Glutamate dehydrogenase (GDH) and respiratory electron-transport-system (ETS) activities in Gulf of Mexico zooplankton

Ratios of GDH activity: NH₄⁺ excretion and ETS activity: oxygenconsumption were measured in western Gulf of Mexico zooplanktonand averaged 18.7 ? 4.3 and 2.65 ? 0.55, respectively. Theseratios were used to estimate NH₄₊ excretion and respirationrates of two natural zooplankton assemblages sampled quantitativelyfor GDH and ETS activity with a Multiple Opening and ClosingNet and Environmental Sensing System (MOCNESS). Greater than80% of the total GDH and ETS activity 0–200 m was concentratedin depth strata above the chlorophyll maximum, suggesting astrong zooplankton-phytoplankton grazing interaction. GDH activityper unit of zooplankton protein biomass was 3-fold greater inthe upper 100 m than between 100 – 200 m, while ETS activityper unit of zooplankton protein biomass showed no consistentpattern with increasing depth. O:N metabolic quotients wereestimated for the zooplankton sampled with the MOCNESS by ratioingGDH-excretion and ETS-respiratory by atoms. Lowest O:N quotientsoccurred in depth strata above the chlorophyll maximum, suggestinga predominance of protein-based grazing and/or predation. ¹Contribution No. 81-013 from the Bigelow Laboratory for OceanSciences