Relationships between benthic animals and detritus deposits in estuarine tidal flats.

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Seasonal investigations of macrofauna, sediment, and deposited particulate matter collected by sediment trap were conducted on two estuarine tidel flats, the Gamo tidal flat at the mouth of the Nanakita River and the ldoura tidal flat at the mouth of the Natori River, Sendai Bay. The macrobenthos detected were very similar between the Gamo and the ldoura tidal flats, probably because of similar salinity ranges, but the dominant species were different. In the Gamo tidal flat, Neanthes japonica and Notomastus latericeus were quantitatively abundant, while in the ldoura tidal flat, the crab Macrophthalmus japonicus and the bivalve Macoma contabulata comprised the most part of the biomass. The higher total biomass of macrofauna, and the shorter lifespan of the dominant species in the Gamo tidal flat may indicate a higher production of benthic macrofauna. The results also suggested that the greater sedimentation of POM (particulate organic matter) of higher nutritional value resulted in a higher production of macrobenthos in the Gamo tidal flat than in the ldoura tidal flat. The C/N ratios of sediment trap samples and surface sediment of tidal flats showed that the sedimentary organic matter of the Gamo tidal flat was derived from phytoplankton, while that of the ldoura tidal flat was of more terrestrial origin. The negative correlation between the biomass and the amount of POM caught in the sediment traps indicated that the benthic macrofaunal production was not food limited in the Gamo tidal flat. It was suggested that severe low Eh (anoxic) and sulfide stresses caused by higher microbial activity of labile POM decomposition combined with long exposure time and a decrease in oxygen concentration in the overlying water resulted in a lower macrofaunal biomass at the inner part of the Gamo tidal flat.