Abstract
Primary production by phytoplankton and biological degradation of organic matter by heterotrophic bacteria are the main factors behind carbon balance in lakes. The purpose of this study is to analyze the relationship between basin loading and carbon balance in lakes using a conceptual food web model. The results showed that biological carbon balance in the lakes changed in a complex manner in accordance with the C:P ratio of the bacterial substrate, which depends on the carbon and the phosphorous load. In the oligotrophic state, the whole food web is inactivated by a reduction in the phosphorus load, and an increase in the TOC concentration in the water is caused by this reduction. In the eutrophic state, a reduction in the carbon load has less effect on the decrease in the TOC concentration. Results obtained using the model suggest that reductions in basin load are not the reasons for a change in the biological carbon balances, and the reduction in the phosphorus load has caused a decrease in the labile TOC in Lake Biwa. The difference between the results obtained using the model and the measured tendency of TOC change suggests that the carbon accumulation in the lake has been caused by the inflow and the production of refractory carbon.
