EXCRETION OF TRACE-ELEMENTS BY MARINE COPEPODS AND THEIR BIOAVAILABILITY TO DIATOMS

We measured the physiological turnover rate (= excretion rate) of five trace elements (Ag, Cd, CO, Se and Zn) in the marine copepod Temora l ongicornis following feeding on radiolabeled diatoms. The turnover rat e constants of trace elements in copepods were high (0.05 to 0.38 d(-1 )) and were comparable to published values for N and P excretion. Ag, Cd and Co were excreted at higher rates than Se and Zn. Turnover rate constants of Ag, Cd, and Co also increased with increasing food concen tration, whereas excretion of Se and Zn was not affected by food conce ntration. There was little evidence that copepod grazing regenerated d iatom Ag, Co and Zn into the dissolved phase during the pre-ingestive phase. Copepod grazing slightly enhanced the release into the dissolve d phase of Cd and Se from diatoms, where they were primarily localized in cytoplasm. Excreted Ag, Se and Zn were less bioavailable to diatom s than when in inorganic form, but the bioavailability of excreted Cd and Co was comparable to inorganic forms. Our study demonstrated that copepod excretion represents a significant route by which particulate metals are regenerated to the dissolved phase. Metal excretion by mari ne zooplankton is analogous to N and P excretion and may thus signific antly affect metal cycling and modify metal speciation in surface wate rs. Regenerated metals may re-enter planktonic food chains and be recy cled several times in surface waters before sinking in particulate mat ter.