One foot in the grave: zooplankton drift into the Westerschelde estuary (The Netherlands)

The net growth rate of marine zooplankton entering the Westerschelde estuary was investigated using an advective-dispersive transport model that simulates zooplankton biomass behaving conservatively in the estuary. Total biomass of marine zooplankters in the Westerschelde was much lower than what would be expected based on transport alone, indicating negative growth rates in the estuary. Including a net consumption term in the transport model allowed the estimation of total net mortality. About 3% of all marine zooplankters that enter the Westerschelde with the flood currents are retained in the estuary, where they die. On average, 5% of the total marine zooplankton biomass in the estuary died per day. Each year a net amount of about 1500 t of zooplankton dry weight (DW) is imported from the sea to the estuary. Thus in the Westerschelde the marine zooplankton persists mainly due to continuous replenishment from the sea. Average net production/biomass rates of the major marine zooplankton species varied from -0.02 g DW (gDW)-1 d-1 (Temora longicornis) to -0.39 g DW (gDW)-1 d-1 (Pseudocalanus elongatus). In the estuary, the differential mortality of these species resulted in shifts in dominance within the zooplankton community relative to that in the sea. Possible causes of this zooplankton mortality are discussed

MOSES: model of the Scheldt estuary: ecosystem model development under SENECA

The ecosystem model was developed for the Westerschelde, and aims to provide a mathematical description of the Scheldt estuary ecosystem, to determine the origin and fate of organic carbon in the estuary, its role in the foodweb, and especially the relative importance of phytoplankton primary produciton, and to test the possibilities of SENECA as a model development too

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