Pelagic and Benthic Nutrient Regeneration Processes in Mussel Cultures (Mytilus edulis) in a Eutrophic Coastal Area (Skive Fjord, Denmark)

Long-line mussel farming has been proposed as a mitigation tool in eutrophic coastal areas as nutrients are removed from the ecosystem upon harvest of the crops and transferred back to land. Further mussels filter the water and thereby increase water transparency and promote benthic plant growth. Intensive mussel farming may, however, negatively affect the nutrient cycling in the local environment through nutrient regeneration in the water column and through sedimentation of biodeposits resulting in organic enrichment of the underlying sediments leading to hypoxic conditions. The objective of this study was to explore the environmental interactions of a long-line mussel farm located in a eutrophic coastal area (Skive Fjord, Denmark) by studying the nutrient cycling in the water column and sediments and assessing their contribution to the nutrient dynamics and oxygen conditions in the fjord. The mussel lines contributed with nutrients, primarily ammonium, to the pool of nutrients in the water column and the contribution increased as the biomass of mussels in the farm increased. The sedimentation of biodeposits was only slightly higher at the farm (51-86 %) compared with a reference site, and the impact on the benthic environment was limited. During most of the production cycle the farm was a net sink of N due to uptake of N in the sediments, but after 1 year, the farm became a net source of N to Skive Fjord. Mussel excretion accounted for similar to 82 % and sediments for similar to 18 % of the N released from the farm. The study shows that mitigation of nutrients by long-line mussel farming will be most efficient, if mussels are harvested within the first year of the production cycle.