Long-term phosphorus trends in Swedish rivers and streams : widespread and persistent nutrient decline

Abstract: Phosphorus (P) is often considered the limiting nutrient for freshwater productivity because of its relevant role in regulating algal production and determining community composition. The concern for increasing anthropogenic P inputs registered since the last century has shaped environmental policies around the world to limit the effects of algal bloom (also known as eutrophication) in freshwater ecosystems through the establishment of maximum threshold nutrient concentrations. In addition to water quality impairments due to eutrophication, a recent number of studies have detected critical P declines occurring in various countries of the northern hemisphere. In Sweden, the few research works within the topic have focused on lakes minimally affected by anthropogenic disturbance, showing persistent P declines across the country. The overall aim of the present study was to determine whether analogous nutrient depletion (also known as oligotrophication) could be found in Swedish streams and rivers (n=34) with comparable conditions. Time trends analyses showed that the majority of the statistically significant (Mann-Kendall test, p value <0.05) watercourses had relevant total phosphorus (Tot-P) and phosphate (PO₄P) declining concentrations between 1980 – 2020, and the most important losses were registered for river stations situated in north Sweden. Despite the failure in identifying any correlational relationship between P trends and land use and land cover variables in the present study, further research work is needed to assess the validity of the hypothesised explanatory factors (vegetation, climate change and soil) for the ongoing oligotrophication. In conclusion, the study confirmed that a significant nutrient depletion is occurring in minimally anthropogenic disturbed catchments across Sweden accordingly to previous research works on the subject matter. In view of the obtained results and the significant evidence that oligotrophication is occurring in freshwater ecosystems of the northern hemisphere, water quality policies should complement their reference values with lower nutrient concentration thresholds to assess ecosystem health and consider possible water quality impairments that might develop from nutrient depletion.