Phosphorus control is critical to mitigating eutrophication

The 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 flux of reactive N to the biosphere from food production has increased from
15 Tg N year1 in 1860 to
187 Tg N year1 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.