Mesozooplankton community structure and variability in the Scotia Sea: A seasonal comparison

Mesozooplankton distribution and community structure was investigated during 3 cruises to the Scotia Sea in austral spring, summer and autumn. Three mesh sizes of Bongo nets were used during each cruise with a 53 μm mesh net yielding on average 1.76 times higher densities (median 923,000 ind. m−2, 0–400 m) than a 100 μm net and 7.42 times more than a 200 μm net across all cruises. Small copepods dominated numerically across all nets with Oithona spp., Oncaea spp., Ctenocalanus citer and Microcalanus pygmaeus being particularly abundant, with sample densities of up to 3.5×106 ind. m−2 recorded within the top 400 m. A more even distribution of biomass among net sizes was apparent, with median net ratios (1.15–1.25) smaller and more even than for abundance. To the south of the Scotia Sea plankton maxima occurred in autumn, consistent with a later spawning in many species, whereas further north, abundance in 53 and 100 μm nets varied little across seasons, although in the 200 μm net there was a clear summer maximum. Median biomass increased through summer and by autumn was twice than found during spring in all parts of the Scotia Sea. Cluster analysis indicated two main station groups in all 3 seasons. To the south of the Southern boundary of the ACC (SB-ACC), Group 1 contained stations, that lay within the seasonal sea-ice zone and where zooplankton abundance and biomass was persistently low. In contrast at Group 2 stations, north of the Southern Antarctic Circumpolar Current Front (SACCF) abundance and biomass was consistently higher. Differences between the two groups were largely apparent at the population rather than at the taxonomic level. LHPR hauls to 1000 m indicated that the large seasonal migrant copepods Calanoides acutus and Rhincalanus gigas were at a more advanced stage of development in the north in spring and summer where they were generally present in the upper water column. In autumn, at all stations, C. acutus was dominated by later stages and was dispersed throughout the water column. Calanus simillimus was only abundant at Group 2 stations with older stages dominant in spring and autumn and younger stages in summer. The influence of environmental factors such as sea-ice, temperature and chlorophyll a biomass (Chl a) which may have influenced the development and seasonal succession of zooplankton populations, is briefly discussed.

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