A role of thermocline on biological activities in Lake Biwa.

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Vertical distribution of chlorophyll a differedl in the pattern between September 1994 when it was extraordinarily water shortage and September 1995 when it was a usual rain. In 1994, chlorophyll a was extremely low in the epilimnion and showed a peak in the thermocline. Phytoplankton growth was interrupted in the epilimnion because of nutrients deficiency due to shortage of river inflow. As a result, solar radiation available for photosynthesis could penetrate sufficiently into the thermocline. Nitrate-N was gradually supplied by the upward diffusion from the hypolimnion to the thermocline and phosphate-phosphorus was supplied continuously by release of phosphate from the phosphorus adsorbed with ferric iron and alminium in the turbid water transperted from share-bottom into the thermocline. In 1995, river water including rich nutrients inflowed just above the thermocline and supplied nutrients to the epilimnion. As a result, phytoplankton growth was enhanced in combination with photosynthesis in the epilimnion. Vertical distribution pattern of chlorophyll a seems to be determined by whether or not nutrients are supplied from river to the epilimnion.