Existing data on environmental impacts related to mussel farming in the Baltic Sea

Mussel farming has been proposed as a measure to reduce symptoms of eutrophication. The present review, produced as an activity of the Baltic Blue Growth (BBG) project, compiles current knowledge about environmental effects of mussel farming in the Baltic Sea. The existing data has also been catalogued and standardized into a GIS-useable format, which will be published online in the Baltic blue mussel farming Operational Decision Support System (ODSS).
The main positive environmental effect of mussel farming is that nutrients are removed from the marine ecosystem upon harvest of the farmed mussels. The uptake of nutrients, through filtration of phytoplankton and other particulate matter, may result in reduced ambient concentrations of nutrients and phytoplankton, resulting in an increased water transparency. The improved light conditions may in turn promote benthic macroalgal growth and increase overall coastal species richness.
Intensive mussel farming may also have local negative impacts on the environmental due to nutrient regeneration in the water column and through sedimentation of biodeposits (e.g. faeces). The organic enrichment of the underlying sediments may lead to hypoxic conditions and release of nutrients from the sediments. The overall environmental impact of mussel farms may differ between sites and change over time during the production cycle. Negative effects of mussel farming can probably be avoided or reduced if choosing a location with welloxygenated sediments. The environmental monitoring of mussel farms within the BBGproject will help in evaluating environmental impact and in identifying key factors to optimize the positive environmental effects of mussel farming.
On-going national environmental monitoring programs will not detect any direct environmental effect of mussel farming, unless the monitoring stations are localized very close to the mussel farms. National monitoring can be used to evaluate long term and large scale changes in nutrient concentrations within the Baltic Sea, and serve as reference to mussel farm monitoring if the same methodologies are being used. National criteria for assessing ecological status within the Water Framework Directive can be used for evaluating environmental status at a mussel farm area, and for mussel site selection. Future studies of environmental impact of mussel farming in the Baltic Sea should have special focus on the sediments (e.g. nutrients, oxygen condition and benthic fauna) and water transparency. Data is also needed for other water quality parameters to achieve a deeper understanding of the overall impact of mussel farming on the marine ecosystem.