Production capacity of estuarine ecosystems and filter feeder stock size: indices for under- or overgrazing

For extensive shellfish culture, the trophic capacity of the culture system is a major production factor and this is determined by the primary production, the flux of food to the culture sites and the role of competing filter feeders. Various control mechanisms play a role in these processes: bottom-up control of primary production may shift to top-down control at increasing filter feeder stocks. Nutrient limitation, hence bottom-up control, may decrease through enhanced nutrient regeneration, while increased grazing pressure will reduce phytoplankton biomass to the benefit of picoplankton and macro-algae. To evaluate the production capacity of a given ecosystem, as well as the potential and the impact of expanding shellfish culture, there is a need for simple parameters, in addition to the development of more sophisticated simulation models. Efforts have been made to develop indices for shellfish culture capacity evaluation, such as the ratios of clearance conjunction with a massive invasion of exotic oysters and razor clams; the outcomes will be discussed. time, residence time, primary production time and nitrogen turnover time, and the ratio between pico- en microphytoplankton. These indices are now further tested in a project in Dutch coastal waters to evaluate the impact of increasing the mussel biomass in