River mouth water chemistry and supporting hydrometeorological data for oligotrophication assessment.

Phosphorus (P) is often a limiting nutrient in freshwaters and oligotrophication (declining nutrient concentrations), associated with reductions in anthropogenic loading, has occurred in many locations. For the past 40 years, P concentrations have been declining in Swedish lakes and rivers sampled in the national monitoring programe.Here, we present data documenting persistent and widespread oligotrophication of northern rivers (56^o-66^o N) that is apparently unrelated to declining anthropogenic P loading associated with, e.g., diffuse inputs or atmospheric deposition. We present P concentrations measured in river mouths whose catchments drain minimally disturbed subarctic, boreal and nemoboreal Swedish catchments. These concentrations have declined by close to 50%, with more rapid declines in already nutrient poor locations. Analysis of supporting data shows that trends are negatively related to increasing air temperature, pH and alkalinity and unrelated to calcium and organic carbon concentrations and runoff. Temporal 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. This finding highlights the need for a new surface water management paradigm addressing the consequences of nutrient scarcity instead of focusing solely on actions targeting nutrient reductions.