Spatial Ecology
Interaction between biological and physical processes is a predominant cause of spatial structure in estuarine landscapes. As an example, the interaction of saltmarsh plants with hydrodynamic forces influences sedimentary processes on salt marshes, and in the end results in the formation of the spatially structured levee-creek saltmarsh landscape.
We investigate these biophysical interactions and their implications for community structure and ecosystem functioning. We use salt marshes, mussel beds, diatom biofilms, seagrass beds and other estuarine ecosystems as a model system, but try to derive from our studies more generally applicable concepts valid in ecosystems in general.
Aims of the research program
- describe, understand and model the interaction between physical and biological forces shaping estuarine ecosystems.
- study how this biological-physical generation of landscape structure affects intra- and interspecific relations among dominant organisms, and determines biodiversity and system characteristics such as resilience, stability and productivity.
- further our general understanding of how biocomplexity determines the dynamics of populations, communities and ecosystems to other ecosystems.
- use this fundamental understanding to evaluate anthropogenic pressure on estuarine systems, by developing appropriate indicators and predictive modeling tools.
Research Themes
Theme 1: Spatial self-organisation in estuaries
The large-scale estuarine landscape, e.g. the ebb-flood channel systems, are largely the result of physical forces. At smaller scales, however, we observe an increasing influence of biological processes on habitat formation. Organisms affect sediment texture by mixing sand and mud, sediment erosion and deposition rates, wave and current intensity. The landscape therefore shows the imprint of both biological, physical and geological processes.
We study processes of self-organisation: the emergence of landscape-scale patterns from individual-scale interactions between organisms. Examples are mussel beds, saltmarshes, tidal flats and seagrass systems. We describe spatial structure at different scales, using lab, field and remote sensing methods. We measure water and sediment movement in flume experiments and field studies, and manipulate communities in field experiments to test hypotheses. We summarize and formalize our knowledge in mathematical models.
Initially, we were most fascinated by the emergence of regularly patterned systems. However, our interest now shifts to the more general question of self-organisation of bio-physical systems along (physical) gradients. As an example, we want to study how seagrass beds and oyster reefs organize themselves along estuarine gradients, and how their presence subsequently influences land-ocean exchange of nutrients.
Thema 2: Ecosystem engineering in estuarine populations
Estuarine systems self-organize because organisms "engineer" their environment. As an example, saltmarsh plants in pioneer vegetation are very effective in damping waves and currents, leading to better living conditions within dense tussocks. We investigate which morphological and ecological traits (e.g. stem stiffness, plant aggregation) contribute to the "ecosystem engineering" capacity of a species, and how they balance with disadvantages such as increased vulnerability (risk of breaking in flow) or competition for resources.
We use direct measurement, in the field and in a laboratory flume, of drag forces, turbulence levels, erosion-deposition rates to describe the interaction between organisms and currents or waves. We are particularly interested in the effect of ecosystem engineering on the interaction between species: can one species facilitate or hinder the establishment of another species? And how does this effect depend on scale, e.g. short-range positive effects balanced by long-range inhibition effects. We use manipulative field experiments to test hypotheses on species interaction mechanisms through modification of the physical environment and attempt to incorporate these mechanisms into hydrodynamic models.
Thema 3: Effective indicators for 'ecosystem health' in estuaries
With the EU Water Framework Directive (for estuaries and the coastal strip) and the EU Maritime Strategy (for Exclusive Economic Zones and open waters), the assessment of "ecosystem health" based mostly on diversity indicators (e.g., indicator species) has gained much societal importance. However, these indicators tend to neglect spatial variability of estuarine habitats at different scales, as well as possible effects of organisms on the physical structure.
We develop techniques to incorporate these gradients into the indicator systems, including the interactions of organisms and the underlying physical processes. Our aim is to translate current knowledge and predictive modeling capacity to better define expected ecosystem structure in undisturbed systems, as a yardstick for currently observed states. We also develop combinations of station-based sampling and remote sensing as new monitoring approaches for the evaluation of the indicators.
Spatial self-organisation of mussels
Watch how 1750 mussels reorganise spatially upon being homogeneously distributed over the bottom of a tank. Spatial self-organization is the main theoretical explanation for the global occurrence of regular or otherwise coherent spatial patterns in ecosystems. Under homogeneous laboratory conditions, mussels developed regular patterns, similar to those in the field. An individual-based model derived from our experiments showed that interactions between individuals explained the observed patterns.