Reexamination of food supply effectual for suspension-feeding bivalves inhabiting shallow waters.

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Understanding of the food supply for each species is a dominant factor to determining carrying capacity in an area. In the fishing ground of the surf clam dredge fishery in Fukushima Prefecture, it was shown, 1. that most infaunal bivalves relied on the overlying water close to the bottom surface as a food source layer, extending less than several cm thick; 2. that pelagic microalgae that were produced in the upper layers rarely entered the food source layer. So, the bivalves might rarely ingest the primary production, and 3, that such food supply system was very common on sandy bottoms in sublittoral waters. These showed the primary production in the near-bottom layer play an important role as the food source for benthic communities.In the upper layers of water column dominated by pelagic microalgae, dominant species groups changed seasonally, and a relatively small number of species groups constituted a large proportion, and their density fluctuated in about a hundredfold. In contrast, the near-bottom layer that forms the food source layer showed smaller seasonal changes in dominant species groups, with low fluctuation in the density of 10-15 folds. Such properties of low fluctuation may support the stability as the food sources.The results obtained in the study showed that the food supply for infaunal bivalves differed from the conventional vague conception for it that phytoplankton produced in the upper layers sink to the bottom, forming the main food source, and that benthic microalgae, especially benthic diatoms, and phyto-detritus derived from them were the effective food sources.