Spatial pattern formation of macrophytes : an integrated model for the management of lowland rivers
31 January 2017
UAntwerpen, Campus Drie Eiken, Promotiezaal Q0.02 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Organization / co-organization:
Department of Biology
Stijn Temmerman & Patrick Meire
PhD defence Veerle Verschoren - Faculty of Science, Department of Biology
The functioning of aquatic ecosystems is influenced to a great extent by the presence of aquatic vegetation, also called macrophytes. Submerged macrophytes are typically abundant in lowland streams. In temperate mid-latitude climate zones, their presence is highly variable in both space and time. Through their above-ground structures they are able to directly influence water flow by modifying current velocity, changing hydrology and promoting sediment trapping (and its associated nutrients). Distinct spatial vegetation patterns are observed, ranging from continuous vegetation cover to patchy vegetation cover. However, it is not clear what the steering mechanisms are that cause these spatial vegetation patterns. Furthermore, there is only limited knowledge on the effect of distinct spatial patterns on the transport of material. The aim of this thesis was to determine on one hand the key factors controlling the spatial distribution of macrophytes and on the other hand the effect of distinct spatial patterns of macrophytes on water flow and transport processes. These aspects were investigated through a combination of field measurements and numerical modelling.
A digital cover photography technique is developed to map macrophytes at the species level with a very-high spatial and a flexible temporal resolution. The method is successfully applied in two lowland rivers in the upper Nete catchment (Belgium). Next, a plant growth model is developed to simulated the spatio-temporal growth of macrophytes in two dimensions. This model is coupled with a two dimensional depth-averaged hydrodynamic model to simulate the reciprocal interactions between macrophytes and water flow. Macrophytes create hydraulic resistance and therefore reduce the flow velocity within vegetation patches. The formulation in our model accounts for the complex morphology and flexibility of natural, submerged macrophytes. Meanwhile, once a critical velocity is exceeded vegetation patches cannot expand further and existing vegetation patches disappear. We found that distinct spatial patterns dominated by different species are the result of the magnitude of the plant-flow interaction and of specific growth strategies of the dominant species. The effect of these distinct spatial patterns on the transport of material is quantified by field measurements. Tracer experiments are conducted with dissolved and particulate organic matter at three vegetation covers in a lowland river in Poland. Our results show that vegetation reduces the longitudinal dispersion coefficient and reduces the transient storage zone, while the retention of particulate organic matter increases. In addition, the most heterogeneous flow field is found in partially vegetated treatments. Finally, a field survey is performed in two river reaches with continuous and patchy vegetation cover. The morphological activity tends to be higher during the summer when macrophytes are abundantly present in both rivers reaches.