Widespread and persistent oligotrophication of northern rivers.

Phosphorus (P) is often a limiting nutrient in freshwaters and most management actions aim to reduce eutrophication associated with excess anthropogenic P inputs. Here, we report on the opposite problem, persistent and widespread oligotrophication (i.e., declining P concentrations) in northern rivers (56-66 N) that appears unrelated to reductions in anthropogenic loading. Over the past forty years, P concentrations and fluxes in rivers draining forest dominated Swedish catchments have declined by nearly 50 %, with steeper declines in nutrient poor locations. Trends are negatively correlated with forest growth, temperature, pH and alkalinity. They are unrelated to trends in calcium, organic carbon and runoff. Declining P trends were strongest in locations draining catchments with shallow, nutrient poor soils and P concentrations in most locations are currently below estimated reference levels. These widespread and ongoing P declines highlight the need for new surface water management paradigms addressing the consequences of both nutrient scarcity and surplus.