Asking the right questions about nutrient control in aquatic ecosystems.

E remains the greatest stressor affecting freshwater ecosystems across North America and Europe. However, over the past five years, the scientific literature has seen a resurrection of the debate over causes of eutrophication, with resulting confusion in management circles. This debate has been poorly defined, fractious, and oftentimes counterproductive. Eutrophication has been defined as the process leading to increased algal productivity in lakes through time. It can be a natural process, as lakes age; or, in the case of cultural eutrophication, it can be facilitated by anthropogenic nutrient inputs. Management concern about eutrophication centers on a change from a desirableoften clear water state, to an undesirable state, exemplified by issues of low hypolimnetic dissolved oxygen, nuisance algal productivity, and in many cases, production of algal toxins. Unfortunately, this may not be a linear change through time, but instead can occur as a state shift or regime change. More unfortunate, is that although eutrophication is reversible, it can take decades or centuries to induce a shift back from a degraded eutrophic state into a more natural, desirable condition. It is this state change and associated degradation of key ecosystem services that matters most to stakeholders and policy makers. Rarely are managers concerned about small, incremental changes in productivity that are of significant interest to science (Figure 1, arrow 1), yet these incremental changes, also correctly termed ‘eutrophication’ are where much the debate appears to derive. The question fundamental to eutrophication control is not “what is the limiting nutrient”, nor is it “can we induce a decrease in algal productivity”, but how can we prevent a state shift into an undesirable state, and how can we push an ecosystem back from a degraded state via nutrient control. If we subscribe to the assumption that a single nutrient is limiting to primary producers, then control of that nutrient will reduce productivity until productivity becomes limited by some other resource. So, where researchers argue that control of nitrogen (N) will reduce algal productivity, this is often correct in ecosystems like many agriculturally impacted prairie landscapes, where phosphorus (P) is replete. However, it remains to be seen whether N control could induce a shift to a desirable state. Although productivity may be controlled, changes may be small, incremental shifts of little interest to managers (e.g., Figure 1, arrow 3). Indeed, to date, it has not been clearly demonstrated that N addition in freshwaters can induce a change to a highly degraded state. More research on this topic is surely forthcoming, and will help clarify the role of N in eutrophication of inland lakes, and the potential importance of N control in remediating eutrophic systems. While the case for N in lake eutrophication, and N control in helping induce a shift back to desired conditions has yet to be made in lakes, the case for N control at a landscape or watershed scale is nuanced, but stronger. Our understanding of responses of rivers to different nutrients is even poorer than our ability to understand responses of divergent lake types. Variable light and flow regimes, temperatures, sediment composition, and nutrient delivery make streams and rivers a more complex mosaic of shifting limitation and variable drivers of eutrophication. As in lakes, there is no question that N affects the algal community, and other aspects of the ecosystem, and the effects of N are likely to vary with N species (nitrate, ammonia, or organic N). Nutrient limitation bioassays (admittedly not at the spatial or temporal scale necessary to fully understand the role