Euphausiid population structure and grazing in the Antarctic Polar Frontal Zone — austral autumn 2004

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The euphausiid community structure and grazing dynamics were investigated in the West Indian sector of the Polar Frontal Zone during the austral autumn 2004. Subsurface (200m) temperature profiles indicated that an intense frontal feature, formed by the convergence of the Subantarctic Front and the Antarctic Polar Front bisected the survey area into two distinct zones, the Subantarctic Zone (SAZ) and the Antarctic Zone (AAZ). Total integrated chlorophyll a (Chl a) biomass was typical for the region (<25mg Chl a m−2), and was dominated by picophytoplankton. Total euphausiid abundance and biomass ranged from 0.1 m−3 to 3.1 m−3 and from 0.1mg dry weight m−3 to 8.1mg dry weight m−3 respectively, and did not differ significantly between the stations occupied in the SAZ and AAZ (p > 0.05). A multivariate analysis identified two interacting mechanisms controlling the distribution patterns, abundance and biomass of the various euphausiid species, namely (1) diel changes in abundance and biomass, and (2) restricted distribution patterns associated with the different water masses. Ingestion rates were determined for five euphausiid species. Euphausia triacantha had the highest daily ingestion rate, ranging from 1 226.1ng pigment (pigm) ind−1 day−1 to 6 029.1ng pigm ind−1 day−1, whereas the lowest daily ingestion rates were observed in the juvenile Thysanoessa species (6.4–943.0ng pigm ind−1 day−1). The total grazing impact of selected euphausiids ranged from <0.1μg pigm m−2 day−1 to 20.1μg pigm m−2 day−1, corresponding to <0.15% of the areal Chl a biomass. The daily ration estimates of autotrophic carbon for the euphausiids suggest that phytoplankton represent a minor component in their diets, with only the sub-adult E. vallentini consuming sufficient phytoplankton to meet their daily carbon requirements.