SEASONAL AND SPATIAL VARIATIONS OF BENTHIC IMPACTS OF MUSSEL LONGLINE FARMING IN A EUTROPHIC DANISH FJORD, LIMFJORDEN

Limfjorden has a long tradition of mussel production based on wild stock fisheries. As a result of environmental concern of adverse effects of fishery, farming of mussels on off-bottom longlines has been well established during the past 5 y. Limfjorden is heavily eutrophic, resulting from intensive agricultural production in the catchment area of the estuary. Stimulation of sedimentation and subsequent benthic impacts of mussel farms were investigated with a focus on seasonal and spatial variations. The seasonal study was performed along a transect from inside one of the mussel farms to a reference station 75 in northeast of the farm every season during I y (summer to summer). The effects depended significantly on the variation in hanging mussel biomass along the transect as the effects on benthic total oxygen uptake, nutrient release, and sulfate reduction rate was at its maximum at the station with the largest hanging biomass. In addition, stimulation of the mussel farm was strongest during the summer when NH4+ and PO43- release was increased by 400% and 740%, respectively, compared with the reference station. The concentration of reduced sulfur was only significantly Stimulated (58%) by the mussel farm during the second summer season, which was characterized by total oxygen depletion in the bottom water and linked to the unavailable oxygen for reoxidation. A significant spatial variation in the benthic impacts was also observed. Eight differently situated mussel farms with variation in water depth and mussel biomass on the longlines were examined. The Stimulation on total sedimentation rates, total oxygen uptake (TOU), and NH4+ and PO43- flux of the mussel farms was in the range of 0-92%, 0-67%, 52-290%, and 0-700%, respectively. Furthermore, stimulation of TOU and NH4+ flux was significantly linked by linear regression to the sedimentation rate of particulate organic carbon and particulate organic nitrogen, respectively. Hence, this study shows strong evidence that mussel farms stimulate the benthic degradation rates in an area that is already highly affected by other sources of organic input.