Plankton community structure and variability in the Scotia Sea: austral summer 2003

Plankton community structure in the Scotia Sea was investigated during January/early February 2003 based on phytoplankton cell counts from 20 m depth and mesozooplankton counts from 0 to 400 m net hauls. Cluster analysis and multi-dimensional scaling revealed 4 major groups of stations within each ordination that broadly corresponded geographically. A grouping of stations to the east of the Antarctic Peninsula was characterised by low phytoplankton cell counts. The corresponding grouping of stations in the mesozooplankton data were characterised by low abundance, overwintered state of many species, low egg production rates, and low carbon mass of copepod instars. In contrast, groupings of stations in the northern part of the Scotia Sea were characterised as chlorophyll and mesozooplankton rich, and the summer generation was well advanced. Latitude was most strongly correlated with mesozooplankton community pattern (rank correlation p = 0.608), whereas surface chlorophyll a was a weaker correlate (p = 0.344) but along with measures of size-fractioned chlorophyll contributed towards explaining variation in species stages carbon mass and egg production rates. Additional hauls to 1000 m with an LHPR indicated copepod populations were broadly in an overwintered state in the south of the region, whereas to the north of South Georgia recruitment had been completed and some species were undergoing a seasonal descent. A comparison with January/February 2000 revealed higher abundances of krill larvae throughout the Scotia Sea in 2000 as well as a more advanced generation of the copepod Calanoides acutus. Ice cover during the 2 years differed considerably; in 2000 the position of the summer ice edge broadly accorded with the 25 yr average, whereas in 2003 the ice edge lay much further north than usual. We suggest that the timing of ice retreat influenced the timing of reproduction with the late retreat in 2003 causing delayed reproduction and reduced population sizes.