Biophysical interactions in coastal habitats

In order to successfully implement nature-based solutions as (part of) coastal defense, there is a need for insight into their efficiency and their reliability. The wave-attenuation capacity of vegetated coastal ecosystems like seagrasses and salt marshes has been widely studied. However, their stability on the medium to long term (i.e., engineering timescales) needs to be quantified. Key wave attenuating ecosystems like salt marshes have a high internal stability. However, their lateral extent is strongly influenced by the vertical (sediment) stability of adjacent mudflats and / or seagrass meadows. In this thesis the sediment stability thresholds of subtidal and lower intertidal ecosystems (from seagrass meadows up to tidal flats) under strong current and wave attack is quantified. Typically such thresholds are measured via field measurements or laboratory flume experiments. The problem with these approaches is, however, that field measurements lack hydrodynamic control, so thresholds can only be measured indirectly, and laboratory experiments require ecosystem transplantation or the use of mimics. Field flumes have therefore been developed as a way to generate controlled hydrodynamics in situ, and have been used to quantify sediment stability thresholds in seagrass meadows and on tidal flats (chapters 2 and 4). We explored the Seagrass – Sediment – Light (SSL) feedback by measuring sediment resuspension for various seagrass densities and sediment types (chapter 3). We were able to establish a general relation between seagrass density and sediment stability. This relation was then implemented in a simplified wave model which was used to assess the effects of changes in typical waves and storminess. We found that seagrasses are resilient to storms, but can be sensitive to changes in typical wind conditions depending on the topographical setting. Benthos influence the stability of tidal flats. This is a complex interaction, because there are many species which all act differently. The effects of different species can be generalized via their energy consumption. We tested this approach in the field, but we still found a large variability (chapter 4). Therefore, we conducted an experiment where we assessed how different combinations of highly contrasting species change sediment stability (chapter 5). We found that the species with the strongest individual effect overruled the effects of other species. When implementing nature-based solutions for coastal defense, there is a risk of accumulation of pollutants like microplastics. We assessed microplastic trapping within various biogenic habitats (chapter 6). We found that the susceptibility of a biogenic habitat to plastic accumulation was determined by their ability to reduce turbulence near the seabed. Furthermore, smaller particles are much more likely to be sequestered as they can ‘hide’ between larger sediment grains.