Spatio-temporal variability in the GDH activity to ammonium excretion ratio in epipelagic marine zooplankton

Abstract

Glutamate dehydrogenase (GDH) activities have been widely used in oceanographic research as an index of in situ NH4 + excretion rates (RNH4 +) in zooplankton. Here we study the variability in the relationship between the enzymatic rates and the actual rates measured in epipelagic marine zooplankton between several marine ecosystems. Although both measures were significantly correlated across zooplankton assemblages, the regression models yielded different GDH/RNH4 + ratios across ecosystems. Accordingly, the error of a general equation increased up to ± 42.5 % when regressing all our data together.