Epsilon Archive for Student Projects

Abstract

During the last years, fish farming in Sweden has increased drastically as a result of a growing interest in locally farmed food. As of today, the environmental consequences of fish farms with open cages is not well studied. In this study, we aim to analyze the effects of open cage fish farming on plankton production in nutrient poor, northern lakes (lake Storuman and lake Hornavan). We hypothesize that nutrient leackage through fish farming activities will cause a gradient in nutrients downstream from the fish farm. We also hypothesize that an increased nutrient concentration at the fish farm will cause an increase in primary production in the lake and that experimental nutrient additions close to the fish farm will not increase primary production due to nutrient limitation relief for plankton. Primary production was measured in the lakes through incubation of water together with nutrient additions (N or P) at 0.10m depth and we sampled water for NO3, NH3 and phosphorus in a transect downstream from the fish farm. We then compared our data together with national monitoring data of both chemistry and plankton biomass for the two lakes to visualize and analyze a longer time period with the plankton biomass aiding in the analysis of primary production that we sampled. While primary production tended to be higher in Storuman compared to Hornavan (average 14 Mmol vs 10 Mmol, peak 36 Mmol vs 14 Mmol) this was not statistically significant. The highest measured primary production was observed in the upstream location of Storuman (mean = 25 μ Mmol·L⁻¹) compared to the other sample points. Combining the national monitoring data for water chemistry with our own, we observed higher mean values for ammonium and phosphate in lake Storuman compared to Hornavan. For our measured transect, we observed a significant linear decrease for ammonium and a small linear increase for nitrate based on distance downstream from the fish farm. No such trend was observed for phosphate. We compared algal biomass in Storuman and Hornavan using the national monitoring data comparing with research. Storuman had an average of 1854 μg L⁻¹ while Hornavan had 91 μg L⁻¹. This suggests that Storuman have increased algal growth due to the fish farm. The resulting water chemistry analysis suggests that ammonium concentrations close to the fish farm are significantly elevated compared to other parts of the lake. Despite us not finding a significant effect of primary production on location, the results from the primary production together with the algae concentrations suggest tendencies for elevated primary production in the lake. Lake Storuman does show elevated nutrient levels, but not across the whole lake. Algae concentrations close to the fish farm are relieved of their nutrient deficiencies since incubated bottles displayed no increases in primary production at this particular location. Management decisions have to take into account the risk of eutrophication when operating fish farms in nutrient deficient waters.