Dead zones are determined as areas where few organisms can survive. The extent of dead zones was estimated in the coastal waters of Mikawa Bay, Japan, to estimate its adverse effects on the bay-wide ecosystem function. Fisherpersons of the fisheries cooperatives holding common fishery rights in the coastal area of Mikawa Bay, were surveyed using a questionnaire to ask opinions as to the type of waters where few organisms can survive. From the survey results, 174 areas were extracted as potential dead zones from the coastal waters of Mikawa Bay. Field observations of macrobenthos biomass were conducted at 44 of the 174 areas. In areas where the number of macrobenthos species was <5, its biomass was very low (4.13 g・m^-2, on an average). The macrobenthos community in these areas was mainly composed of pollution indicator species, such as polychaetes Paraprionospio patiens and Sigambra hanaokai, and bivalves Macoma tokyoensis, Macoma incongrua and Theora fragilis. These areas were thus defined as dead zones. Multiple linear regression analysis indicated a significant relationship between the number of macrobenthos species and environmental factors. These factors included not only local ones (such as degree of closeness to the coastline, the depth, presence of influent water, presence of near-by tidal flat, and speed of tidal flow), but also large-scale hypoxia. The number of macrobenthos species was estimated based on a multiple linear regression analyses of the 174 potential dead zones. The estimated number of macrobenthos species was <5 at 80 out of the 174 areas. These 80 areas, comprising of 27.8 km², were designated as dead zones. Overall, 79.2% of the total dead zones was represented by large-scale ports, and waterways created at closed-off section of the bay. Our results suggest that environmental restoration of the dead zones is essential to the recovery of the bay-wide ecosystem function.

Culture production of nori (Porphyra) in the central part of Bisan Strait in the Seto Inland Sea is decreasing recently. The decrease is mainly due to the discoloration of nori caused by deficiency of dissolved inorganic nitrogen (DIN). We conducted two field observations to investigate the transport process of river water and accompanying DIN supply from rivers to the central part of Bisan Strait. River-water was flowing out to the sea only during ebb tide; the discharge reached its maximum at low tide. The emitted riverine water mass was transported westward by the flood current in the sea, and the river water on the estuary zone was pushed back into the river during flood tide. As the river discharges increased, spread of the river plume became wider whereas movement of riverine water mass did not change. In winter, nori cultivation season, the lower-salinity water was containing higher DIN concentration. These results suggest that nori cultured in this area was intermittently supplied with DIN from rivers. When the river discharge increases, the DIN supply increases.