A simulation analysis of continental shelf food webs

Abstract

Energy flow through continental shelf food webs was examined using a simulation model. The model structure expands the two traditional marine food chains of phytoplankton-zooplankton-pelagic fish and benthos-demersal fish into a complex web which includes detritus, dissolved organic matter (DOM), bacteria, protozoa, and mucus net feeders. Simulation of energy flux for different shelf systems using the expanded web revealed that heterotrophic microorganisms and their predators account for a significant component of the energy flux in the continental shelf ecosystem. Contrary to previous models, where all phytoplankton were considered to be grazed by zooplankton, our simulation results indicate that only slightly more than 50% of the annual net primary production is grazed. A substantial quantity of the phytoplankton production directly becomes detritus. Bacteria mineralize detritus and DOM produced by phytoplankton and other components of the food web, converting these to biomass with high efficiency. Consequently, the model predicts that planktonic bacterial production is equivalent to zooplankton production. Exclusion of the bacteria requires the assumption that all DOM is either exported from the system or consumed by another component of the food web. Neither of these assumptions can be supported by present knowledge of the dynamics of DOM in the sea. Model simulations were also employed to test the hypothesis that production exceeds consumption on continental shelves, resulting in exports of 50% of the annual primary production. Simulations of shelves with high rates of primary production resulted in a particulate export of 27% and realistic estimates of secondary production. Results of other simulations suggest that shelves with lower primary production cannot export production and still maintain the macrobenthos and their predators. General properties about continental shelves can also be inferred from the model. From simulations of shelves of differing primary production, nanoplankton are predicted to account for a greater proportion of the primary production in nutrient limited systems. Benthic production appears to be related to both the quantity of primary production and the sinking rates of the phytoplankton. The model indicates that zooplankton fecal inputs to the shelf benthos are only a small portion of the total detrital flux, leading to the prediction that fecal pellets are of little significance in determining benthic production. Finally, the model generates production efficiencies that are highly variable depending on the type of system and kind of populations involved. We argue that the assumed ecological efficiency of 10% should be abandoned for continental shelves and other ecosystems.