2017 年 71 巻 2 号 p. 102-110
Estuaries, the main zones of transition between riverine terrestrial and marine ecosystems, are characterized by environmental fluctuations including changes in salinity, water temperature, and sediment biogeochemistry. In this review, I summarize a series of studies conducted in a freshwater-affected brackish lagoon (Idoura Lagoon, Sendai City, Northeastern Japan) in Sendai Bay. This lagoon system is considerably affected by irrigation water discharge from its catchment area during the rice-farming season (late April to early September). River-derived terrestrial materials are the main source of sediment organic matter in the lagoon. We found a distinct spatial change in the macrozoobenthic community structure along a salinity gradient from the freshwater inflow. Annual monitoring also revealed that the population dynamics of the polyhaline polychaete Pseudopolydora spp. were affected by seasonal changes in the salinity regime. Stable isotope ratios of the polychaetes Hediste spp. and the bivalve Laternula marilina also suggested that river-derived organic matter was a major component of the diets of these species during the irrigation period. Our results show that fluctuations in irrigation discharge have considerable effects on estuarine soft-bottom habitats, suggesting a significant linkage between terrestrial, riverine, and estuarine ecosystems under anthropogenically modified water-use conditions.
