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Influence of oceanic fronts on mesozooplankton abundance and grazing during spring in the south-western Atlantic

We investigated the influence of oceanic fronts on mesozooplankton distribution and grazing activity in the south-west Atlantic. Sampling was conducted during late spring 1993 along a north–south transect between subtropical waters off Brazil and Antarctic waters. Mesozooplankton abundance and biomass were ~10-fold higher in the Subtropical Confluence Zone (SCZ) and in the Antarctic Polar Front (APF) relative to subtropical and subantarctic waters beyond the influence of those frontal systems. Mesozooplankton was dominated by calanoid and cyclopoid copepods. Community ingestion rates ranged between 0.1 and 0.3mg chlorophyll-a m–2day–1 in interfrontal areas, increasing to 2.0 and 9.0mg chlorophyll-a m–2day–1 in the APF and SCZ respectively. Mesozooplankton grazing removed up to 40% of the total chlorophyll stock in the SCZ, and 22% in the APF, on a daily basis. These estimates suggest that mesozooplankton exert a significant grazing impact on phytoplankton over much of the frontal areas studied. Recent investigations have shown that the geographic position of oceanic fronts and plankton biomass maxima in the study area have remained basically the same from the time of our sampling effort to date, meaning that our results apply to present conditions.