Spatial Modelling of Mussel Farm Production and Nutrient Mitigation Potential in the W Baltic Sea

Eutrophication of coastal water bodies by massive anthropogenic nutrient inputs is a serious global challenge. Suspended mitigation cultures of blue mussels have been suggested as a tool to remove nutrients through harvesting from eutrophic systems like the Western Baltic Sea. The general idea of mitigation mussel farming is that the mussels remove nutrients contained in particles (mainly phytoplankton) directly from the water through their feeding activity and incorporate them into animal tissue during growth. In the present study, mussel farm production and nutrient mitigation potential was estimated for the Western Baltic Sea.

We integrated data from field experiments and national monitoring programs within a sequence of numerical, statistical, and spatial models. Mussel individual growth was estimated by a dynamic Energy Budget (DEB) model and the results were used to make a more simple statistical model of mussel growth versus monthly data of temperature, salinity and chlorophyll-a (Chl a) concentrations. A spatial model estimated long-term (2008-2017) monthly means of environmental variables on 1km² scale based on monitoring data from Denmark, Sweden and Germany. The statistical growth model was imposed on the spatial environmental data and up-scaled to farm production taking bathymetry (farm depth) and mussel densities into account. For the whole study area, water quality affects the growth of individual mussels in the following order of impact: salinity > Chl-a > temperature. When selecting the 10% of sites with highest farm production and nutrient mitigation potentials, the north-west coast of Sweden, the Limfjorden, Mariager Fjord, Isefjord and the area ranging from the Danish Belt Sea to the Kiel Bay in Germany are part of the selection. Site-selection for marine mitigation aquaculture can be an important part of sustainable marine spatial planning considering both farm production potential, as well as environmental and socio-economic goals and interests.