It Takes Two to Tango: When and Where Dual Nutrient (N & P) Reductions Are Needed to Protect Lakes and Downstream Ecosystems

Preventing harmful algal blooms (HABs) is needed to protect lakes and downstream ecosystems. Traditionally, reducing, phosphorus (P) inputs was the prescribed sciintion for lakes, based on the assumption that P universally limits HAB formation. Reduction of P inputs has decreased HABs in many lakes, but was not successful in others. Thus, the P-only paradigm is overgeneralized. Whole-lake experiments indicate that HABs are often stimulated more by combined P and nitrogen (N) enrichment rather than N or P alone, indicating that the dynamics of both-nutrients are important for HAB control. The.-changing paradigm from P-only to consideration of dual nutrient control is supported by studies indicating that (1) biological N fixation cannot always meet lake ecosystem N needs, and (2) that anthropogenic 14 and P loading has increased dramatically in recent decades. Sediment P accumulation supports long-term internal loading,, while N may escape via denitrification, leading to perpetual N deficits. Hence, controlling both N arid P inputs will help control HABs in some lakes and also reduce N export to.downstream N-sensitive ecosystems. Managers should Consider whether balanced control of N and P will most effectively reduce HABs along the freshwater-marine continuum.