How bioturbating lugworms and stabilizing seagrasses shape the morphology of a Wadden Sea tidal basin.
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Ecosystem engineering species are mostly known for their ability to affect abiotic conditions and change the morphology of their habitat. Most often to improve habitat suitability for the species itself. Within the Wadden Sea a bioturbating lugworm digs burrows into the tidal flat bed where it feeds on the nutrients which these soils contain. The bioturbating activities of the lugworm facilitate the erosion of mud and fine sediments from the mudflat. Biostabilisers, like seagrasses, facilitate the deposition of mud in their canopy, since these species locally reduce flow velocities. The lugworm and seagrasses compete over space as they both find their suitable habitat in the intertidal range of the tidal flat, where low-energy conditions are prominent and mud concentrations are above 5\%. Extensive research has already been conducted into how these species interact with the abiotic environment and in what ways they affect sediment dynamics and morphology. Most of these studies are carried out on the scale of single mudflats or within flume experiments. However, studies have shown that these species also affect the environment on a larger scale (estuaries and tidal basins). To be able to understand the mechanisms behind these large-scale effects further research is required. Furthermore, there is still a lack of understanding on how the interactions between ecosystem engineers affect morphology on such scales. The objective of this research is to identify what the combined effect of the stabilising seagrasses and the bioturbating lugworm is on the morphology of tidal basins in the Dutch Wadden Sea. To tackle this objective and answer the main research question, the hydrodynamic FLOW module of Delft 3D is coupled to a species model (MATLAB). The study is carried out in a model domain inspired by one of the smaller tidal basins near the outlet of the Ems river in the Northeastern part of the Dutch Wadden Sea. The species model establishes the suitable areas in the model domain for lugworm and seagrass settlement, and accounts for competition between the two species. The species are parameterised to change certain physical conditions on the locations of settlement. The lugworm lowers the critical bed shear stress for the erosion of mud and increases the erosion parameter, whereas the seagrasses locally increases the drag coefficient. These changes are fed into the Delft 3D environment which computes sediment dynamics and basin morphology. A coupling between the Delft 3D environment and the species model is repeated every morphological month, which represents one tidal cycle. The scenarios with only lugworms, only seagrasses or both species with competition accounted for are compared to each other and to the control scenario without ecosystem engineers. The results of the model scenarios show how the lugworm erodes the higher elevations of the tidal flats. Generally, the bioturbating activities result in a decrease of mean mud fraction in the tidal basin. Seagrasses, on the other hand, locally increase bed elevations and mud fraction. However, scale-dependant feedbacks result in an increase in flow velocity around the seagrass fields. This results in the fixation and incision of tidal channels. The stabilising activities of the seagrass show a positive feedback on habitat suitability for this species, whereas eco-engineering activities of lugworms have a negative impact on their habitat. When competition is considered, the lugworm colonises a significantly larger area of the tidal basin than the seagrasses. Where the latter are restricted to the lower intertidal zones. The sediment eroded from the higher elevations where bioturbation occurs, are captured in the canopy of the seagrasses which settle on the lower elevations. Promoting incision and erosion of the tidal channels.
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