Abstract
Lake Biwa had eutrophicated till the 70s, but total phosphorus and chlorophyll a concentration began to improve around 1980 due to various efforts by the government and residents. Along with the decrease in chlorophyll a concentration, Secchi depth in Lake Biwa has been improved. It is expected that the decline in chlorophyll a concentration decrease primary productivity by phytoplankton. However, the phytoplankton community in Lake Biwa has also been changed along with the oligotrophication. Therefore, the response of the productivity against the change in chlorophyll a concentration may not be linear. In this research, areal primary productivity in the northern basin of Lake Biwa was estimated using phytoplankton census data and environmental data collected by Lake Biwa Environmental Research Institute from 1980 to 2008. Surprisingly, the estimated areal productivity tended to increase over time, whereas chlorophyll a showed a reverse trend. Two influential factors were deduced: the one was the expansion of euphotic zone with the improvement of Secchi depth and the other was the shift of phytoplankton to smaller species, which had higher productivity than larger ones. The latter had a stronger effect than the former. Thus, a change in phytoplankton community should be considered for the accurate evaluation of productivity.
