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

Ratios of GDH activity: NH4+ excretion and ETS activity: oxygen consumption were measured in western Gulf of Mexico zooplankton and averaged 18.7 ± 4.3 and 2.65 ± 0.55, respectively. These ratios were used to estimate NH4+ excretion and respiration rates of two natural zooplankton assemblages sampled quantitatively for GDH and ETS activity with a Multiple Opening and Closing Net and Environmental Sensing System (MOCNESS). Greater than 80% of the total GDH and ETS activity 0–200 m was concentrated in depth strata above the chlorophyll maximum, suggesting a strong zooplankton-phytoplankton grazing interaction. GDH activity per unit of zooplankton protein biomass was ∼3-fold greater in the upper 100 m than between 100 – 200 m, while ETS activity per unit of zooplankton protein biomass showed no consistent pattern with increasing depth. O:N metabolic quotients were estimated for the zooplankton sampled with the MOCNESS by ratioing GDH-excretion and ETS-respiratory by atoms. Lowest O:N quotients occurred in depth strata above the chlorophyll maximum, suggesting a predominance of protein-based grazing and/or predation.