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

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 × 10² to 1.9 × 10⁵. 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.