A preliminary environmental impacts assessment of future production sites of mussels on the seabed (culture banks) or in the water column (mussel farms ) in the Danish strait, Limfjorden , has been conducted by DTU AQUA in collaboration with DHI A/S. The assessment was based on a modelling tool developed to assess if mussel productions sites may have a potential significant impact on the nearby Natura 2000 areas or nearby stone reefs, biogenic reefs and eelgrass beds.
The modelling tool is used to simulate the dispersal of faeces from the mussel production sites using a dispersal modelling framework based on DHI’s integrated hydrodynamic and ecological modelling system MIKE 3 FM ECO Lab / ABM Lab. Preliminary results of the model calculations show that the direct sedimentation from the mussel farms takes place just belo w the farm and within approx. 100 m from each farm . Resuspension of mussel faeces due to wave strikes and currents under mussel farms and from culture banks can spread faeces to a larger area. For areas close to production sites with the highest exposure to currents and wave strikes , the calculations show that there may probably be some accumulation of organic matter of up to between 10-20 g C/m2 on average at a distance of 100-500 m from the production sites, depending on the location. In general, the accumulated material will occur in an area from the outline of the production site to deeper water, where the general sedimentation conditions favor sedimentation, or along the depth curve in the dominant current direction along the coast. For comparison, the natural average accumulation of sedimented material from the water column in Limfjorden is estimated to 10 g C/m2. Based on expert judgement, oxygen consumption associated with the settling material can be assumed to be very local under or with in the production sites but may also occur locally and briefly outside the primary impact zone and will not in itself give rise to oxygen depletion in the water column. The calculations in the analysis can be assumed to be generally conservative in the sense that there will be a greater accumulation of material on the seabed and a greater oxygen consumption in the model than can be expected in reality . Positive effects of mussel farming on general sedimentation and basin-scale oxygen consumption have also not been included in the calculations. Also, no cumulative impacts including existing mussel production sites were included. These considerations are currently being addressed in ongoing studies.
Accumulated organic C (gC/m2) as mean of means of May and October 2014 - 2016.