Sediment quality and benthos biomass were observed for 3 years in and around a borrow pit (1-3 m deeper than the natural bottom) in the innermost Mikawa Bay. In the natural shallow bottom (less than 3.2 m in depth) near the pit, although the organic matter and sulfide contents of the sediment were lower than those of the pit during the observation period, the biomasses of macro- and meio-benthos fluctuated with seasonal change in dissolved oxygen saturation in the bottom water. In the borrow pit (6.7 m in depth) where the organic matter and sulfide contents of the sediment were very high, the biomass and number of species/groups observed were very low or zero, but some macro- and meio-benthos species/groups occurred only during fall to early spring when the bottom dissolved oxygen saturation was relatively high. In a deeper natural bottom with the same depth of the pit, the organic matter and sulfide levels were lower, and the macro- and meio-benthos biomasses were higher than those in the pit. Results of the 3-year observation suggest that the environment for benthic organisms in the pit largely deteriorated with the accumulation of organic matter in the sediment.

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