Mesozooplankton metabolism and its effect on chlorophyll and primary production in slope waters of the Canary Islands

Abstract

Chlorophyll, primary production and mesozooplankton biomass (> 200 μm), electron transfer system (ETS), glutamate dehydrogenase (GDH) and aspartate transcarbamylase (ATC) activities were measured at selected profiles, in slope waters south of Gran Canaria Island (Canary Islands) at different times of the year. A deep chlorophyll maximum (DCM) was characteristically found above the compensation depth, while the primary production maximum (PPM) and the assimilation number maximum (ANM) were generally above the DCM. The mesozooplankton biomass maximum (ZBM) coincided with the peaks of primary production and assimilation number. The vertical distribution of mesozooplankton specific ETS, GDH and ATC activities showed an almost uniform vertical pattern, suggesting that the related processes (respiration, ammonia excretion and growth) were rather homogeneous with depth. Although Zooplankton biomass maxima showed an inverse relationship with chlorophyll during the fertile period (late winter), suggesting a herbivorous control of the primary production, our estimates of potential grazing based on the indices of metabolism (assuming herbivorous feeding) accounted for 10 to 46% of the primary production. The calculated ammonium excretion by mesozooplankton would support 8-19% of primary production. These two latter estimates suggest that microzooplankton (< 200 μm) must play a more important role than mesozooplankton in controlling primary production in Canarian waters.