Abstract
Recent research indicates that many ecosystems, including intertidal marshes, follow the alternative stable states theory. This theory implies that thresholds of environmental factors can mark a limit between two opposing stable ecosystem states, e.g. vegetated marshes and bare mudflats. While elevation relative to mean sea level is considered as the overall threshold condition for colonization of mudflats by vegetation, little is known about the individual driving mechanisms, in particular the impact of waves, and more specifically of wave period. We studied the impact of different wave regimes on plants in a full scale flume experiment. Seedlings and adult shoots of the pioneer Scirpus maritimus were subjected to two wave periods at two water levels. Drag forces acting on, and sediment scouring occurring around the plants were quantified, as these are the two main mechanisms determining plant establishment and survival. Depending on life stage, two distinct survival strategies emerge: seedlings present a stress avoidance strategy by being extremely flexible, thus limiting the drag forces and thereby the risk of breaking. Adult shoots present a stress tolerance strategy by having stiffer stems, which gives them a higher resistance to breaking. These strategies work well under natural, short period wind wave conditions. For long period waves, however, caused e.g. by ships, these survival strategies have a high chance to fail as the flexibility of seedlings and stiffness of adults lead to plant tissue failure and extreme drag forces respectively. This results in both cases in strongly bent plant stems, potentially limiting their survival.
Highlights
Intertidal marshes provide many important ecosystem services
A three-way ANOVA showed that there were no significant differences in sheet erosion between experiments with different life stages, water levels and wave periods nor between all possible combinations of these conditions which means that results of self-scour can be compared for all conditions
We focused on drag forces, sediment scouring and bending that individual plants experience under wave exposure, showing that both wave period of incoming waves and life stages of plants can act as thresholds for individual marsh plant establishment on the bare mudflat
Summary
Intertidal marshes provide many important ecosystem services. Apart from ecological functions such as carbon sequestration, water quality regulation, contribution to fishery and providing irreplaceable habitats for specialized organisms [1] they are important for flood and wave attenuation [2,3,4] which is why they play a crucial role in coastal defence [5]. In recent years there has been increasing evidence that seaward marsh expansion by colonization of marsh vegetation onto bare mudflats can be explained by the alternative stable states theory [8,9,10,11,12,13]. When—as a result of sedimentation—a tidal flat reaches a threshold elevation where vegetation can establish, this vegetation will subsequently reduce waves and currents and thereby accelerate sedimentation and increase elevation. This will in turn further stimulate vegetation growth leading to enhanced sedimentation etc. This will in turn further stimulate vegetation growth leading to enhanced sedimentation etc. until a high vegetated marsh state is reached [14,15]
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