The Influence of Benthic Diatoms and Invertebrates on the Erodibility of an Intertidal Mudflat, the Danish Wadden Sea

The erodibility of mudflat surfaces has been investigated in the Lister Dyb tidal area. A description is given of the short-term erosional, depositional history and the main biological factors governing the stability of the sediment surface. The erosion threshold seems mainly to be controlled by the relationship between algal biomass, expressed as chlorophylla content and the abundance of deposit feeders. Benthic microalgae are important for the sediment stabilization due to their production of extracellular polymeric substances (EPS) during locomotion. The deposit feeder Hydrobia ulvae on the other hand limits the influence of microalgae because diatoms are the main part of their diet. Additionally, H. ulvae produces fecal pellets which can be more easily eroded than the cohesive bed since they seem to behave as individual units losing cohesive properties. Freshly deposited material was more stable than eroded areas, explained by the occurrence of benthic microalgae, which stabilize the sediment surface in areas of accretion. There was a positive correlation between the water content of the surface material and erosion threshold, interpreted as a result of the dominance of biological stabilizing and destabilizing factors at the site. The variation in algal mass and species abundance causes a marked cross-shore variation in erosion threshold with an increase of stability towards the salt marsh line. The reason for this is argued to be the cross-shore variation of exposure time, which governs the growth of microphytobenthos since light exposure declines towards the low-water line. The cross-shore variation of the erosion threshold is discussed in relation to the suspended sediment transport and it is argued that the result of this variation is a tendency for net landward transport of suspended sediment.