Mesozooplankton community structure in the Southern Ocean upstream of the Prince Edward Islands

A meso-scale oceanographic grid survey was conducted during the first cruise of the Marion Offshore Ecosystem Variability Study in the upstream region of the Prince Edward Islands in austral autumn (April/May) 2001. Mesozooplankton samples, collected using a Bongo net (fitted with 200-µm and 300-µm mesh nets), were separated into three size fractions, 200–500 µm, 500–1,000 µm, 1,000–2,000 µm, by reverse filtration. Total surface (depth<5 m) chlorophyll-a concentration during the study ranged between 0.11 and 0.34 µg l–1 and was always dominated by picophytoplankton (0.45–2.0 µm). Total mesozooplankton abundance and biomass during the survey ranged between 49 and 1,512 ind. m–3 and between 0.7 and 25 mg Dwt. m–3, respectively. Throughout the survey, the 200 to 500 µm class numerically dominated the mesozooplankton community, with an average of ~69% (SD=±12.3%). The dominant species in the 200- to 500-µm size fraction were the copepods, Oithona similis, Calanus simillimus and Metridia lucens, and the pteropod, Limacinaretroversa. However, in terms of biomass, the 1,000- to 2,000-µm group was predominant, with dry weight values constituting an average of ~66% (SD=±10.2%). The most well-represented species in this group were the carnivorous Euphausia vallentini, Thysanoessa vicina, Sagitta gazellae and Eukrohnia hamata. Three distinct groupings of stations were identified by numerical analysis. The different station groupings identified reflect changes in the relative contributions of the dominant species, as opposed to the presence/absence of species.