Responses of intertidal and subtidal communities of the macrobenthos to organic load and oxygen depletion in the Seto Inland Sea, Japan.

The physico-chemical characteristics of low-tide water and emerged sediments along a transect line set between the LWL (low water level) and the ELWL (extreme low water level) were monitored monthly for a year. At the sampling stations, the seasonal changes of the macrobenthic communities were investigated simoultaneously. A similar survey on the chemical characteristics of the sediment and the macrobenthic communities was carried out on an adjacent subtidal station (water depth ca. 10 m). From spring to summer, both primary and secondary production on the intertidal flat were extremely high. Minimum ammonium nitrogen concentration and oversaturated oxygen concentration (up to 224%) in the water column related to the development of a high biomass of the macroalgae Ulva sp., which covered large areas of the tidal flat. During the same period, the biomass of two dominant bivalves, Ruditapes philippinarum and Musculista senhousia, increased sharply from 58.7 gWW m-2 (April 1994) to 937 gWW m-2 (August 1994). However, in early autumn, the deterioration of meteorological conditions and the rapid decomposition of the macroalgae lead to a distrophic crisis. A sharp increase of ammonium nitrogen, pheo-pigments and particulate organic carbon (POC) in the water and pheo-pigments, total organic carbon (TOC) and acid-volatile sulfide (AVS) at the sediment level coincided with the catastrophic mortality of R. philippinarum. The biomass of this bivalve crashed from 563 ± 163 gWW m-2 (September 1994) to 77.0 ± 61.2 gWW m-2 (October 1994). In contrast, polychaeta profited from the increased amount of dead organic materials and increased both in density and biomass through the next months. At the adjacent subtidal station, the organic load from the intertidal zone simoultaneously caused anoxia at the bottom sediments. Unfavourable benthic conditions had a deleterious effect on the macrobenthos, whose biomass drastically declined from 179 gWW m-2 (July 1994) to 11.3 gWW m-2 (October 1994). This work highlights the coincident and drastic shift from an hypertrophic to a distrophic period of both primary producers and primary consumers. We suggest valuable countermeasures for sustainable use of tidal estuaries, such as a reduction of nutrient export from the land and control of macroalgal development on the intertidal zone