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.

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In Lake Biwa, the gap between biochemical oxygen demand (BOD) and chemical oxygen demand (COD) is widening, so the increase in the amount of refractory organic matter has been of concern. The objective of this study is to estimate the source and the cause of the increase in the amount of refractory organic matter. In order to investigate the material flow using the box model, the generation (or inflow) load in the watershed and internal production are calculated by monitoring COD and total organic carbon (TOC) by biodegradation assay in drainage from each pollution source and lake water. As a result, it is revealed that although the generation load of organic matter has been reduced by domestic and industrial measures, the main cause is the reduction of labile organic matter. Model results indicate that autochthonous organic matter is the dominant source of refractory organic matter in the lake but is at the same level as the allochthonous organic matter in the case of the dissolved fraction. It is most likely that the change in the amount of the autochthonous organic matter caused the increase in the amount of refractory dissolved organic matter.

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