Organic carbon accumulation in Hardangerfjorden - A micropaleontological and geochemical study of benthic environmental impact from fish farming

Abstract

The present study investigated the impact of organic loading from fish farm waste on the benthic environment surrounding a fish farm, and the organic carbon accumulation in fjord sediments in a Western Norwegian fjord. One sediment core collected 100 m from the fish farm was compared with a control core collected 500 m from the farm. Both cores were radiometrically dated to 1927. Interpretation of depositional environment was done based on grain size distribution analysis. Benthic foraminifera (protists) were utilized to assess the ecological quality status (EcoQS) of the benthic environment in each sample, based on foraminiferal species diversity, foraminiferal assemblage composition and benthic foraminiferal accumulation rates (BFAR). Total organic carbon- and total nitrogen content in the sediments, together with stable C- and N-isotope ratios and C- and O-isotope ratios in the tests of selected foraminiferal species, were used to interpret the source of organic matter in the sediments. Heavy metal concentrations were measured and the sediments classified according to official guidelines for sediment pollution. Stable isotope signals and carbon- and nitrogen content was stable throughout the period since 1927 until present-day in both cores, and showed no evidence for increased organic loading related to fish production. Both cores had C/N-ratios characteristic of marine organic matter in sediments deposited after 1927. Foraminiferal species assemblage compositions reflected naturally different benthic environments in the two cores, and showed no clear response to fish production. EcoQS was ‘good’ in both cores. BFAR in the control core increased since the onset of fish production.