Effects of food quality and quantity on nitrogen excretion by the copepod, Acartia tonsa

Nitrogen excretion by marine crustacean zooplankton can provide inorganic and organic nitrogen to both phytoplankton and bacterioplankton. In this study, excretion of ammonium, urea and dissolved primary amines (DPA) by the calanoid copepod, Acartia tonsa, was measured in laboratory and field experiments as a function of the amount and type of food ingested.; In laboratory experiments, excretion by A. tonsa was measured when the copepods were fed high and low concentrations of foods which differed in their N/C ratios. Food concentration and N/C ratio both influenced nitrogen excretion, especially that of ammonium. In general, nitrogen excretion increased at high food concentrations only when the food N/C ratio resembled that of the copepods. When food N/C ratio was lower than that of the copepods, excretion was higher at low food concentrations. At low concentrations of all food types copepods ingested less than they excreted and apparently catabolized body tissue for survival.; When fed natural assemblages of phytoplankton and microzooplankton, A. tonsa ingested 4-fold more microzooplankton than phytoplankton on 2 of 3 dates and ingested only very small amounts of phytoplankton on the other date, making it difficult to discern any effect of food quality on excretion. Nitrogen-specific excretion was related to total nitrogen ingestion below 60% body N ingested/copepod/day. Total ingestion did not always balance excretion. Such uncoupling between ingestion and excretion could be due to inadequate descriptions of time-averaged ingestion or insufficient acclimation periods to food conditions. However, in 2 experiments, ingestion of microzooplankton alone balanced excretion, indicating the importance of microzooplankton to the nitrogen nutrition of A. tonsa.