EFFECTS OF ZOOPLANKTON ON SEDIMENTATION IN PELAGIC ECOSYSTEMS - THEORY AND TEST IN 2 LAKES OF THE CANADIAN SHIELD

A simple heuristic theory based on conservation of matter and describi ng the fate of autotrophic production in pelagic ecosystems was develo ped to assist in conceptualizing how zooplankton affect sedimentation processes in lakes and oceans. The theory predicted that effects of zo oplankton on the fraction of autotrophic incorporation that sediments (the ''export ratio'') will be a function of factors related both to z ooplankton digestion and egestion and to tendencies of particulate mat ter to sediment directly prior to mineralization. As a result, effects of zooplankton grazing on the export ratio were predicted to be site- dependent, a function of physical conditions and zooplankton communiti es characterizing an ecosystem. The theory was tested by monitoring au totrophic production of C, N, and P, sedimentation of C, N, and P, and zooplankton biomass in two lakes of the Canadian shield characterized by contrasting morphometry and food-web structure. In Lake 110, a sma ll elongate lake protected from wind exposure, export ratios of C, N, and P declined strongly with zooplankton biomass. In contrast, in L240 , a larger lake with considerably greater wind exposure, export ratios increased with zooplankton biomass. These results were consistent wit h predictions of our theory that effects of zooplankton on sedimentati on processes will depend on the tendency of particulate matter to dire ctly sediment relative to the tendency of egested materials to sedimen t. However, no significant differences in relationships between export ratios for C, N, and P and zooplankton biomass were found.