| The Toyo River is one of the major rivers flowing into Atsumi Bay, a part of Ise Bay, central Japan. We measured water depth, temperature, salinity, turbidity, and dissolved oxygen levels and analyzed bottom sediments in the estuary of the Toyo River to determine the relationship between benthic macroinvertebrate distributions and these variables. We also performed stable isotope analysis for benthic invertebrates, sediments, and suspended solids to investigate the food sources of the benthic macroinvertebrates. Bivalves such as Japanese littleneck (Ruditapes philippinarum), Japanese corbicula (Corbicula japonica), and mussel (Musculista senhousia) were commonly found, and their abundance was mainly controlled by salinity. Water depth also affected the distribution of macroinvertebrate taxa. The biomass of these three bivalves was large in relatively shallow areas characterized by sandy sediments. Bottom anoxia in the area offshore of the river mouth can negatively impact the suitability of habitat near the river mouth. Stable isotope analysis showed that bivalves feed on organic compounds supplied from both the river and bay. Organic compounds can be food for bivalves and also the cause of oxygen consumption. This suggests that the topography of the estuary substantially affects the value of the habitat for bivalves. Since parts of this area have been excavated and deepened, the restoration of shallow estuary areas will improve the quality of the habitat and increase the biomass of bivalves, leading to the accelerated consumption of organic compounds without causing anoxia |
