INGESTION OF N-15(2)-LABELED TRICHODESMIUM SPP AND AMMONIUM REGENERATION BY THE HARPACTICOID COPEPOD MACROSETELLA-GRACILIS

The pelagic harpacticoid copepod, Macrosetella gracilis (A. Scott), is found in association with colonies of the nitrogen-fixing (diazotroph ic), bloomforming cyanobacterium Trichodesmium spp, in tropical and su btropical waters, M. gracilis is one of the few direct grazers of thes e often toxic cyanobacteria. Experiments investigating NH4+ regenerati on by M. 4 gracilis were conducted in the Caribbean in September 1992 and the Coral Sea, Australia in November 1994. Rates of M. gracilis in gestion of Trichodesmium thiebautii labelled with N-15(2) measured in the eastern Caribbean indicated that M. gracilis could consume 33 to 4 5% of total T. thiebautii colony Nd-1 and > 100% of new N fixed d(-1). We also measured the release of NH4+ by M. gracilis feeding on T. thi ebautii, 4 as well as by non-feeding copepods, using N-15 isotope dilu tion methods. In non-feeding copepods, rates of NH4+ release increased as numbers of copepods were 4 increased. In feeding copepods, the rat e of NH4+ release 4 increased as both copepod numbers and food availab ility increased. In the presence of T. thiebautii colonies, M. gracili s had an average rate of NH4+ regeneration of 7.7 +/- 1.5 nmol N copep od(-1) h(-1) (+/-SE), which was significantly higher than when food wa s absent (1.9 +/- 0.7 nmol N copepod(-1) h(-1)). Rates of M. gracilis excretion were relatively high based on excretion:ingestion ratios-whi ch could be due to having a high-N food source readily available, to ' 'sloppy-feeding'' effects, or as a response to toxins in the cyanobact erium. Incubations of M. gracilis with and without T. erythraeum resul ted in significant increases in [NH4+] as a function of copepod densit y only. Ammonium leakage from the cyanobacterium and/or microheterotro ph associates was relatively low. M. gracilis, through excretion and p ossible mechanical breakage of cells while grazing, appears to provide a direct link between atmospherically derived ''new'' nitrogen and re generated NH4+ in the oligotrophic systems where Trichodesmium spp. ar e abundant.