Effects of organic effluents from a salmon farm on a fjord system. III. Linking deposition rates of organic matter and benthic productivity

Abstract

The vertical flux of organic matter and infauna production, biomass and abundance at six stations along a gradient away from a salmon farm, located in a western Norwegian fjord at a depth of 230 m and which produced 2910 tonnes of fish in 19 months, was measured repeatedly during an entire on-growth cycle. The study showed that large increases in benthic secondary production can result from the loading of organic waste in nutrient-poor systems like fjords and a strong correlation between annual sedimentation rates of particulate organic matter (carbon and nitrogen) and annual benthic secondary production was found. At the stations that received the bulk of the waste matter (stations situated within 250 m from the mooring point of the farm) infauna production followed feeding regimes at the farm and in 2004 annual benthic secondary production was 40 g ash-free dry weight m − 2 , 50 times as high as the production 550 to 3000 m away from the farm. Production close to the farm was mainly due to the polychaete Heteromastus filiformis and the bivalve Abra nitida in periods with moderate loadings of organic matter, and by the polychaete Capitella capitata in periods with high loadings. The results of the study further indicate that the threshold for increased infauna production in this deep benthic ecosystem had been reached at an annual flux of 500 g C m − 2 and that continuous loadings at this magnitude over time might cause overloading of fish farm localities.