Using Blue Mussels as a Tool for Mitigating Eutrophication in the Baltic Sea

Sammanfattning: Eutrophication is a consequence of excess nutrients in the water which leads to increased algaegrowth, reduced water transparency and hypoxic bottoms. This is the biggest environmental problemfor the Baltic Sea which recently has resulted in stricter legislations and other initiatives to help theBaltic Sea to recover. However, the actions to reduce the nutrient input to the Baltic Sea have so farmainly been land-based. These actions seem to not be enough since the eutrophication continues tobe a problem for the Baltic Sea. Farming blue mussels has shown to have a mitigating effect on theeutrophication and could thus be a complementary action. Blue mussels are filter-feeding specieswhich means that they filter water for food and thus eat phytoplankton and accumulate nutrients atthe same time. When the blue mussels are removed from the sea, so is the nutrients accumulated inthe mussels, resulting in a mitigation of nutrients and thereby the eutrophication. Due to the brackishwater with the low salinity in the Baltic Sea, the blue mussels farmed there do not grow bigger thanaround 3 cm. This means that the mussels are not suitable for human food production and theharvested mussels need to be used for something else, even though the farming itself is anenvironmental action. Three possible mussel products from valorisation of the Baltic Sea blue musselshave been identified; producing mussel meal, biogas or compost.Region Östergötland is involved in a project, Baltic Blue Growth, with the main objective to study howto use mussel farming as an environmental measure and which of the three valorisation options is themost beneficial from an environmental perspective. This study is a part of their investigation to reachtheir goal and will study their mussel farm in St. Anna and the three valorisation options from anenvironmental perspective. The aim of this study is thus to investigate the net nutrient reduction froma mussel farm in the Baltic Sea in combination with the contribution to climate change. This is donefrom a life cycle perspective to include the valorisation of the mussels into the different productsmussel meal, biogas or compost. For this, an existing farm in the archipelago of St. Anna, Östergötland,Sweden is studied. The main results show that there is a nutrient reduction from the mussel farm andthis is not majorly affected regardless of which valorisation option that is chosen. However, the musselfarm does have an impact on climate change and the magnitude of the impact varies for the threevalorisation options. The results of the sensitivity analysis show that the result from the life cycle canbe improved with future improvements of the mussel farm and transportation. The nutrient reductioncan become larger and the impact on the climate change can be reduced. Outside the result from thelife cycle analysis it is discussed that there are other future improvement possibilities in the productionof the mussel products, which would impact the result. The mussel farm and the mussel products alsohave other positive impacts that is not included in the life cycle analysis but discussed in the study,such as increased water transparency, recycling of nutrients and reduction of over fishing. However,the mussel farm could also have negative impacts, such as emissions of microplastics and locallyincreased sedimentation which affect the hypoxia. Those are discussed in this study but the probabilityand possible impact of them are not fully investigated and need further research.