Fates of diatom carbon and trace elements by the grazing of a marine copepod

Radiotracer experiments were performed to quantify the carbon assimilation and efflux in a subtropical coastal copepod Acartia spinicauda, and the importance of copepod grazing on the release of Cd, Fe, Se and carbon, from diatoms of different sizes (4 to 100 μm) at different food concentrations (0.04 to 9.0 mg C l^-1). Carbon assimilation was not significantly affected by the diatom food concentration (Thalassiosira pseudonana, T. weissflogii and T. rotula) or the amount of food ingested, but was lower for the large diatom T. rotula than for the small diatom T. pseudonana. No significant relationship between carbon assimilation and gut passage time of food materials was observed. The efflux rate-constant (physiological turnover rate) of carbon ranged between 0.134 and 0.372 d^-1 and was not significantly affected by diatom concentration (T. weissflogii). During the physiological turnover period, over 50% of the copepod's carbon metabolic loss was in the form of dissolved organic carbon, whereas only a small fraction of carbon was lost in the form of feces. A significant fraction of the copepod's carbon was also lost through respiration, the relative importance of which decreased with increasing period of depuration. The retention of carbon, Cd, Fe, and Se by the diatoms (T. pseudonana, T. weissflogii, T. rotula, and Cosinodiscus sp.) was further compared in the presence and absence of the copepods. In general, there was no increase in the release of metals and carbon from the diatoms in the presence of grazing copepods regardless of diatom concentrations, suggesting that copepod grazing does not trigger significant releases of diatom metals and carbon directly into the ambient environment. Our results suggested that the excretion by zooplankton as well as leakage from fecal pellets presumably account for the majority of the zooplankton-mediated production and cycling of dissolved organic matter in the ocean.