Bioturbation, sediment fluxes and benthic community structure around a salmon cage farm in Loch Creran, Scotland

Abstract

This study examined bioturbation along an organic carbon gradient away from an Atlantic salmon farm and sought to determine relationships between benthic fluxes, mixing intensity and the infaunal community structure. Macrofaunal community structure, abundance and biomass were examined at stations with varying quantities and qualities of organic matter input. In situ benthic chambers were used to determine oxygen and nutrient fluxes and mixing parameters were derived from down core profiles of chlorophyll a (chl a). Mean oxygen demand of sediments ranged between 8.8 and 467.8 mmol m −2 day −1, being highest beneath the fish farm and indicating very high rates of community respiration and organic matter diagenesis. Oxygen and nutrient fluxes followed similar trends to community abundance and biomass, declining with increasing distance from the farm. Mixing intensity increased with distance from the farm until returning, at the farthest station, to values similar to those measured beneath the farm. The differences in the community structure between sediments beneath the farm and furthest from it suggest that similar diffusive mixing coefficients are generated by different mechanisms. These results generally follow the successional model of Pearson and Rosenberg [Oceanogr. Mar. Biol. Ann. Rev. 16 (1978) 229.], with the exception of the farthest station, but suggest that the bioturbation potential of the community over short time scales is greatest at stations with intermediate qualities and quantity of organic matter. However, the methods used here to assess mixing over short time scales (i.e. diffusive mixing coefficient and the mixed layer depth) do not account for the activities of deep burrowing infaunal animals, such as Maxmuelleria lankesteri, known to be present at the farthest station.