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 mas s and water temperature in measurements made from April to October 198 9 and 1990. Rates ranged between 3.5 and 5.0 mug atoms NH4+-N (g dry w t)-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 t he size distributions and biomasses of gelatinous zooplankters and wat er temperature were used to estimate the contribution of ammonium by m edusae and ctenophores to mesohaline Chesapeake Bay waters on several dates during April to October 1989 and 1990. We then compared the esti mated contributions to direct measurements of (NH4+)-N-15 uptake by mi croplankton. The maximum estimated regeneration by gelatinous zooplank ton was 5.8 mug atoms NH4+ N m-3h-1 at night in August 1990, when medu sae biomass was greatest. This represents about 4 % of the ammonium re quired by the microplankton. During the daytime on all dates, less tha n 1 % of the ammonium required by microplanktion was supplied by gelat inous zooplankton. Therefore, gelatinous zooplankton appear to play a minor role in the ammonium cycle of Chesapeake Bay.