Spatiotemporal variation of extracellular polymeric substances (EPS) associated with the microphytobenthos of tidal flats in the Yellow Sea.

The physical functions of extracellular polymeric substances (EPS), viz., by-product of microphytobenthos (MPB), in tidal flat system are well documented, but some ecological aspects remain unknown. We investigated MPB biomass (Chl-a), EPS, diatom assemblage, and erodibility in two contrasting tidal flat environments (megatidal vs. macrotidal flat) in the Yellow Sea. Thick biofilms were observed when MPB bloomed, with high Chl-a and increased EPS concentrations. Among diatom genera, Navicula was the most dominant taxa found over the year (mean 41%) in both areas. Compared with non-bloom periods, the erodibility decreased by 54-73% as biofilm thickened during the blooms. It was attributed to the elevated abundance of large-sized (>40 µm) Navicula, which was expected to secrete large amounts of EPS. Overall, we successfully demonstrated spatiotemporal differences of sediment stabilization that significantly related to ecological variations of MPB, and identified the key diatom genus as a "sediment stabilizer" in the typical tidal flats of the Yellow Sea.

Influence of tidal forcings on microphytobenthic resuspension dynamics and sediment fluxes in a disturbed coastal environment.

The resuspension dynamics of microphytobenthos (MPB) and sediment fluxes were investigated in a disturbed coastal environment by employing an in situ mooring system. We aimed to identify the interrelationship between microalgal biomass and sediment particles in aspect of their (de)coupling mechanism in a tidal channel system. In specific, the Chl-a (as a proxy of MPB biomass) and the suspended sediment concentration (SSC) were simultaneously measured under natural tidal conditions at different time scales, encompassing daily to fortnightly variations. Looking for the sediment dynamics, the SSC showed a strong positive correlation (p < 0.001) with the Reynolds stress; however, this relationship was not observed for benthic Chl-a. This could be due to more dynamic characteristics of the smaller biological cells, i.e., the decoupled benthic Chl-a from the sediment particles might randomly distribute in the turbid water column above the tidal channel. Notably, the iteration between MPB coupling (prevailing in spring) and segregation (prevailing in neap) with the sediment particles across the flood and ebb tidal cycles was evident during the study period. Meantime, the onshore flux of suspended sediment was almost balanced by its offshore flux, but that of Chl-a appeared to be unbalanced due to an excessive onshore transport. Altogether, the study area seems to experience a time lag in resuspension between MPB and sediment, followed by biological trapping in the tidal channel system, which would support a productive shallow water environment. The present study is the first to address the tidal resuspension of benthic microalgae in relation to sediment dynamics in a disturbed coastal environment of the Yellow Sea.

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.

Effects of vegetation and fecal pellets on the erodibility of cohesive sediments: Ganghwa tidal flat, west coast of Korea.

Although the Korean tidal flats in the Yellow Sea have been highlighted as a typical macrotidal system, so far, there have been no measurements of the sediment erodibility and critical shear stress for erosion (τce). Using the Gust erosion microcosm system, a series of field experiments has been conducted in the Ganghwa tidal flat to investigate quantitatively the effects of biogenic materials on the erodibility of intertidal cohesive sediments. Four representative sediment cores with different surficial conditions were analyzed to estimate the τce and eroded mass. Results show that τce of the "free" sediment bed not covered by any biogenic material on the Ganghwa tidal flat was in the range of 0.1-0.2 Pa, whereas the sediment bed partially covered by vegetation (Phragmites communis) or fecal pellets had enhanced τce up to 0.45-0.6 Pa. The physical presence of vegetation or fecal pellets contributed to protection of the sediment bed by blocking the turbulent energy. An inverse relationship between the organic matter included in the eroded mass and the applied shear stress was observed. This suggests that the organic matter enriched in a near-bed fluff layer is highly erodible, and the organic matter within the underlying sediment layer becomes depleted and less erodible with depth. Our study underlines the role of biogenic material in stabilizing the benthic sediment bed in the intertidal zone.