2019 Volume 42 Issue 2 Pages 53-65
We investigated water quality and sessile animals in Dokai Bay, Japan, during periods of heavy eutrophication (1991-1992) and improved conditions (2010-2011) to evaluate the usefulness of sessile animals as environmental indicators. Hypoxia disappeared and total nitrogen and phosphorus decreased following the control of industrial discharges into the bay. Reduced eutrophication led to the spread of oxygenated water to the inner bay, accompanied by expanded distributions of marine sponges and bryozoans. In addition, the improvement of water quality appears to have added 40 new species to the community of gastropod mollusks (snails) . The Shannon diversity index for mollusks increased, and the total number of sessile animal species increased to 1.6 times that before water quality improvement. However, the total wet weight of two species of bivalve mollusks, mussels and Japanese oysters, decreased to 1.2% that before the improvement; statistics confirmed that this drastic decrease was a result of increasing water temperature. These results show that the community and population of sessile animals can be used as indicators of eutrophication and increasing water temperature, because of their dependence on environmental factors.
