Eten en gegeten worden in het Schelde-estuarium

Like other estuaries, the Schelde estuary is characterized by a diversity of organic matter inputs. These inputs can be classified as ‘autochtonous’, i.e. produced in situ (e.g. phytoplankton and microphytobenthos), or ‘allochtonous’, i.e. transported into the estuary via river and seawater inflow. Together, these organic matter sources form an enormous energy pool, that is utilized by huge numbers of living organisms. The Schelde is a largely heterotrophic estuary, implying that allochtonous sources fuel the majority of the metabolic processes taking place within the system. This is most pronounced in the upstream reaches of the estuary, and is strikingly illustrated in the maximum turbidity zone. All this allochtonous carbon fuels a decomposer food web where bacteria, their protistan grazers, and rotifera and copepods feeding on protists form the link between detritus and the higher trophic levels like shrimps, fish and birds. The high bacterial activity results in oxygen depletion, which in turn causes a relatively species-poor food web. Downstream, the relative importance of autochtonous carbon increases, and an autotrophic, grazer food web develops in addition to the decomposer food web. In the water column, phytoplankton is grazed by copepods, which in turn are food for fish and shrimps. In the benthos, a large variety of algal grazers and detritus feeders exist, forming the basis of complex, interlinked food webs. Inspite of the high concentrations of available carbon and nutrients, much of the consumed sources are constantly being recycled within and between different trophic levels. Unraveling these food webs is often a complex task requiring sophisticated methods such as stable isotope tracer techniques

Analysis of multibeam echo sounding data on bed forms near the Walsoorden sandbar, a first phase in the subtidal habitat classification for the Western Scheldt

Within the scope of the long term vision of the Scheldt estuary, a new disposal strategy was proposed by an international expert team appointed by the Antwerp Port Authority. After the feasibility study, two in situ disposal tests were carried out near the Walsoorden sandbar in the Western Scheldt. Both tests were thoroughly monitored, both morphological as ecological.A project was defined to investigate the impact of bed forms, hydrodynamics and sediment properties on the ecological value in the shallow water area near the Walsoorden sandbar. The goal of this project is to make a habitat classification for the Western Scheldt based on its ecological value, allowing to define which areas are best suited for disposal of dredged material and which areas should be avoided.In the first phase the multibeam echo sounding data were analysed in respect to the occurrence of bed forms. The analysis was executed in four steps, resulting in a limited number of classes.A large spatial variation was found for the analysed parameters. The average length of bed forms ranged from 5 m to 50 m, the average height from 0,10 m to almost 2,00 m, while, considering the asymmetry due to tidal currents, most of the sections were flood dominated.In a second phase the seasonal variation of the bed forms was investigated, analysing the multibeam echo sounding data of the summer condition (September 2007).In the third phase of the project the relation between the occurrence of bed forms and hydrodynamic parameters (using a validated numerical hydrodynamic model) was investigated. With the addition of sediment properties (grain sizes) a classification based on the physical parameters is made. These “physiotopes” were finally validated with the ecological data (i.e. Macrobenthos and fish), trying to make a classification in “ecotopes”