Intensified Mussel Farming; Impacts on Nutrient Budgets and Ecology in a Eutrophic Semi‐Enclosed Fjord System

Abstract

Compared to land‐based sources, mussel aquaculture provides food products with a high‐quality protein content and a low carbon footprint. At the same time, mussel cultures store nutrients in their tissue that are removed from the system through harvesting. However, increasingly development of suspended bivalve aquaculture in the coastal zone also comes with a price as the ecological carrying capacity of the ecosystem may be exceeded. The present study aims to support future fjord‐management by estimating the nutrient budgets and ecological impacts of intensified mussel aquaculture in a shallow semi‐enclosed system, the Limfjorden, using 3D ecosystem modeling. Model results showed a net removal of nutrients by suspended mussel cultures at basin scale, whereas at farm scale the efficiency was lower due to increased sediment fluxes. An increase in mussel farming from the current 4 kt‐fresh weight to a future projection of 104 kt‐fresh weight did not exceed the ecological carrying capacity with respect to impacts on sediment chemistry but could cause local declines in benthic bivalve populations. Intense mussel farming provided ecosystem services such as better oxygen conditions and higher Secchi depth together with lower nutrient‐ and chlorophyll a concentrations on basin scale. In addition, there was a redistribution of nutrients, chlorophyll a concentrations, and Secchi depth between sub‐basins in the fjord depending on farming location and intensity. Overall, intensified mussel farming could contribute to the mitigation of eutrophication effects by removing nutrients from land sources and by reducing the local sediment loading.

Plain Language Summary

Mussel aquaculture provides food products with a high‐quality protein content and a low carbon footprint. At the same time, mussel cultures store nutrients in their tissue that are removed from the system through harvesting. However, increasingly development of suspended bivalve aquaculture in the coastal zone also comes with a price as the ecological carrying capacity of the ecosystem may be exceeded. We used a spatial ecosystem model to estimate the impacts of intensified mussel aquaculture on nutrient cycling, natural benthic populations, and water quality in a eutrophic semi‐enclosed fjord system. On basin scale, there was a high nutrient removal by mussel aquaculture that can contribute to the mitigation of eutrophication effects. We found that mussel aquaculture mostly showed positive effects on water quality such a better oxygen conditions and higher water clarity. The carrying capacity was not exceeded in the study area since mussel aquaculture improved the water quality.

Key Points

• Intense mussel farming provided several ecosystem services such as better oxygen conditions, higher water clarity, and lower nutrients

• Nutrient budgets on basin scale showed an efficient nitrogen and phosphorous net removal with intensified mussel farming

• Intensified mussel farming caused a redistribution of water quality elements and benthic bivalves