A model study of the regulation of blue mussels by nutrient loadings and water column stability in a shallow estuary, the Limfjorden

Danish estuaries are characterised by being shallow, eutrophic and with high benthic biomass dominated by suspension feeders. However, the standing stock of benthic fauna is often higher than would be expected from the relation to residence time of sea water or to annual primary production found in other estuaries. The present study investigated how the biomass of benthic suspension feeders was regulated by food supply produced due to eutrophication and supplied to the benthos by vertical mixing. This was tested using a 1D water column model calibrated and validated against monitoring data from a 5 years period in the Limfjorden. The model described vertical hydrodynamics and -pelagic biogeochemistry, sediment dynamics and was coupled to a Dynamic Energy Budget model for blue mussels. The modelled biomass of blue mussels showed a dome-shaped relationship to water column stability that was most pronounced combined with high nutrient loadings. At strong stratification, there was little coupling between the upper productive layer and mussels causing food limitation. In addition, hypoxia induced mass mortality of mussels during summer at high nutrient loadings. Under well-mixed conditions, mussels were capable of depleting phytoplankton in the whole water column and food limitation occurred resulting in a lower annual mussel biomass. The optimal conditions for growth of benthic suspension feeders in the long term would therefore be periods of stratification that are long enough to allow the phytoplankton to bloom at the surface layer and short enough to prevent benthic suspension feeders from starving or suffer from hypoxia.