Goods and services of extensive aquaculture: shellfish culture and nutrient trading

Bivalve shellfish play an important role in top-down control of primary symptoms of eutrophication. This short-circuits the process of organic decomposition and promotes an enhancement of underwater light climate, improved oxygenation of bottom water, and restoration of submerged aquatic vegetation. This review analyses this ecosystem service as a potential actor in watershed-level nutrient credit trading programmes and explores the possibilities of implementation of such programmes in Europe. We examine the different components of the issue, including the eutrophication status of European coastal waters, legal and management instruments, and the use of mathematical models at both the ecosystem and farm scales to evaluate the potential removal of nitrogen by cultivated shellfish such as oysters, mussels, and clams. The annual European bivalve shellfish production of over 700,000 metric tons is estimated to generate a nitrogen removal of 46,800 t year(-1), equivalent to 14 x 10(6) population equivalent, and a minimum value of 507 x 10(6) a,not sign. We discuss future directions for this topic in Europe, drawing from ongoing research in the USA and elsewhere, in the light of the twin challenges of European aquaculture expansion and implementation of EU directives.