Secondary biological pump driven by food web interactions : A case study in Lake Biwa for gloval environmental issues

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To understando responses of aquatic ecosystems to global environmental change, we studied biological and decomposition processes in relation to environmental variables such as nutrients, light and water temperature, and relations between sedimentation flux of biological products and food chains.1) Primary production of phytoplankton was limited by phosphorus in the layr above 5 m deep but by light intensity below 5 m deep. 2) Decomposition rate by bacteria was limited by phosphorus in the epilimnion and by water temperature in the hypolimnion.3) Sedimentation flux corresponded with 12% of daily production of phytoplankton and most of the phytoplankton production were consumed by zooplankton and bacteria in the epilimnion. 4) 20-30% of the sedimentation flux were assimilated by Amphipoda gammarid in the aphotic zone. 5) The sedimentation flux changed seasonally depending on precipitation and propagation of microplanktonic algae such as Fragilaria crotonensis.Biological production-decomposition rates and the sedimentation flux in Lake Biwa were strongly influensed by physical variable like light and water temperature, and food web structure. Further, they related closely to nutrients inflowing from rivers. These results suggest that climate change controlling precipitation, light, water temperature and nutrients influences directly acquatic ecosystems through processes of biological production-decomposition and vertical transportation of particulate organic matter.