This dissertation describes the results of a 4-year research into the dynamics of biophilic elements (carbon, nitrogen, phosphorus and silicon) in a tidal estuary of the Seto Inland Sea, Japan. Both on the intertidal and the subtidal zone, major physicochemical parameters of water and sediments were examined at fixed transect-line stations monthly from April 1994 to April 1996. Density and biomass of the macrozoobenthic communities were investigated simultaneously at both sites. Additionally, laboratory experiments were performed on the nutrient excretion rate of the dominant species of macrozoobenthos. Based on the results of these experiments and the biomass found on the field, the animal contribution on the processes of nutrient regeneration within the estuary was quantified. Finally 24-hour surveys were conducted along the estuary to assess the short-term (hours) effect of a tidal cycle (mixed-semidiurnal type) on the balance of dissolved and particulate compounds.

An upper intertidal station was a major source of microphytobenthos to the estuary (241 +/- 121 mg m-2). Estimated microalgal primary production was 634 gC m-2 year-1, two times higher than that by phytoplankton in the eutrophic Seto Inland Sea. On the intertidal zone, high primary production in the sediments and mass fluxes in the water column (up to 50g m-2 hour-1) sustained a large macrozoobenthic biomass up to 1 kgWW m-2. The fresh water runoff was a major source of new nitrogen (nitrate + nitrite), while the processes of nitrogen regeneration (ammonium) were indeed higher within the estuary. The dominant bivalve Ruditapes philippinarum and Musculista senhousia contributed up to 39 mM m-2 day-1 and 29 mM m-2 day-1 to the regeneration of ammonium and silicate, respectively. Nutrient regeneration through diffusional fluxes was 10 times (winter) to 100 times (summer) lower than that due to animal excretion. The pheo-pigment content in the water and sediments correlated significantly with the organic carbon and nitrogen pools, but not that of Chl a. Grazing pressure and algal degradation by macrozoobenthos was discussed.

Nutrients and particulate compounds progressively decreased along the subtidal zone. Whereas at a deeper station (10 m), unutilized organic matter periodically raised the acid-volatile sulfide levels in the sediments up to > 2 mg g-1. Physicochemical factors influenced the spatial/temporal variability of the macrozoobenthic communities. Small poychaeta were abundant in muddy and organically enriched sediments, following benthic mortality. In contrast, bivalve biomass was up to 600 gWW m-2 in less hypoxic and shallower sediments. The high productivity and the self-purifying potential of the intertidal zone were compared to those of the subtidal zone. The ecological peculiarity of the intertidal zone was highlighted as a very resourceful boundary between the land and the sea which should be preserved and ameliorated.

The author was supported by a Japanese Government fellowship and is presently expanding this work under a European Union S&T fellowship (1999-2000).