Study finds controlling phosphorus pollution in wetlands more important than believed

A study led by a Duke University scientist suggests that the current emphasis on controlling upstream nitrogen pollution fails to adequately address the impacts on water quality of another potential contaminant, phosphorus. Thus, according to the scientists, current strategies used by environmental managers to control excessive nutrients in coastal wetlands may not achieve their intended goals.

The finding was published in a report in the Friday, Jan. 24, 2003, issue of the journal Science by Pallaoor Venkatesh Sundareshwar, a research associate and instructor at the Duke University Wetland Center in the Nicholas School of the Environment and Earth Sciences and co-authors James Morris and Brandon Fornwalt from the University of South Carolina at Columbia, and Eric Koepfler from Coastal Carolina University in Conway, S.C. The study was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration.

Sundareshwar and his co-authors worked in a pristine wetland at the University of South Carolina’s Baruch Marine Field Laboratory, near Georgetown, where organisms’ natural interactions could be studied in the absence of human-caused pollution.

Both the phosphorus originating in upstream fertilizer applications, and the nitrogen derived from lawn and agricultural fertilizers or animal livestock operations can run off the land and flow downstream to shallow wetland estuaries, where they can cause algae blooms and fish kills that can threaten critical seafood nursery areas.

Managers have emphasized controlling nitrogen because that nutrient can lead to highly visible algae “blooms” in estuaries, which can turn the water green, Sundareshwar said in an interview. “People tend to be driven by what they see. But what we have shown is that’s not the whole truth; there is a major response to phosphorus by bacteria, which you can’t see.”

By treating test plots with measured amounts of nitrogen and phosphorus, and comparing those results with untreated plots, the scientists learned that whereas plants visibly respond to nitrogen fertilization, bacteria in saturated wetland soils respond to phosphorus, not nitrogen. Bacterial responses to phosphorus pollution thus inconspicuously mimic the response of algae to nitrogen.

When nitrogen pollution leads to a surge of algae in coastal waters, subsequent algae die-offs release nutrients and carbon that the bacteria use for growth, and in doing so rob the water of needed oxygen, he added.

Extra phosphorus causes the bacteria to undergo a growth spurt and also consume any available organic matter, Sundareshwar said. In removing the carbon from the organic matter the bacteria take up oxygen as well. When coastal waters are over-enriched with phosphorus, bacteria can thus consume available carbon and remove enough oxygen from the water to potentially harm fish, even if there is no excess nitrogen in the water to cause algae blooms.

“Gone are the days of saying ’nitrogen, that’s the only thing,’ or ’phosphorus, that’s the only thing,’” Sundareshwar said. “I’m saying it’s high time we start looking at a more integrated approach to coastal management.”

Not only do plants and bacteria in a coastal wetland respond to different nutrients; the tie between phosphorus supplies and bacterial growth also affects inputs and outputs of nitrogen in a wetland ecosystem, Sundareshwar and his co-authors report.

Among certain “legume” plants such as soybeans that grow on dry land, phosphorus fertilization increases nitrogen fixation by “symbiotic” bacteria residing in plant roots. These bacteria convert nitrogen from the air to a chemical form that acts as a plant fertilizer. Fixing that nitrogen is also an energy-intensive process requiring the symbiotic bacteria to use carbon from their host plants as an energy source.

In contrast to how symbiotic bacteria respond to phosphorus in dry land plants, Sundareshwar’s group found that adding extra phosphorus to a pristine coastal wetland can prompt the non-symbiotic bacteria that reside there to “shut down nitrogen fixation instead of promoting it,” he said.

At Duke, Sundareshwar has designed a new course on the biogeochemistry of estuaries based on his personal studies. “As I teach this course, I always promote the integrated view, to get away from isolating-out nitrogen and phosphorus,” he said.