Drivers of hypoxia variability in a shallow and eutrophicated semi-enclosed fjord

Impacts of climate change on water quality, benthic mussels, and suspended mussel culture in a shallow, eutrophic estuary

Climate change is a global problem that causes severe local changes to marine biota, ecosystem functioning, and ecosystem services. The Limfjorden is a shallow, eutrophic estuary influenced by episodic summer hypoxia with an important mussel fishery and suspended mussel culture industry. Three future climate change scenarios ranging from low greenhouse gas emissions (SSP1-2.6), to intermediate (SSP2-4.5) and very high emissions (SSP5-8.5) were combined with nutrient load reductions according to the National Water Plans to investigate potential impacts on natural benthic mussel populations and suspended mussel culture for the two periods 2051-2060 and 2090-2099, relative to a reference period from 2009 to 2018. The FlexSem model combined 3D hydrodynamics with a pelagic biogeochemical model, a sediment-benthos model, and a dynamic energy budget - farm scale model for mussel culture. Model results showed that the Limfjorden was sensitive to climate change impacts with the strongest responses of physics and water quality in the worst case SSP5-8.5 scenario with no nutrient reductions. In the two low emissions scenarios, expected improvements of bottom oxygen and Chlorophyll a concentrations due to reduced nutrient loads were counteracted by climate change impacts on water physics (warming, freshening, stronger stratification). Hence, higher nutrient reductions in the Water Plans would be needed to reach a good ecological status under the influence of climate change. Suspended mussel culture was intensified in all scenarios showing a high potential harvest, whereas the benthic mussels suffered from reduced food supply and hypoxia. Provided the environmental changes and trends in social demands, in the future, it is likely that suspended mussel cultivation will become the primary source of mussels for the industry. Model scenarios can be used to inform managers, mussel farmers, fishermen, and the local population on potential future changes in bivalve harvesting and ecosystem health, and to find solutions to mitigate climate change impacts.

Modelled dispersal pathways of non-indigenous species in the Danish Wadden Sea

The introduction-rate of non-indigenous species (NIS) to coastal water bodies has accelerated over the last century. We present a model study assessing the fate of NIS released in likely point sources of the Danish Wadden Sea. We show that NIS-particles released in the deep North Sea are generally transported away from the Wadden Sea, while those released in the coastal North Sea and the Wadden Sea show large variability in track pattern and settlement location. Consequently, the introduction of NIS from ships entering the port of Esbjerg pose a threat to the Wadden Sea through primary and secondary spreading, while transport of species from sources in the south likely causes a slow and steady settling of NIS in the Wadden Sea and coastal North Sea. The study points to the importance of enforcing an efficient monitoring system to ensure early detection of changes to the species composition of the Wadden Sea.