Thorleifur Eiríksson, Leon Moodley Gudmundur Vídir Helgason, Kristján Lilliendahl, Halldór Pálmar Halldórsson, Shaw Bamber, Gunnar Steinn Jónsson, Jónatan Thórdarson and Thorleifur Águstsson .2017. Estimate of organic load from aquaculture - a way to increased sustainability. RORUM 2017 011.

Abstract

Aquaculture derived organic matter (AOM) is an aberrant supply of OM to the seafloor and environmental sustainable aquaculture and fish health depend on a sound understanding of quantity and impact of AOM entering the seafloor ecosystem. In Iceland, there are a very limited number of direct measurements of organic carbon in sediments around fish farms. In this pilot study, we examined sediment organic carbon(Corg) content and reactivity along three transects around a fish-farm in Berufjordur, east Iceland. Utilizing stable carbon and nitrogen isotopes in sediment and dominant macrofauna taxa, the spatial impact and trophic resource significance of AOM was estimated. Sediment close to cages are sites of Corg accumulation characterized by extremely high Corg content of very high reactivity. The number of species is lowest nearest to the cages and that the number of species increases further away except in transect 3 (in opposite direction of the current) where the number of species on station 3-B is highest. The same trend is seen in Shannon diversity indices. Sediment Corg content at reference sites ranged from 135 – 148 gC m-2 and 220-325 close to the cages. While highest values are found in direct vicinity of cages, they are less than twice that of reference sites. Sediment Corg content are relatively high in these fjord sediments supporting rich macrofauna. Sediment oxygen consumption rates is ranging from 31 to 1153 µmol O2.gC-1hr-1 . Extremely high contents of very labile Corg is found in close vicinity of the cages. Reactivity of organic carbon (measured as sediment oxygen consumption rates) is thus a very sensitive indicator of ecological footprint along transects. Strongest impact were along transect 1, with direction of residual current. Capitella capitata was most dominant macrofauna species close to cages and its fauna isotope signatures suggest AOM as trophic resource. Away from cages fauna isotope signatures suggest utility of natural marine sources. The different tracers used in this study suggest limited spatial seafloor impact of AOM and trend suggest that impact from fish farm is restricted to area close to cages. There is a strong negative correlation between species diversity and Reactivity of Corg in surface sediment which makes that measurement a potential tool for monitoring impact from aquaculture in sea cages.