Phosphorus control is critical to mitigating eutrophication.

T he Midwest floods of 2008 added more than just water to the region’s lakes, reservoirs, and rivers. Runoff from farms and towns carries a heavy load of silt, nutrients, and other pollutants. The nutrients trigger blooms of algae, which taint drinking water. Death and decay of the algae depletes oxygen, kills fish and bottom-dwelling animals, and thereby creates ‘‘dead zones’’ in the body of water. The syndrome of excessive nutrients, noxious algae, foul water, and dead zones—which ecologists call eutrophication—is depressingly familiar to those who depend on water from rich agricultural regions. The cure sounds simple: decrease inputs of nutrients, especially nitrogen (N) and phosphorus (P). But which nutrient, and how deeply should the inputs be cut? In this issue of PNAS, Schindler et al. (1) present a remarkable 37-year experiment on nutrient management in Canadian lakes which shows that P inputs directly control algae blooms. Surprisingly, however, the authors also observed that algae blooms are made worse if N inputs are decreased without also decreasing P inputs. This finding is of critical importance for current programs aimed at mitigating eutrophication of both freshwaters and coastal oceans. Human activity has greatly increased the inputs of reactive N and P to the biosphere. Reactive N (biologically active forms such as nitrate, ammonia, or organic N compounds, in contrast to N2 gas, which is not used by organisms except for a few nitrogen-fixing species) is supplied by natural sources, as well as by human activities such as industrial N2 fixation, combustion, and planting of soybeans and other N2-fixing crops. Global f lux of reactive N to the biosphere from food production has increased from 15 Tg N year 1 in 1860 to 187 Tg N year 1 in 2005 (2). Additional reactive N is fixed for industrial or household use or is inadvertently created as a byproduct of fossil fuel combustion. Excess reactive N enters groundwater, surface water, or the atmosphere. P enters the biosphere by natural weathering of rock, as well as through mining and other land disturbances by humans. Mined P is used in fertilizers and a host of other products. The global P flux to the biosphere increased from 10–15 Tg P year 1 in preindustrial times to 33–39 Tg P year 1 in 2000 (3). Excess P added to cropland accumulates in soil, which can be eroded to surface water. Global P production appears to be in decline (http://energybulletin.net/ node/33164), suggesting that conservation and recycling of P could help sustain crop production and reduce pollution of surface waters. N and P from farmland runoff or industrial and municipal discharges are associated with widespread and expanding eutrophication of freshwaters and coastal zones (4) (Fig. 1). Globally, over long time scales of centuries to millennia, P appears to be the nutrient that constrains biotic production of freshwater and ocean ecosystems (5–7). However, long-term global averages fail to express the enormous heterogeneity of reactive N and P supplies to particular sites over days to decades—the space and time scales of ecosystem management. Reactive N and P differ greatly in their mobility in the environment. Reactive N is transported rapidly in the atmosphere and hydrosphere. For example, nitrate is highly mobile in groundwater, and ammonia can move far through the atmosphere before entering aquatic ecosystems. In contrast, P tends to be bound to soil or sediment particles or tightly conserved by organisms. Atmospheric transport of P is limited, and erosion and transport of P in particles can be slow. Differences in mobility, combined with great spatial heterogeneity in abundance, lead to considerable variability among ecosystems in supply rates of reactive N and P (8). Therefore, it is difficult to infer drivers of eutrophication from global f luxes alone.