Impact of dam operations on bottom sediment and macrobenthic community in the Kurobe Coast over a quarter century.

Reservoir sedimentation has become a global problem, yet many uncertainties remain regarding the impact of sediment control measures on marine ecosystems. This study examines the effects of dam operations on marine ecosystems through long-term monitoring. The operations mainly transport silt to the marine area, as the downstream Unazuki Dam, which began operations in 2001, prevents larger particles such as gravel and sand from passing through. Over time, this could result in coarser coastal bottom sediment (i.e., a decrease in clay content) from the accumulation of river-supplied material. Our results indicate no serious impact on macrobenthic communities; instead, there has been a long-term increase in macrobenthic density and taxa richness at the monitored site, as a result of organic matter and nutrients supplied by the river. Near the river mouth, minimal change in coastal bottom sediment was observed, probably as a result of the influence of a complex estuarine plume that prevents silt accumulation. At this site, macrobenthic densities and taxa richness were more affected by the maximum river discharge than by the quantity of flushed sediment, suggesting that natural disturbances play a key role in shaping macrobenthic community structure near the river mouth. The current sediment flushing and sluicing operations appear sustainable for the marine environment and economic activities, as their impact is widespread but gradual. Since 2016, the sedimented downstream dam has allowed more sediment, including larger particles, to pass through, potentially restoring the natural sediment regime of the Kurobe River and benefitting marine ecosystems.