Temporal and spatial trends in eutrophication status in the Oslofjord, Norway

The eutrophication status of the Oslofjord, Norway, was assessed using 1) existing monitoring data, 2) established assessment criteria, and 3) an updated version of the HELCOM Eutrophication Assessment Tool (HEAT), which is widely employed in Europe. The analysis, covering the period from 1960 to 2023, revealed persistent 'eutrophication problem areas' in specific regions of the Oslofjord, particularly in inner areas and those situated downstream of large catchments and urban centres. This assessment demonstrated that the Oslofjord has been classified as a 'eutrophication problem area' for more than five decades. These findings, including basin- and criteria-specific distance-to-target estimates, provide a foundation for establishing evidence-based nutrient management strategies in the Oslofjord and its upstream catchments. In the absence of reduced nutrient inputs, a significant portion of the Oslofjord will continue to be classified as a 'problem area'.