations in August (-6 m) differs significantly from that for the three drogue stations in February ( + 23 m), so there is an increased overall diel migratory activity during the latter

Zooplankton Changes in Eastern Banda Sea and Aru Basin 7 BANDA SEA AUGUST BANDA SEA FEBRUARY 160 80 0 80 160 80 40 0 40 80,,:] ~l V.,A 400 400 600 A,,:l ":l rr,.e 400 8,;l,;l Vc c 400 400 22.8 400 20:l 17,,:l,,, D 400 400 179 16.8 N.C. Figure 6. Profiles of zooplankton biomass during night (black bars) and day (white bars) at the drogue stations in the Banda Sea area A: August cruise; B: February cruise. Vertical axis: depth in metres, horizontal axis: displacement in cc.m - 3.10-3 Figures at left and right side of the histograms give the total displacement in cc.m- 2 of the upper 500 m for the night and day catches, respectively. Table 2 Migratory activity of the zooplankton at the drogue stations. First two columns give the arithmic means of depth of the zooplankton, ''the centres of gravity,'' for the night and day series. Third column gives the differences between the mean day and night depths; a negative sign points to reversed migration. Last column gives the shifted biomass as a sum of the differences in zooplankton biomass between the night and day time catches per stratum Cruise Banda Sea August 84 12.4 14.7 14.7 ST Night Day.:lDepth I.:lBiomass A 183 170-13 10.5 B 172 154-18 15.4 c 134 160 26 6.7 D 178 158-20 4.2 Mean 167 ill - 6 9.2 D Banda Sea Febr. 85 A B c D Mean 148 166 163 182 159 164 208 174 182 16 42 11 6.0 5.2 3.6 4.9

8 P. H. Schalk period. In addition to the vertical distance covered by the diel migration, the amount of biomass migrating has also been taken into account. The summed differences in zooplankton biomass between day and night catches per stratum of 100 m have been calculated for the various stations and compared for the two seasons (Table 2). In August, there is a large variation in these figures between the stations. About two times as much biomass migrated in August (an average of 9.2 cc.m - 2 ) compared to February (an average of 4.9 cc.m - 2 ), but, in relation to the total biomass present per m 2, there is no proportional difference between the seasons. Discussion The increased surface temperatures and enlarged temperature gradient during the February cruise in comparison with the August cruise point to strong hydrographical changes between the two monsoon seasons, which is in agreement with the findings of Wyrtki (1961) and Birowo and Ilahude (1977). Profiles obtained with a conductivity temperature depth (CTD) probe, repeated in time, indicate upwelling conditions in large parts of this area during the August cruise, while this is not the case in February (pers. comm. M. A. Baars and J. J. Zijlstra). As a measure of these changes in the water column, the vertical stability E = 1/p dcra/ dz oo-s.m- 1 ) (Knaus 1978) have been calculated (p = density; cr 9 = potential density; z = depth), clearly showing an increased vertical stability for the February data compared to August (Figure 3). The average zooplankton biomass of 20.0 cc.m- 2 in the upper 300m in this area during the south-east monsoon is comparable to the average of24.0 cc.m- 2 in the upper 200 m reported by Birowo and Ilahude (1977) for almost the same area in September 1972. This is about twice as high as the average of9.5 cc.m- 2 during the north-west monsoon. The seasonal variability in zooplankton biomass reflects the change in average phytoplankton biomass between the south-east and north-west monsoon in this area, which amounts to about a factor of two (Gieskes et al. in press). Comparable seasonal changes in zooplankton biomass (Table 3) have been reported for adjacent areas with a monsoon climate: the Indian Ocean south of Java (Tranter and Kerr 1969) and the South China Sea and the Gulf of Thailand (Brinton 1979). The overall seasonal variability in biomass in these tropical ecosystems seems to amount to a factor of two to three. The standing crop of zooplankton in the eastern Banda Sea and Aru Basin is about as much as in adjacent waters (Table 3). In the Gulf of Thailand, higher values were recorded (Brinton 1979), but this is a rather shallow area. Although these observations result from the use of different nets with different mesh sizes (200-600 ~J.m), it is believed that in general these figures are comparable since mesh sizes in this range do not seem to influence the biomass measured as displacement volumes very much (Beet al. 1971). Spatial variation in zooplankton biomass was greater in August, differences in zooplankton biomass between the various stations amounting to about a factor of 4 to 5, compared to only a factor of 2 to 3 in February, and presenting a greater heterogeneity during the south-east monsoon. The large variability in August is caused by a zone of low zooplankton biomass, based on stations 45 and 51, in the centre of the area (Figure 4a), which was not present in February. All stations

Zooplankton Changes in Eastern Banda Sea and Aru Basin 9 Table 3 Zooplankton biomasses (in cc/m 2 ) in the two monsoon seasons in the Banda Sea and adjacent areas. Last column gives the sampled depth Gulf of Thayland 78.3 Aug. 33.8 Feb. 0-150 (Brinton 1979) South China Sea 10.2 Sep/Oct. 5.1 Dec. 0-150 (Brinton 1979) Ind. Ocean, south of Java 20.5 Sep. 11.0 Dec. 0-150 (Tranter and Kerr 1969) Banda Sea 24.1 Sep. 0-200 (Birowo and Ilahude 1977) Banda Sea 20.0 Aug. 9.5 Feb. 0-300 (present study) Tropical North Atl. Ocean (unpubl. data) 31.4 Upw. 10.3 Eq. Curr. 0-300 showed higher biomasses in August than in February except for the two stations 45 and 51, where, to the contrary, higher biomasses were found in February. Both stations are located directly south of the strait between Ceram and Irian Jaya. Wyrtki (1957) presents two different current patterns in the area, related to the two monsoon seasons. He suggests an average flow during the south-east monsoon to the west and southwest. This flow is probably fed by water coming from the Arafura shelf north of the Kai and Aru Islands, by upwelling water, and by water coming out of the strait between Ceram and Irian Jaya, which is in open connection with the oligotrophic waters of the Pacific Equatorial Current. Flow from there may explain the very low zooplankton biomasses at stations 45 and 51 during the rich south-east monsoon season, and this possibility is confirmed by the drift of the drogues. Zoogeographic connections between the tropical Pacific Ocean and Indian Ocean through the Banda Sea are described by Vander Spoel and Heyman (1983). In February, the surface currents flow in an eastern direction as indicated by Wyrtki (1957) and confirmed by the drogues, and no Pacific influence occurs. The diel migratory activity of the zooplankton shows differences between the various drogue stations of one cruise. Three out of four stations in August showed reversed diel vertical migration, while in February all stations showed the normal pattern. This agrees with the conclusions of Vinogradov (1968), Vinogradov et al. (1970) and Longhurst (1976b) that differences in intensity of diel vertical migration are related to hydrographic conditions and that in general intensities are larger in poor central gyre waters than in areas of equatorial divergences. In the area under discussion, the difference in migratory activity is expressed by the direction and depth of the migratory movement and not by the relative amount of biomass migrating. Closely related to the diel migration activity is the depth distribution of the zooplankton biomass at night. In August, with upwelling conditions and a low vertical stability, the zooplankton is relatively deeper at night than in February, with downwelling conditions and a high vertical stability. A similar phenomenon is encountered in the tropical North Atlantic Ocean, where, in the upwelling area

10 P. H. Schalk 80 00 75 0 70 65 0 60 ooo 00 0 0 55 50 0 45 40 35 5 10 15 20 25 30 35 40 BIOMASS Figure 7. Relation between vertical distribution (biomass 0-100 mlbiomass 0-300 m as percentage) and total biomass at the station (in cc/m- 2 in the upper 300 m) in the two seasons of the Banda Sea (diamonds). Black symbols; August cruise with low vertical stability; white symbols: February cruise with high vertical stability. Squares represent data from the tropical North Atlantic; black symbols: upwelling area; white symbols: stratified area. off north-west Mrica, 57% of the zooplankton biomass was caught in the upper 100m compared to the 0-300 m range, against 72% in the stratified waters of the North Equatorial Current (Schalk, unpublished data). Vertical distribution of zooplankton generally seems to relate to the vertical stability of the water column as suggested by Vinogradov (1968). Biomass and vertical distribution of zooplankton show distinct differences between stations with a low and a high vertical stability (Figure 7) and may be used as biological indicators for hydrographic conditions. The similarity in standing crop of zooplankton as well as the seasonal variability therein of the Banda Sea and Aru Basin with adjacent waters suggests that the present data may be representative for other tropical monsoon-governed waters. The alternating monsoons influence the hydrographic conditions, which, in their tum, cause changes in zooplankton biomass, depth distribution, and diel migratory activity. In the area under discussion, the south-east monsoon (June September) induces upwelling conditions with low vertical stability in the water column, characterized by high zooplankton biomass, relatively deep depth distribution at night, reduced or reversed diel vertical migration, and, in general, a greater spatial variation. The north-west monsoon (December-March) induces downwelling conditions with a high vertical stability in the water column, characterized by low zooplankton biomass, a relatively shallow depth distribution at night, enhanced diel vertical migration, and, in general, less spatial variation. Acknowledgements The author is indebted to Dr. J. J. Zijlstra and Dr. S. van der Spoel for critical reading of the manuscript and helpful suggestions; to S. Oosterhuis and J. Y. Witte for their help in operaitng the gear; to the technicians T. Buisman, H. Van Veen, A. Balk, F. Schilling, J. Wagenaar, and P. Alkema, who made the sampling possible; and to the captain and crew of MS Tyro, who performed a great job in

Zooplankton Changes in Eastern Banda Sea and Aru Basin 11 maneuvering the ship through these waters. The research described here was supported by Grant W 85-186 of the Netherlands Foundation for the Advancement of Tropical Research. Research has been carried out as a part of the Snellius II expedition, organized by the Netherlands Council of Sea Research (NRZ) and the Indonesian Institute of Science (LIPI). References Baker, A. de C., M. R. Clarke, and M. J. Harris. 1973. The NIO combination net RMT (1 + 8) and further developments of rectangular midwater trawls. J. mar. bioi. Ass. U.K. 53:167-184. Be, A. H. W., J. M. Forns, and 0. A. Roels. 1971. Plankton abundance in the North Atlantic Ocean. In Fertility of the sea edited by J.D. Costlow, Jr., 17-50. New York: Gordon and Beach Scient. Pub!. Birowo, S., and A. G. Ilahude. 1977. On the upwelling of the eastern Indonesian waters. In Papers presented at the 13th Pacific Science Congress, 69-89. LIPI: Jakarta. Brinton, E. 1979. Euphausiids of the southeast Asian waters. Naga Report 4 (5). Gieskes, W. W. C., G. W. Kraay, A. Nontji, D. Setiapermana, and Sutoma. In press. Regional and monsoonal differences in taxonomic composition and biomass of phytoplankton in the Band-Arafura Sea region (Indonesia) revealed by the multiple regression and cluster analysis of algal pigment fingerprints. Heyman, R. P. 1981. Narcotisation, fixation and preservation experiments with marine zooplankton. Verst. techn. Geg. 28. ITZ: Universiteit van Amsterdam. Knaus, J. A. 1978. Introduction to physical oceanography. Englewood Cliffs, N.J.: Prentice Hall Inc. Longhurst, A. R. 1976a. Vertical migration. In The ecology of the seas, edited by D. H. Cushing and J. J. Walsh, 116-137. Blackwell Scient. Pub!. Longhurst, A. R. 1976b. Interaction between zooplankton and phytoplankton in the eastern tropical Pacific Ocean. Deep-Sea Res. 23:729-754. Nontji, A. 1975. Distribution of chlorophyll a in the Banda Sea by the end of the upwelling season. Mar. Res. Indonesia 14:49-59. Omori, M., and W. M. Hamner. 1982. Patchy distribution of zooplankton: Behaviour, Population assessment and sampling problems. Mar. Bioi. 72:193-200. Ortner, P. B., P. H. Wiebe, and J. L. Cox. 1980. Relationships between oceanic epizooplankton distributions and the seasonal deep chlorophyll maximum in the Northwestern Atlantic Ocean. J. Mar. Res. 38:507-531. Rochford, D. F. 1962. Hydrology of the Indian Ocean II. The surface waters of the southeast Indian Ocean and Arafura Sea in spring and summer. Aust. J. mar. Freshw. Res. 12(2):226-251. Roe, H. S. J., A. de C. Baker, R. M. Carson, R. Wild, and M. Shale. 1980. Behaviour of the Institute of Oceanographic Sciences' Rectangular Midwater Trawls: Theoretical aspects and experimental observations. Mar. Bioi. 56:247-259. Rudyakov, Y. A. 1971. Details of the horizontal distribution and diurnal vertical migration of Cypridana (Pyrocypris) sinuosa (G. W. Muller) (Crustacea, Ostracoda) in the western equatorial Pacific. In Life activity of pelagic communities in the ocean tropics, edited by M. E. Vinogradov. Tranter, D. J., and J. D. Kerr. 1969. Seasonal variation in the Indian Ocean along l10 E V: Zooplankton biomass. Aust. J. mar. Freshw. Res. 20:77-84. Vander Spoel, S., and R. P. Heyman. 1983. A comparative atlas of zooplankton: Biological patterns in the oceans. Utrecht, Nederland: Wetenschappelijke uitgeverij Bunge. Vinogradov, M. E. 1968. Vertical distribution of the oceanic zooplankton. Moscow: Nauka.

12 P. H. Schalk Vinogradov, M. E., I. I. Gitelzon, andy. L Sorokin. 1970. The vertical structure of the pelagic community in the tropical ocean. Mar. Bioi. 6:187-194. Wyrtki, K. 1957. The water exchange between the Pacific and the Indian Oceans in relation to upwelling processes. Proc. 9th Pac. Sci. Congr. 16:61-66. Wyrtki, K. 1961. Physical -oceanography of the southeast Asian waters. NAGA rept. Scripps Inst. Oceanogr. 2:1-163.