Mussel transplantation as a tool to mitigate hypoxia in eutrophic areas

Mass mortality of blue mussels occurs every summer due to hypoxia in eutrophic estuaries and the oxygen consumption of decaying mussels is suspected to accelerate the hypoxia. In the present study, the solution of transplanting 3600 and 6000 t-WW of mussels from donor areas with frequent hypoxia to more suitable growth areas was tested as a tool to mitigate hypoxia in the Limfjorden. The environmental effects in the donor area were estimated by a 3D ecological model in FlexSem. Model results showed that hypoxia mitigation was most efficient at water depths of 3–5 m with highest changes in mussel biomass and oxygen levels close the ecosystem hypoxia threshold of 3.4 mg l⁻¹. Mussel biomass soon recovered from the fishing pressure during summer due to the better oxygen conditions. These effects led to reduced Chl a concentrations and increased Secchi depths in the model. Negative effects of dredging on Secchi depth and oxygen levels were short-lived, locally and exceeded by the positive effects during the following summer period. In conclusion, mussel transplantation can prevent mass mortalities and limit the extent of the hypoxic events and the associated negative environmental effects, which is important knowledge for the management of eutrophic systems.