Effects of diminished dissolved oxygen supply on the benthic communities were studied in eutrophicated Hiuchi-Nada and Harima-Nada of the Seto Inland Sea, where stratification was formed in summer, 1981.
The dissolved oxygen concentration was more markedly diminished at the bottom-layer, located 0.5 meter above the soft muddy sediment, in late summer than in early summer, but not so above the other sediments. Most of the benthos, found commonly in the bays or coastal waters of Japan, were molluscs and polychaetes with Theora lubrica and Paraprionospio sp. dominating, respectively.
Species composition of the benthic communities didn't indicate a striking change during the summer. On the other hand, the number of species and individuals decreased logarithmically with depletion of ambient dissolved oxygen, and also the species diversity decreased gradually. This was due to a decline in number of the small bivalve Theora lubrica, inhabiting the extreme surface sediment.

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