Widespread and persistent oligotrophication of northern rivers

Abstract Phosphorus (P) is often a limiting nutrient in freshwaters1 and oligotrophication (declining nutrient concentrations), associated with reductions in anthropogenic loading, has occurred in many locations2,3,4,5. Here, we report on persistent and widespread oligotrophication of northern rivers (56o-66o N) that is apparently unrelated to declining anthropogenic loading. Over the past forty years, P concentrations in rivers draining minimally disturbed subarctic, boreal and nemoboreal Swedish catchments have declined by close to 50%, with more rapid declines in already nutrient poor locations. Trends are negatively related to increasing air temperature, pH and alkalinity and unrelated to calcium and organic carbon concentrations and runoff. Trends were strongest in rivers draining catchments with shallow, nutrient poor soils lying above the highest postglacial coastline. Today, P concentrations in most study rivers are below expected background (reference) levels, suggesting we are entering a new water quality regime. These widespread and ongoing P declines are likely to further increase oligotrophication of northern rivers and highlight the need for a new surface water management paradigm addressing the consequences of nutrient scarcity instead of focusing solely on actions targeting nutrient reductions.