Environmental monitoring in Sørfjorden (Osterøy)

Summary

Fish farming have been conducted in the fjords around Osterøy since the 1970s. There are now two companies operating farms in Sørfjorden; Lerøy Vest/Sjøtroll Havbruk AS and Fjord Drift AS Tombre. Due to poor results on the mandatory B and C surveys on some of the sites, a more extensive monitoring program was initiated. The program aims to monitor the environmental conditions in the area based on water, sediment chemistry and benthic fauna, and will run for five years. During the program's first year, 2016, samples were taken from three stations in Sørfjordens outer (station Ytre), middle (station Midtre) and inner part (station Indre). Samples were also taken on a reference station in a part of the fjord system where there has never been aquaculture activity. This report summarizes results from 2016 and includes a compilation of previous surveys done in this area. Oxygen reduction was observed in the bottom water at all stations, but the oxygen levels were lowest in the middle and inner parts of the fjord. With the exception of the reference station, the monitoring in Sørfjorden in 2016 showed sediment organic carbon content (nTOC) in class III (Moderate) or IV (Poor), where the amount of organic carbon was higher in station Midtre and Indre compared to Ytre. However, the organic loads had the most pronounced negative effect the softbottom communities at station Midtre. Here the soft-bottom community was dominated by opportunistic polychaetes, and were classified as class III (Moderate) based on fauna indices. The general pattern from the oxygen measurements and the measurements of organic carbon to some extent appeared in the fauna samples that were generally poorer in middle and inner parts of the fjord compared to outer parts and the reference station. In addition to the standardized monitoring program, we used isotope studies to investigate the origin of the organic matter in the sediments and further, to investigate whether there have been changes in the isotope profiles between present time and the period before fish farming activities were started in the fjord. Radioactive isotopes were used to date sediment cores and stable natural isotopes (15N and 13C) were used to investigate the origin of organic matter. There was a gradient from the innermost to the outermost part of the fjord, where the isotope profiles indicated a more marine and less terrestrial influence in station Ytre compared to the other stations. The dating of the cores showed a high sedimentation rate in the fjord, especially in the inner parts. Some of the cores were therefore not long enough to cover the period without any farming in the fjord. However, all cores were long enough to cover the period when aquaculture was in its very early beginning in the fjord. There were very small differences in the isotope profile between the upper parts of the sediment core (present), and the deepest parts of the core that represented the period with little or no aquaculture activity. An isotope mixing model was run for Midtre station, where the sources included river sediment, soil, plant residues, marine POM, POM brackish water, sludge from fish farms and sewage. The results indicate that soil and marine POM were the most prominent sources, while sludge from fish farming constituted a relatively small contribution.