Trophodynamics of selected mesozooplankton in the west-Indian sector of the Polar Frontal Zone, Southern Ocean

Mesozooplankton community structure and grazing impact were investigated at 15 stations in the west-Indian sector of the Polar Frontal Zone during the third dynamics of Eddie impacts on Marion's ecosystem cruise, conducted during April 2004. An intense frontal feature, likely the convergence of the Sub-Antarctic and Antarctic Polar Fronts, was identified running in a north-eastward direction across the survey area. Total integrated chlorophyll-a (chl-a) biomass ranged from 4.15 mg m(-2) to 22.81 mg m(-2) and was dominated by picophytoplankton at all stations. Mesozooplankton abundances ranged from 163.84 ind m(-2) to 2,478.08 ind m(-2) and biomass between 6.70 mg Dwt. m(-2) and 23.40 mg Dwt. m(-2). The mesozooplankton community was dominated almost entirely by copepods, which contributed between 35% and 79% (mean=63%; SD=+/- 12%) of the total numbers. The pteropoda, Limacina retroversa, contributed up to 30% (mean=10%; SD=+/- 8%) of the total numbers. Numerical analysis identified two distinct mesozooplankton communities separated by the intense frontal feature, namely the Antarctic and the Sub-Antarctic Zone Groups. Ingestion rates of the four numerically dominant copepod species (Calanus simillimus, Clausocalanus spp., Ctenocalanus spp. and Oithona similis) and the pteropod, L. retroversa, were estimated using the gut fluorescence technique. Total grazing impact ranged from 0.156 mg (pigm) m(-2) to 2.958 mg (pigm) m(-2) or between 1% and 29% of the available chl-a per day. The four copepods contributed approximately 36% of the total daily grazing impact, while the pteropod contributed to a mean of 64%, indicating that this zooplankton group may play an important role in the Southern Ocean carbon cycle. In general, the highest daily grazing impact was exhibited in the Antarctic Zone Group (mean=12% phytoplankton standing stock per day).