Nitrogen excretion by the calanoid copepod Acartia tonsa: results of mesocosm experiments

Rates of excretion of NH4+, urea and dissolved primary amines (DPA) by Acartia tonsa were measured over two 24-h periods in estuarine mesocosms. Animals (similar to 20 non-gravid females) were collected from the mesocosms at intervals ranging from 3 to 6 h, isolated by micropipette, and incubated in vials. Concentration changes of each substrate were determined over incubation periods lasting 2-4 h. Rates of NH4+ excretion ranged from non-detectable to 2 ng atom N copepod(-1) h(-1), and 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 atom N copepod(-1) h(-1), but these rates tended to be higher during early morning and early evening. Our results suggest that the relative proportions of organic and inorganic nitrogen excreted by A. tonsa can be highly variable, both with time of day and with other treatment factors that may be related to the physiological and nutritional state of the copepods. These findings are consistent with other field studies suggesting that direct nitrogen excretion by copepods is a small flux relative to other pathways of microbial nitrogen regeneration.