Scavenging and retention of metals by zooplankton fecal pellets and marine snow

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The scavenging and retention of nine metals in different batches of zooplankton fecal pellets and marine snow were studied in the laboratory using radiotracer techniques. Dissolved 60Co, 65Zn, 106Ru, 109Cd, 110mAg, 141Ce, 154Eu, 237Pu and 241AM were scavenged from seawater onto euphausiid fecal pellets, marine snow and copepod fecal pellets recovered from sediment traps and onto copepod fecal pellets freshly produced in the laboratory. K d values for the different radioisotopes ranged from 8.4 × 10 2 to 1.9 × 10 5. The radiolabeled fecal pellets and marine snow, resuspended into unlabelled seawater at 2 and 15°C, displayed metal depurations curves generally conforming to a two-compartment model, with overall retention half-times ( t r 1 2 s ) varying with each metal and type of debris. 109Cd was retained least effectively and 106Ru was generally retained for the longest periods, with t r 1 2 s up to 66 days. The t r 1 2 s of 141Ce, 154Eu, 237Pu and 241Am in each type of debris were comparable and were not appreciably affected by temperature. 60Co, 65Zn, 109Cd and 110mAg were released into the dissolved phase more rapidly at 15 than at 2°C. Metal binding on copepod fecal pellets and marine snow was generally greater than on euphausiid fecal pellets. These results provide direct evidence that fecal pellets and marine snow can effectively scavenge metals from seawater; of the metals examined, zinc and cadmium are likely to be remineralized most rapidly in surface waters, while the others are likely to be vertically transported hundreds to thousands of meters.