The ecology of larger microzooplankton in the Weddell-Scotia Confluence Area: Horizontal and vertical distribution patterns

The distribution of microzooplankton \u3e 15 μm (large dinoflagellates, foraminifers, radiolarians, tintinnids, microcrustaceans and various invertebrate larvae) was studied in samples retrieved from 10 to 400 m in two overlapping transects along 49W, between 57S and 61°30′S (27 Nov.–12 Dec. 1988, and 27 Dec. 1988–4 Jan. 1989). Dinoflagellates and tintinnids concentrated at 50–90 m (10–400 m weighted averages, dinoflagellates: 103 ind./I, 131 mg C/m2; tintinnids: 9.7 ind./I, 53 mg C/m2). Copepod nauplii had a more variable vertical pattern with maximum numbers at 100–200 m (10–400 m av.: 2.6 ind./I, 27 mg C/m2). Foraminifers and radiolarians were most abundant in noticeably deeper waters peaking below 150 m (10–400 m av., foraminifers: 0.2 ind./I, 11 mg C/m2; radiolarians: 2.7 ind./I, 12 mg C/m2). Large dinoflagellates accounted, on the average, for 55% of the biomass of the heterotrophs considered in the 10–400 m layer, followed by the tintinnids (23%), copepod nauplii (11%), foraminifers (5%), and radiolarians (5%). The 100–400 m layer hosted up to 87% (mean: 49%) of total 10–400 m integrated microzooplanktonic biomass. The distribution of loricate ciliates was strongly correlated with those of chlorophyll a, and especially dinoflagellates (r = 0.832, for log-transformed data), suggesting close trophic relationships between these two groups. The northern sites were generally richer in microzooplankton than the area closer to the ice-edge, and the southernmost ice-covered zone yielded the lowest microplanktonic values. This biological pattern, which was but loosely coupled with the Weddell-Scotia Confluence, with the vertical stability of the water column, and with near-surface concentrations of chlorophyll a, can at least partly be explained by differential grazing pressure by crustacean mesozooplankton. The time elapsed between the two transects did not affect the microzooplanktonic assemblages noticeably. Comparisons with previous abundance estimates carried out earlier and later in the growth season suggest that microzooplanktonic abundances increase toward the late summer-fall, probably in response to enhanced availability of nano- and pico-sized producers, characteristic of Antarctic post-bloom conditions