Size-fractionated mesozooplankton biomass, metabolism and grazing along a 50 N-30 S transect of the Atlantic Ocean

Abstract

Size-fractionated mesozooplankton grazing and metabolism were investigated along the wide latitudinal range (50°N-30°S) covered doring the Atlantic Meridional Transect (AMT) 11 cruise. Five different oceanic provinces were traversed in this cruise: North Atlantic Drift (NADR), North Atlantic Subtropical Gyral (NAST), Canary Coastal (CARY), Eastern Tropical Atlantic (ETRA), and South Atlantic Gyral (SATL). CNRY and ETRA were affected by the upwelling Mauritaian and equatorial respectively and primary production in these provinces was higher than in the oligotrophic subtropical gyres (NAST and SATL). Both mesozooplankton and phytoplankton biomass were highest around the equator. The amount of chlorophyll a ingested daily by copepods was noticeably higher in mesotrophic than in oligotrophic provinces as shown by the spatial distribution of gut content values and the high abundances of copepods recorded at the equator. Grazing impact along the transect ranged from 0.2 to 5.6% of the phytoplankton standing stock and from 1.6 to 14.5% of primary production. If only phytoplankton >2 μm are considered, the ranges are 1.0-19.4% (stock) and 3.4-44.7% (primary production). Grazing impact upon both phytoplankton biomass and primary production followed a spatial distribution similar to that of chlorophyll a ingestion, with higher values in upwelling zones than in the gyres. Weight-specific rates of respiration and NH 4 + and PO 4 3- excretion showed large variability both along the transfect and within provinces, but did not differ between provinces. Therefore, zooplankton assemblages inhabiting the different provinces visited in the AMT 11 seem to be adapted to the prevailing thermal conditions. Given the substantial proportion of nitrogen and phosphorus that are supplied to primary producers through the excretory activity of mesozooplankton (the percentage of nitrogen and phosphorus requirements of phytoplankton accounted for by mesozooplankton excretion was >30% in all the provinces) it follows that they may play a crucial role as nutrient regenerators, especially in the oligotrophic gyres where regenerated production dominates.