Abstract
Study region: River basins draining into the Baltic Sea, known as the Baltic Sea Drainage Basin (BSDB).
Study focus: Dramatic shifts in water quality have been observed in the Baltic Sea in past decades. This study investigated the spatial distribution of trends in nitrogen (N) and phosphorus (P) in relation to societal, land cover and climatic changes. A 31-year record of observed catchment scale nutrient concentration and discharge data for the period 1970–2000 was combined with climate and land cover data. A Mann–Kendall test was applied to reveal trends in N and P, the N:P ratio, discharge, temperature and precipitation. Classical factor analysis and Kendall's rank correlation identified the most important relationships between nutrients, land cover and climate.
New hydrological insights for the region: A large spatial variability in N and P trends was observed with a notable difference between the east and west of the BSDB. The existence of regional trend variations are important for nutrient load reduction management strategies. Specifically, it is recommended that strategies targeting seawater eutrophication should focus more on P rather than N reduction because increasing P in the eastern catchments is responsible for the overall declining trend in the N:P ratio, an important trigger for algal blooms.