Zooplankton biomass and metabolism: Response in the Namibian upwelling.

Abstract

Zooplankton biomass and potential metabolic rates in terms of electron transport system (ETS) and glutamate dehydrogenase (GDH) were analyzed in four size fractions (100-200 µm, 200-500 µm, 500-1000 µm and >1000 µm) along a cross-shelf transect in waters off northern Namibia. The highly variable dynamics of upwelling filaments promoted short-term fluctuations in the zooplankton distribution. Maximum values were characteristically found over the shelf-break, where zooplankton biomass as dry mass (DM) reached peaks of 64.5 mg m−3 within the upper 200 m in late August. Two weeks later, the zooplankton-DM decreased by more than a third (19 mg DM m−3). Zooplankton potential respiration and NH4+ excretion averaged 234 µmol O2 m−3 d−1 and 169 µmol NH4+ m−3 d−1 in the Namibian shelf, respectively. High protein-specific ETS activities even in the low-chlorophyll waters outside the filament suggested a shift into greater omnivory towards the open ocean. This would also explain the decrease of zooplankton biomass and the higher protein-catabolism seaward. In this light, zooplankton elemental and isotopic compositions were used to investigate the pelagic food web interactions. They evidenced spatial changes in the carbon resource for zooplankton as well as in the form of nitrogen that fueled the biological production in aging advected waters. Overall, the zooplankton respiration consumed a small proportion of the daily primary production (0.9 - 9.3%), which argued for the autotrophy of the system during the intense wind-forcing season. Parallel with the respiration, zooplankton NH4+ excretion only contributed from 1 to 12% to the total nitrogen requirements for primary production. This may reveal a zation pathway through microheterotrophs in these Namibian waters.