Study Finds Controlling Phosphorus Pollution
In Wetlands More Important Than Believed
DURHAM, N.C. -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
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,
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
"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
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.