EFFECTS OF SINKING AND ZOOPLANKTON GRAZING ON THE RELEASE OF ELEMENTS FROM PLANKTONIC DEBRIS

Laboratory radiotracer experiments compared the effects of zooplankton grazing and microbial decomposition on the release of organic C and Ag, Cd, Co, Pb and Po from phytoplankton cells. After 40 h incubation of diatoms with copepods + microorganisms, 70 % of the diatom cellular C was remineralized to CO2 (40 %), assimilated in copepod tissue (20 %), excreted in fecal pellets (5 %) or released as DOC (5 %); microorganisms alone removed half this amount of diatom C. Copepod grazing enhanced the conversion of Ag from diatom cells to other forms (particulate and dissolved) by 27 %, Po by 25 %, Pb by 20 %, Cd by 13 % and Co by 10 % over those cells incubated with only microorganisms; zooplankton grazing increased by 5 to 15 % the release of elements from diatom cells into the dissolved phase. Decomposing copepod fecal pellets, held free-falling on a spinning wheel, lost about 20 % more C, Ag, Co and Pb than did undisturbed fecal pellets, while no appreciable difference was observed for release of metals from copepod carcasses treated similarly. The results suggest that retention of an element contained primarily in the cytoplasm of phytoplankton cells (such as C or Cd) is largely governed by microbial activity and leaching. Microbial activity has a smaller effect on particle-reactive elements (Ag and Pb) bound to structural components of cells; zooplankton are effective in removing these unassimilable elements by grazing and packaging them into rapidly sinking fecal pellets.