Rainfall effects on the erodibility of sediment and microphytobenthos in the intertidal flat

The frequent rainfall during the low tide would erode and transport the surface sediment and microphytobenthos (MPB) in the intertidal flat. In order to quantify the rainfall effects on the erodibility of sediment and MPB at the salt marsh and mixed flat, a series of erosion experiments have been conducted with a Gust erosion microcosm system. Surface sediments were sampled for analyzing algal biomass (Chl-a) and primary production (PP) during three typical weather (without rain, rain, and post-rain) conditions. The results of erosion experiment, in both salt marsh and mixed flat, showed that the sediment erodibility under rain condition was higher than that under without rain condition, with increased total eroded mass by 37-86%. It indicated that the rainfall effects caused to significantly disturb the surface sediment. After the rainfall events, the removal of highly-erodible sediments resulted in the reduction in eroded mass. The MPB erodibility under rain condition was lower than that under without rain condition, with decreased total eroded Chl-a by 29%. At the mixed flat, the rainfall effects caused to significantly decrease biological activities of MPB (biomass and PP) associated with surface sediment. The surface Chl-a concentration under post-rain condition decreased by 73%, compared to that under rain condition. At the salt marsh, in contrast, the rainfall effects were barely shown when the biological activities of MPB were rather stable. This implied that the eroded MPB induced by rainfall was retained within the marsh system due to vegetation canopy.