Ecological Recovery Following Net-Pen Aquaculture : An investigation of nitrogen and phosphorus levels in fish farm-impacted coastal waters of the Bothnian Sea

Eutrophication is a severe problem in the Baltic Sea, particularly due to phosphorus and nitrogen runoff. Previously unaffected areas like the Bothnian Sea are showing increasing signs of nutrient overload. Open-water fish farming may exacerbate eutrophication locally and in the open sea due to excessive nutrient release. This study investigated two sites along the Swedish coast of the Bothnian Sea that were impacted by fish farming for over 20 years before closing in 2020, aiming to assess ecological recovery related to nutrient levels in fish farming affected locations. Water chemical parameters analyzed included dissolved inorganic nitrogen (DIN) and its components ammonium, nitrite, and nitrate, as well as dissolved inorganic phosphorus (DIP), total nitrogen (TOT-N), and total phosphorus (TOT-P). Trend tests, statistical comparisons and ecological status assessments were used to evaluate and compare sites between the impacted locations and an unaffected environmental monitoring station. A limited analysis of temperature effects was conducted. The study found significantly higher quantities of DIN and nitrite in the areas affected by fish farming. No significant trends were observed regarding TOT-P or TOT-N. The ecological assessment showed continued “Good Status” in all sites as well as higher status of TOT-N, although lowering of status regarding TOT-P and DIP in one of the fish farming affected locations. Large nutrient fluctuations suggest internal loading. Oxygen, chlorophyll-a, and macrofauna are discussed as essential to be assessed for thorough status evaluation. Prolonged sampling and sediment testing is found to be needed to estimate recovery time.