The effect of human activities on benthic macroinvertebrate diversity in tributary lagoons surrounding Lake Biwa

In aquatic ecosystems, tributaries play an important role in maintaining the populations and diversity of aquatic organisms throughout the drainage basin, but because of their small catchments, these ecosystems are often more susceptible to the effect of land-use changes and flow-regime alterations. Here, we examined anthropogenic effects on benthic macroinvertebrate diversity in the tributary lagoons, collectively called "Naiko," surrounding Lake Biwa. We sampled macroinvertebrates and determined the environmental characteristics of 20 tributary lagoons. We identified the environmental factors determining the diversity of macroinvertebrates and found that turbidity significantly diminished species diversity. We assessed the anthropogenic stressors that contributed to the increase in turbidity and found that human population density and the proportion of paddy fields in the watershed area were positively correlated with turbidity, most likely caused by erosion of terrigenous organic matter from the paddy fields and urban areas. In addition, the presence of sluice gates and the lengths of channels connecting to the main lake were positively correlated with turbidity, suggesting that these factors lowered hydrologic connectivity and retained organic matter. We concluded that changes to the landscape and decreased hydrologic connectivity caused by human activity increased turbidity, which in turn decreased macroinvertebrate diversity. The identification of these factors in tributaries is vital for developing a strategy for habitat restoration to conserve the entire ecosystem of the Lake Biwa basin.