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Microphytobenthos influence sediment dynamics via their growth and covering on the surface sediment of saltmarshes. To understand the sediment transport and morphological changes resulting from microphytobenthos in a saltmarsh, two parallelly located sites were established to measure the hydrodynamic forces, sediment characteristics and microphytobenthos properties both in winter and summer. Diatoms were not observed at any of the two sites in winter because of their growth cycles. In summer, however, diatoms were found in both sites, with a chlorophyll a (Chl-a) concentration of 4.1 μg/g in the surficial sediment at the unvegetated site and a Chl-a concentration of 1.4 μg/g at the vegetated site. Interactions between diatoms and fine particle accumulation induced positive feedbacks between the seabed stable states and the growth of diatoms. Furthermore, the critical erosion threshold value in the vegetated area was 0.33 N/m 2 , whereas the value in the adjacent unvegetated area reached up to 0.43 N/m 2 because of the dense diatom armouring in summer, which increased 50% and 126% critical erosion threshold values than those at the corresponding sites in winter. Consequently, net sediment accumulation occurred in the unvegetated area in summer but opposite to that in winter. These results reveal that high coverage of diatoms contributes to substantial impacts on bed erodibility and sedimentary processes, which is vital for the understanding of the morphological evolution in saltmarshes, and provide valuable suggestions for coastal saltmarsh reconstruction.

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Wave parameters, e.g., wave height, near-bed wave orbital velocity, and wave-induced shear stresses, are important hydrodynamic parameters for sediment processes in coastal oceans. Wave orbital velocity is particularly critical in sediment resuspension. Several algorithms to calculate wave orbital velocity have been proposed, including linear wave theory, spectrum, and Joint North Sea Wave Project methods, but the validity of these algorithms in relatively shallow waters is not well understood. In this study, we compared the wave parameters obtained by different instruments and algorithms at four sites, one within the intertidal zone with a mean depth of 1 m and the remainder three in deeper offshore water with mean depths of 15-30 m. We found a high consistency of the estimated wave height, peak wave period, and wave orbital velocity among different datasets and different algorithms at the offshore sites, while there were significant discrepancies at the shoreline site. Using Ursell number, our study suggests that it is reliable to apply any of the three algorithms and different instruments (acoustic Doppler velocimeter and buoy) in deeper water. However, for very shallow water, it is recommended to use the measured highfrequency velocity and spectrum method to calculate wave orbital velocity, and use wave gauge instrument or zero-crossing algorithm to obtain wave height and period information. Finally, the effect of turbulence and bedform morphology on wave-induced shear stress is discussed: without removing the turbulence or taking onto account bedforms (e.g., ripples), the orbital velocity will be remarkably over-estimated or under-estimated.

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