Spatial patterns and bio-geomorphological effects of vegetation loss in a submerging coastal marsh
19 December 2017
Stadscampus, R0.07 - Rodestraat 14 - 2000 Antwerpen (route: UAntwerpen, Stadscampus
Organization / co-organization:
Faculty of Science
PhD defence Lennert Schepers - Faculty of Science
Coastal marshes are situated at the interface between land and sea. Their ecological value is of great importance as they provide important ecosystem services. For example, they provide protection against storm surges and water quality regulation. They also provide important habitat for specialized organisms and nursing grounds for fishery species. Furthermore, marshes are one of the most efficient ecosystems for carbon storage. However, when marshes cannot keep up with increasing rates of sea level rise, they will subsequently drown and eventually disappear. This results in large-scale marsh loss, which severely affects their valuable ecosystem functions.
Currently, little is known about how this marsh loss process develops spatially from intact marshes to large open water areas. Also, little is known about the effect of these spatial patterns on tidal flow and sediment dynamics, which is crucial for the recovery of the marsh vegetation.
In this thesis we determined the spatial patterns of loss and of studied the bio-geomorphological effect of these losses on flow and sedimentation dynamics. We also studied how the geomorphology, species composition, soil strength and belowground biomass change with increasing marsh loss. These points were investigated by a combination of methods, covering aerial image analysis, a large-scale mowing experiment (the Scheldt estuary, Belgium), and a wide range of field measurements in the Blackwater Marshes (MD, USA). The Blackwater Marshes are a large micro-tidal marsh system exhibiting a gradient from fairly intact marshes to vast open water where all marshes have been lost.
Our findings show that mashes disappear in particular patterns, starting from small bare patches far from the tidal channels that evolve to large ponds which will eventually be connected to the tidal channel system. Initially, the inner bare patches have little effect on flow and sediment dynamics. However, once ponds deepen below the firm marsh root mat, the underlying material is very soft and prone to erosion, which probably amplifies marsh edge loss and pond deepening. Furthermore, flow velocities might increase once ponds have been connected to the tidal channels, which might lead to export of the loose sediments from the soft layers out of the ponds. This results in steep and abrupt transitions between marshes and permanently submerged ponds which impede recovery of marsh vegetation in ponds.
These feedbacks highlight the importance of identifying marsh vulnerability as soon as possible to foresee pending shifts from the vegetated marsh state to the pond state. Our research demonstrated that two new indicators, the skewness of the marsh elevation distribution and the co-occurrence of species in mixtures, can be used to assess marsh vulnerability to sea level rise. However, the indices failed to correctly assess the most degraded marsh site by neglecting lateral erosion of the marshes that can lead to a runaway process of permanent marsh loss.
Our findings imply that management of vulnerable marshes should primarily focus on retaining existing marshes rather than trying to restore ponds. Such strategies should pay particular attention to the sediment budget of the whole marsh complex in order to enable remaining marshes to keep up with sea level rise.
While this thesis identifies important bio-geomorphological feedbacks in areas that experience marsh loss, further research is needed to fully understand the processes and interactions between the complex vegetation patterns, flow and sediment dynamics in tidal wetlands.