Development of benthic microalgal assemblages on a tidal flat in the Seto Inland Sea, Japan: effects of environmental variability

A bi-weekly survey lasting 13 months was carried out at low-tide on an intertidal flat in the Seto Inland Sea, Japan. We monitored the environmental conditions at a river and a low-tide shore-line stations and examined the factors affecting the development of benthic microalgal assemblages. At the two stations, the physico-chemical parameters of both emerged sediment (i.e. nutrient concentrations of the pore water) and the nearby low-tide water (i. e. salinity and dissolved oxygen concentration) showed strong but similar short- (days) and long-term (interannual) variability. However, the benthic microalgal standing stock was significantly higher at the river station (240.5±121.1mg m-2, n=107) than at the low-tide shore line station (121.9±41.4 mg m-2 n=108). Accordingly, estimated annual primary production was 634 g C m-2yr-1(1.74±0.65 g C m-2day-1) and 259 g C m-2 yr-1 (0.71±0.27g C m-2 day-1), respectively. At the river station, more elevated than the low-tide shore-line station, the development of benthic microalgal assemblages was significantly limited by the washout caused by rainfall, but greatly enhanced during calm and fine weather. At the low-tide shore-line station, the reduced emersion of the surface sediment and the hydrodynamic energy (tidal currents, waves) were major factors responsible for keeping lower and less fluctuating microalgal biomass. Comparable primary production by intertidal microalgae (447gC m-2 yr-1) and phytoplankton in the Seto Inland Sea (285gC m-2 yr-1) is discussed as evidence of the high primary productivity of the intertidal zone, most likely underestimated in light of the grazing pressure by the macro-zoobenthos which is similarly abundant on this intertidal flat.