Phosphorus + nitrogen: Let no good deed go unpunished

ENLARGE The Why Files

Two elements have taken center stage in the fight against surface water pollution: nitrogen and phosphorus. Both are essential plant fertilizers, and both can cause serious water pollution.

Overloads of nitrogen can wash downstream to feed algae that monopolize oxygen when they decay, creating fish-fatal “dead zones ” near the mouth of the Mississippi and hundreds of other rivers. Overloads of phosphorus in lakes can cause overgrowth of choking weeds and contributes to disgusting “blooms” of toxic cyanobacteria.

In general, levels of both nutrients have been rising in surface waters for decades.

After years of focusing on nitrogen, regulators turned their attention to phosphorus. Ever-tightening restrictions on releases from sewage-treatment plants, combined with controls on erosion and development, and smarter use of fertilizer, have started a trend toward lower phosphorus levels in U.S. waters.

Now they tell us!

In Science today, however, we read that reducing phosphorus may, ironically, impair removal of nitrogen. After looking at pollution levels in large lakes – including four Great Lakes, Jacque Finlay, of the University of Minnesota, and colleagues found when phosphorus fell, the level of nitrogen rose in 10 of the 12 lakes studied.

The relationship had already surfaced in smaller studies, Finlay says, “but we have made a more general case for the effect.” In many cases, “If you add phosphorus to a lake, you draw down the nitrate.”

Although correlation does not prove causation, Finlay says a mechanism can explain the link: When phosphorus accelerates plant growth, those plants also remove nitrogen from the water. When they die, the plants carry nitrogen to the lake bottom, where it can enter long-term storage.

Nitrogen pollution: An unintended consequence of controlling phosphorus!

Finlay notes that the same high-nitrogen, low-phosphorus relationship appears in the St. Lawrence River, which drains the Great Lakes. But it may not prevail in smaller lakes and rivers, which are frequently flushed by new water.

What is to be done?

Like the effort to control acid rain that began killing forests in the 1980s, the unwelcome phosphorus-nitrogen relationship follows the law of unintended consequences. Acid-rain regulations helped save forests dying in a bath of acid, but the sulfur oxides were, ironically, reflecting solar heat back to space. And so a pollutant that was killing forests was also retarding a planetary warming that is also harming forests.

Against this background, we had to ask Finlay to clear something up: Should we dump phosphorus into large lakes so plants will absorb more of the nitrogen that is entering those lakes from fertilizer, runoff or vehicle exhaust? Finlay wasn’t buying the idea. “I think phosphorus control is a victory; I want to be clear – it’s done a lot of good things. But for large lake systems, we have to make sure we are paying attention to nitrogen, because by controlling phosphorus, we may be making the nitrogen problem worse.”

— David J. Tenenbaum