The Westerschelde estuary: two food webs and a nutrient rich desert
Hamerlynck, O.; Mees, J.; Craeymeersch, J.A.; Soetaert, K.; Hostens, K.; Cattrijsse, A.; Van Damme, P.A. (1993). The Westerschelde estuary: two food webs and a nutrient rich desert, in: Progress in Belgian Oceanographic Research, Brussels, January 21-22, 1993. pp. 217-234
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| Available in | Authors |
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Document type: Conference paper
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| Keywords |
Aquatic communities > Plankton > Phytoplankton
Aquatic communities > Plankton > Zooplankton
Biological production > Primary production
Cycles > Chemical cycles > Geochemical cycle > Biogeochemical cycle > Nutrient cycles
Food webs
Nutrients (mineral)
Nutritional types > Autotrophy
Nutritional types > Heterotrophy
Population characteristics > Biomass
ANE, Netherlands, Westerschelde [Marine Regions]
Marine/Coastal; Brackish water |
| Abstract |
Hummel et al. (1988) hypothesised the concomitant existence of two separate food chains in the Westerschelde: a photo-autotrophic coastal food chain in the marine part and a heterotrophic chain in the brackish part. The present study intends to re-examine the hypothesis on the basis of recently published data. Biomass gradients of the important functional units along an estuarine transect were observed to differ from those reported by Hummel et al. (1988) in some important aspects. The biomodal primary production gradient reported by Spaendonck et al. (in press) does not resemble the phytoplankton biomass curve, gradually increasing from the sea to Antwerp proposed by Hummel et al. (1988). Estimates of mesozooplankton biomass were found to be about an order of magnitude lower than reported by Hummel et al. (1988) and to display a completely different and more complex spatial pattern. However, the new gradient found is more in line with the hypothesis of two food chains than the gradient reported by Hummel et al. (1988). In the macrobenthos the biomass peak in the brackish part reported by Hummel et al. (1988) could not be confirmed. This finding does not falsify the original hypothesis as the function of this detritus dependent macrobenthic fauna is largely taken over by the hyperbenthic mysids, a group of previously unknown importance in the system. The existence of two food chains is also supported by the gradients observed in fish and epibenthic invertebrates, functional units not addressed by Hummel et al. (1988). In the zone between the two different food chains the dominant animal groups of the pelagic system have only a low biomass, this is the nutrient rich desert of the title. The zone upstream of the Dutch-Belgian border supports no hyperbenthos, no epibenthos and no mesozooplankton because of the low dissolved oxygen concentrations (less than 40% saturation), but there is a prominent peak in the microzooplankton. Clearly, in the brackish part, the richness of most functional units can only be explained on the basis of an input of organic matter from outside, consumed through a heterotrophic food chain. A second, smaller peak is observed close to the mouth of the estuary and is dependent on the primary production in the marine part of the estuary. Even for individual species this clear bimodal pattern can be observed. This disqualifies simplistic physiological models of estuarine succession as a basis for the findings. In the oxygenated part of the system there is no good general correlation between macrobenthic biomass (mostly suspension-feeders) and primary production. Macrobenthic biomass is highly variable in this zone, probably as a result of local differences in current velocity maxima. The new data confirm the view of Hummel et al. (1988) but it is concluded that these authors must have formulated their hypothesis intuitively and could not have done so from the data available at the time. |
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