Ammonium Excretion by Gelatinous Zooplankton and Their Contribution to the Ammonium Requirements of Microplankton in Chesapeake Bay

Ammonium excretion rates of recently collected specimens of gelatinous zooplankton. the scyphomedusan Chrysaora quinquecirrha DeSor and the ctenophore Mnemiopsis leidyi A. Agassiz, were correlated with body mass and water temperature in measurements made from April to October 1989 and 1990. Rates ranged between 3.5 and 5.0 mug atoms NH4+-N (g dry wt)-1 h-1 for C. quinquecirrha and 3.0 to 4.9 mug atoms NH4-N (g dry wt) -1 h-1 for M. leidyi. Excretion rate equations and in situ data on the size distributions and biomasses of gelatinous zooplankters and water temperature were used to estimate the contribution of ammonium by medusae and ctenophores to mesohaline Chesapeake Bay waters on several dates during April to October 1989 and 1990. We then compared the estimated contributions to direct measurements of (NH4+)-N-15 uptake by microplankton. The maximum estimated regeneration by gelatinous zooplankton was 5.8 mug atoms NH4+ N m-3h-1 at night in August 1990, when medusae biomass was greatest. This represents about 4 % of the ammonium required by the microplankton. During the daytime on all dates, less than 1 % of the ammonium required by microplanktion was supplied by gelatinous zooplankton. Therefore, gelatinous zooplankton appear to play a minor role in the ammonium cycle of Chesapeake Bay.

Keywords: partially stratified estuary, chrysaora-quinquecirrha, food, availability, metabolic rates, phytoplankton, nitrogen, regeneration, , respiration, temperature, ctenophores