Invertebrate micronekton and macrozooplankton in the Marguerite Bay region of the Western Antarctic Peninsula

Invertebrate micronekton and macrozooplankton in the Marguerite Bay region of the Western Antarctic Peninsula (WAP) were sampled using a 10-m2 MOCNESS as part of the Southern Ocean Global Ecosystem Dynamics (SO GLOBEC) program. A total of 62 trawls were completed during four separate cruises in the austral fall (April-June) and winter (July-September) of 2001 and 2002. Crustaceans dominated the system in both seasons, accounting for 32 of the 55 species captured in the fall and 30 of the 48 species captured in winter. In both seasons, a very few species made up the majority of the catch. In the fall, the euphausiids Euphausia crystallorophias, Euphausia superba, and Thysanoessa macrura, and the mysid Antarctomysis ohlinii numerically dominated the assemblage, contributing over 85% of the total. In the winter, the same three euphausiids and the chaetognath Pseudosagitta gazellae were the numerical dominants, comprising over 90% of the catch. A significant increase in total abundance and biomass was observed from 2001 to 2002. The invertebrate micronekton/macrozooplankton communities found in the Marguerite Bay region of the WAP were a mixture of oceanic and neritic fauna: a direct result of local hydrographic conditions. Near the shelf break and in the outer reaches of the Marguerite Trough, a deep canyon transecting the shelf in a south-southeast direction, the communities were more diverse, dominated by oceanic species such as Euphausia triacantha, Salpa thompsoni, and Themisto gaudichaudi. The assemblages present in the nearshore fjords exhibited lower diversity and were dominated by neritic species such as E. crystallorophias and A. ohlinii. At the mid-shelf and mid-trough locations, the assemblages were composed of a variable mixture of oceanic and neritic fauna. The faunal mixing and overall species composition in those areas is the result of episodic Circumpolar Deep Water (CDW) intrusions onto the shelf via deep bathymetric features such as the Marguerite Trough. Species diversity and integrated abundance for the upper 200 m of the water column were similar between seasons in the WAP study region, but integrated biomass was nearly three times greater in fall than in winter. Integrated estimates from the WAP study region were similar to those from other studies conducted in the Scotia and Weddell Seas, but were orders of magnitude lower than estimates from a study in Croker Passage, primarily due to a large catch of E. superba. In contrast, species diversity in the WAP was higher than recorded in any of the previously mentioned studies, which is due to the mixing of typical oceanic fauna with endemic nearshore fauna.