THE ECOLOGY OF LARGER MICROZOOPLANKTON IN THE WEDDELL-SCOTIA CONFLUENCE AREA - HORIZONTAL AND VERTICAL-DISTRIBUTION PATTERNS

The distribution of microzooplankton > 15 mum (large dinoflagellates, foraminifers, radiolarians, tintinnids, microcrustaceans and various i nvertebrate larvae) was studied in samples retrieved from 10 to 400 m in two overlapping transects along 49W, between 57S and 61-degrees-30' S (27 Nov.-12 Dec. 1988, and 27 Dec. 1988-4 Jan. 1989). Dinoflagellate s and tintinnids concentrated at 50-90 m (10-400 m weighted averages, dinoflagellates: 103 ind./l, 131 mg C/m2; tintinnids: 9.7 ind./l, 53 M g C/m2). Copepod nauplii had a more variable vertical pattern with max imum numbers at 100-200 m (10-400 m av.: 2.6 ind./l, 27 Mg C/m2). Fora minifers and radiolarians were most abundant in noticeably deeper wate rs peaking below 150 m (10-400 m av., foraminifers: 0.2 ind./l, 11 mg C/m2; radiolarians: 2.7 ind./l, 12 Mg C/m2). Large dinoflagellates acc ounted, on the average, for 55% of the biomass of the heterotrophs con sidered in the 10-400 m layer, followed by the tintinnids (23%), copep od nauplii (11 %), foraminifers (5%), and radiolarians (5%). The 100-4 00 m layer hosted up to 87% (mean: 49%) of total 10-400 m integrated m icrozooplanktonic biomass. The distribution of loricate ciliates was s trongly correlated with those of chlorophyll a, and especially dinofla gellates (r = 0.832, for log-transformed data), suggesting close troph ic relationships between these two groups. The northern sites were gen erally richer in microzooplankton than the area closer to the ice-edge , and the southernmost ice-covered zone yielded the lowest microplankt onic values. This biological pattern, which was but loosely coupled wi th the Weddell-Scotia Confluence, with the vertical stability of the w ater column, and with near-surface concentrations of chlorophyll a, ca n at least partly be explained by differential grazing pressure by cru stacean mesozooplankton. The time elapsed between the two transects di d not affect the microzooplanktonic assemblages noticeably. Comparison s with previous abundance estimates carried out earlier and later in t he growth season suggest that microzooplanktonic abundances increase t oward the late summer-fall, probably in response to enhanced availabil ity of nano- and pico-sized producers, characteristic of Antarctic pos t-bloom conditions.