Quantification and valuation of the potential of shellfish ecosystem services in mitigating coastal eutrophication

Eutrophication of coastal waters due to anthropogenic nutrient enrichment is a significant problem in European seas and worldwide. Bivalve shellfish remove nutrients from the water through filtration of suspended particles. This 'bioextraction' capacity is a key regulating ecosystem service that contributes to eutrophication control, but the extent and value of this ecosystem service has not been well established. This study aims to assess the potential of shellfish ecosystem services in mitigating coastal eutrophication in European coastal waters. The estimation of the amount of nutrients removed by different bivalve species under specific culture practices and locations is relevant due to the variety, geographical distribution, and scale at which shellfish species are farmed in Europe.The approach used here consisted of (a) estimation of nutrient loading to European regional seas, and source apportionment where possible; (b) evaluation of nutrient removal by key species of bivalve shellfish; (c) analysis of the role of shellfish in top-down control of eutrophication as a complement to the established bottom-up approach of emissions reduction.The nitrogen removal capacity of bivalves was assessed using two complementary approaches: (i) elemental analysis of N concentration in soft tissues and shell; and (ii) modelling the physiology of shellfish at the typical farm. In both cases, the results were upscaled to the shellfish aquaculture production in Western Europe.Our results show that European shellfish aquaculture can help reduce negative water quality impacts of excess nutrients (nitrogen and phosphorus) in coastal communities. Different shellfish species have different removal rates, which can also be influenced by environmental conditions at the distinct locations and culture practices. The species responsible for the largest N removal (3356-3491 tonnes y-1) was the Mediterranean mussel (accounting for 53-70% of the total) which is also the farmed species with highest production in Europe. Our study shows that the EU annual current production of over half a million metric tonnes of bivalves removes between 4.8 and 6.5 kilotonnes of N per year. The annual cost of removing the same amount of nutrients using other measures would be between 15.9 and 21.6 billion euro. This ecosystem service has not been used in Europe as part of a nutrient management framework. The results of this study aim to provide the basis for strategic guidelines to include shellfish aquaculture in watershed-scale nutrient management policies in the EU, following the same principles as the on-going programmes in the United States.