Effect of Nutrient Loads on Upper Trophic Level Species in Lake Biwa: Analysis Using Food Chain Model by Monte Carlo Method

It has been pointed out that some waterbodies face “oligotrophication” owing to a decrease in nutrient loads. In this study, we predicted the effect of a change in future nutrient loads on variables such as the biomass of upper-trophic-level species, using the food chain model. The target area is Lake Biwa, where fish catches have fallen in recent years concurrent with a decrease in nutrient loads. Three models with different structures were developed and 100 patterns of parameter sets were selected for each model by the Monte Carlo method. As a result, the change in the biomass of phytoplankton, regardless of the model structure, tended to be roughly proportional to the change in nutrient loads in general. On the other hand, for the fish, differences in model structures and their prey strongly affected the biomass, and although inflow load increased, the biomass decreased in some cases. Considering the uncertainty of the prediction, we suggested that the addition of nutrients with the aim of increasing the fish populations would cause undesirable results in Lake Biwa such as a combination of deteriorating water quality and no increase in the fish populations.