NITROGEN-EXCRETION BY THE CALANOID COPEPOD ACARTIA-TONSA - RESULTS OFMESOCOSM EXPERIMENTS

Rates of excretion of NH4+, urea and dissolved primary amines (DPA) by Acartia tonsa were measured over two 24-h periods in estuarine mesoco sms. Animals (similar to 20 non-gravid females) were collected from th e mesocosms at intervals ranging from 3 to 6 h, isolated by micropipet te, and incubated in vials. Concentration changes of each substrate we re determined over incubation periods lasting 2-4 h. Rates of NH4+ exc retion ranged from non-detectable to 2 ng atom N copepod(-1) h(-1), an d tended to be higher during the day than at night. Rates of urea and DPA release also ranged between non-detectable and similar to 2 ng ato m N copepod(-1) h(-1), but these rates tended to be higher during earl y morning and early evening. Our results suggest that the relative pro portions of organic and inorganic nitrogen excreted by A. tonsa can be highly variable, both with time of day and with other treatment facto rs that may be related to the physiological and nutritional state of t he copepods. These findings are consistent with other field studies su ggesting that direct nitrogen excretion by copepods is a small flux re lative to other pathways of microbial nitrogen regeneration.