Environmental impacts of coastal fish farming: carbon and nitrogen budgets for trout farming in kaldbaksfjørour (Faroe Islands)

ABSTRACT: Flow of organic carbon (OC) and nitrogen through a sea cage trout farm was calculated on
the basis of detailed studies of the farming operation, water circulation, OC and nutrient transport and
recycling processes in sediment. A third of the OC and nitrogen provided by fish food was incorporated
into fish biomass, which is more than has been found in previous studies. Most OC input was respired
by the fish (52 to 70%), and ~63% of the associated nitrogen was lost as dissolved inorganic nitrogen
(DIN), potentially stimulating pelagic primary production. Approx. 6% of carbon and 5% of nitrogen derived from fish food settled on the seabed, where it was either mineralized or accumulated in the sediment. Based on transect measurements of diagenetic activity, the farm footprint was found to cover an
area ~10 times the farm area. OC mineralization in the sediment increased linearly with increasing food
input; the divergence between carbon efflux and oxygen uptake in sediment likewise increased with increasing food input, reflecting an increasing level of sediment reduction. Directly below the farm, the
dissolved organic carbon (DOC) efflux was high (on average 53% of dissolved inorganic carbon efflux),
indicating that DOC efflux is an important pathway for benthic carbon release below aquaculture farms.
Overall, microbial processes removed 56 and 38% of OC and nitrogen, respectively, that settled to the
seabed. During a 39 d break in farming activity, due to the combined effect of mineralization and resuspension of surface sediment, sediment conditions improved considerably