A bio-physical coastal ecosystem model for assessing environmental effects of marine bivalve aquaculture

A simple lower trophic level, bio-physical marine ecosystem model is developed for the purpose of assessing the environmental effects of bivalve aquaculture in coastal embayments. The ecosystem box model includes pelagic and benthic components and describes the cycling of a most-limiting nutrient. The pelagic compartment is comprised of phytoplankton, zooplankton, nutrients and detritus. These populations interact following predator-prey dynamics and biogeochemical processes. Mixing processes within the bay, and exchange of waters with the adjacent open ocean, are included. The pelagic ecosystem is coupled to a simple benthos containing a dynamically active organic matter pool. Benthic-pelagic coupling includes episodic resuspension, remineralization, sinking, and permanent burial. A population of grazing bivalves is superimposed on this system as a diagnostic variable. The model is applied to a coastal bay and used to determine how bivalve populations affect nutrient cycling in the ecosystem. This is done by examining changes in the standing stock of the various populations, as well as associated nutrient (mass) fluxes, for cases both with and without intensive bivalve culture. It was demonstrated that bivalves divert production from the pelagic to benthic food webs. Phytoplankton and detritus are depleted from the water by bivalve filter feeding and biodeposited to the benthos as fecal matter. This organic loading causes order of magnitude changes in the benthic detrital pool and the associated benthic-pelagic fluxes. It was also shown that water motion and mixing is important in structuring the ecological dynamics in the bay. To faciliate future applications and observational studies, a retrospective analysis of parameter identifiability and uncertainty was also undertaken.