EFFECTS OF SINKING AND ZOOPLANKTON GRAZING ON THE RELEASE OF ELEMENTSFROM PLANKTONIC DEBRIS

Laboratory radiotracer experiments compared the effects of zooplankton grazing and microbial decomposition on the release of organic C and A g, Cd, Co, Pb and Po from phytoplankton cells. After 40 h incubation o f 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 %); microorga nisms alone removed half this amount of diatom C. Copepod grazing enha nced the conversion of Ag from diatom cells to other forms (particulat e 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 ce lls into the dissolved phase. Decomposing copepod fecal pellets, held free-falling on a spinning wheel, lost about 20 % more C, Ag, Co and P b than did undisturbed fecal pellets, while no appreciable difference was observed for release of metals from copepod carcasses treated simi larly. The results suggest that retention of an element contained prim arily in the cytoplasm of phytoplankton cells (such as C or Cd) is lar gely governed by microbial activity and leaching. Microbial activity h as 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 rapid ly sinking fecal pellets.