Abstract

We measured the physiological turnover rate (= excretion rate) of five trace elements (Ag, Cd, Co, Se and Zn) in the marine copepod Temora longicornis following feeding on radiolabeled diatoms. The turnover rate 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 concentration, whereas excretion of Se and Zn was not affected by food concentration. There was little evidence that copepod grazing regenerated diatom Ag, Co and Zn into the dissolved phase during the pre-ingestive phase. Copepod grazing slightly enhanced the release into the dissolved phase of Cd and Se from diatoms, where they were primarily localized in cytoplasm. Excreted Ag, Se and Zn were less bioavailable to diatoms 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 marine zooplankton is analogous to N and P excretion and may thus significantly affect metal cycling and modify metal speciation in surface waters. Regenerated metals may re-enter planktonic food chains and be recycled several times in surface waters before sinking in particulate matter.

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