Ammonium excretion and glutamate dehydrogenase activity of zooplankton in great South Bay, New York

In Great South Bay, nanoplankton, (<20 sμm) accounted for the largest fraction (56%) of zooplankton glutamate dehydrogenase (GDH) activity over a one year period. Microzooplankton (20-200 μm) and macrozooplankton (>200 μm) accounted for 20% and 24%, respectively. Total zooplankton ammonium regeneration in Great South Bay could account for 74% of the ammonium requirement by phytoplankton in winter, but in summer when phytoplankton demand was greater, and zooplankton population was low, it supplied less than 5%. This study suggests that the smallest zooplankton fraction, less than 20 μm, can be the most important as regards nitrogen regeneration in estuarine environments. Macrozooplankton GDH activity in Great South Bay ranged from 0.18 mg atoms NH⁺₄-N m^-3 d^-1 in winter to 3.34 mg atoms NH⁺₄-N m^-3 d^-1 in spring. Over an annual period, the averaged GDH/excretion ratio was 20.4 ± 3.5 (n = 10), and this ratio agrees well with observations by other investigators. Observed macrozooplankton excretion rates showed a strong correlation with the excretion rates indirectly estimated from GDH activities. The GDH/excretion ratio seems to vary depending on the internal physiological states of zooplankton as well as food availability.