Vegetation Properties and Sedimentation in Salt Marshes: Implications for Wave Attenuation and Marsh Survival

Abstract

Salt marshes are tidal ecosystems that form a transition area between the land and the sea. A central ecosystem service of the vegetation adapted to salt- and flooding-stress is the wave attenuation potential. By attenuating the waves, salt marsh vegetation significantly contributes to coastal protection. The extent of the wave attenuation depends on the biophysical properties of the salt marsh vegetation and generally correlates positively with the shoot density, the rigidity of the stems, the height of the vegetation, and the biomass. When calculating and investigating the wave attenuation over salt marshes with numerical models or physical laboratory models, the biophysical plant properties must be taken into account. To date, only a few studies have quantified the biophysical properties of salt marsh plants. The temporal and spatial variability has not been considered adequately and was not taken into account for all biophysical properties. Salt marsh vegetation also contributes substantially to sediment deposition and therefore to the vertical accretion of salt marshes. On the one hand, the vegetation-induced wave attenuation and reduction of flow velocity leads to faster gravity- induced deposition of suspended sediment particles from the water column onto the marsh surface. On the other hand, suspended sediments are deposited directly on plant surfaces. Livestock grazing leads to massive alterations of the vegetation such as a reduced biomass or vegetation height. Consequently, grazing has the potential to cause considerable changes in biophysical plant properties and thus in the vegetation’s influence on sediment deposition. This thesis aims to improve the database and the understanding of the spatial and temporal variability of biophysical plant properties in Wadden Sea salt marshes. Furthermore, it aims to investigate and understand the influence of grazing and vegetation structure (short vs. high vegetation) on sediment deposition. This thesis consists of five chapters. A general introduction (Chapter 1) is followed by three manuscripts (Chapters 2-4) and a synthesis (Chapter 5) discussing the overall context and links between the manuscripts. Furthermore, this chapter derives concrete implications from the research results presented in the three manuscripts. Chapter 2 deals with the seasonal and spatial variability of the biophysical properties of Spartina anglica (pioneer zone/low marsh) and Elymus athericus (high marsh) within a mainland marsh in Northern Frisia. A pronounced seasonal variability of the biophysical properties (e.g. biomass, stem density and stem flexibility) with significant intraspecific differences was found in this study. Compared to Elymus athericus, Spartina anglica showed a greater seasonal variability. Also, with regard to the spatial variability, Spartina anglica showed clear differences in biophysical properties between a seaward and a landward-located zone. Chapter 3 investigates the influence of livestock grazing on sediment deposition and accretion on the marsh island Hallig Langeness in the Wadden Sea. The results show significantly reduced sediment deposition and accretion on grazed areas compared to adjacent non-grazed areas. This difference was more pronounced at the marsh edges adjacent to the mudflats compared to areas in the interior of the marsh island. Chapter 4 presents the results of a study on the influence of small-scale vegetation patterns on the suspended sediment concentration and on the sediment deposition in a mainland salt marsh in Dithmarschen. Partial mowing of the vegetation resulted in a pattern of mown subplots and control subplots with a size of 4 m2 in various combinations adjacent to a creek. Based on the results, it can be concluded that on the spatial scale of 4 m2, there is no effect of the vegetation on the waves and water flow to be expected since the sediment deposition between mown and control subplots did not differ neither in the high nor in the low marsh. Furthermore, a mown or a control subplot next to the creek had no influence on the sediment deposition on a mown or control subplot behind.