Agricultural Runoff Linked to Oceanic Algae Blooms
Concrete data that connect the dots between
fertilizer runoff from farms to blooms of algae in coastal waters have
been hard to come by, although scientists have long suspected this
relationship. Now a study just published in
Nature
presents the first direct evidence that runoff from large scale coastal
farming causes algae growth in the ocean, as populations of these tiny
plants boom in response to the excess nitrogen and phosphorus that runs
off the land into coastal waters — sometimes draining oxygen out of the
ecosystem and putting marine life at risk.
Algae blooms occur naturally in marine ecosystems in response to
nutrients that are brought to the surface by upwelling, which occurs
when wind-driven currents lift bottom waters from the depths. Algae
form the base of the marine food web, providing nourishment for grazing
zooplankton, fish and ultimately higher order predators that feed on
fish.
But too much of a good thing can be a problem. In ecosystems like the
Chesapeake Bay and the Gulf of Mexico, excessive algal blooms each
summer create areas of low or no oxygen inhospitable to most marine
life. Patterns of land use, such as agriculture and development,
clearly affect algae growth in coastal waters, but most ecosystems —
like the Chesapeake Bay — are "noisy," meaning that they experience too
much variability due to climate and other sources to readily build
evidence for such causal relationships.
The Gulf of California, also called the Sea of Cortez, now provides more clear-cut evidence of that agriculture-algae link.
A team of scientists from Stanford University suspected that Yaqui
Valley in Mexico might be a well-suited to test the hypothesis that
fertilizer runoff from large-scale coastal farming practices is
connected to episodic large algae blooms in the Gulf of California's
very diverse marine ecosystem, one of Mexico's most important
commercial fishing centers. Farmers in the Yaqui Valley, a 556,000 acre
wheat-growing area, irrigate and fertilize their fields in very short
windows of time during a six-month cycle. The region's predictably dry
climate provides a perfect backdrop for detecting a fresh water signal,
explains Ph.D candidate J. Michael Beman, the study's lead author. That
signal, produced by nitrogen-rich pulses of fresh water that enter the
Gulf of California, can be readily detected by satellite.
Beman and his colleagues analyzed changes in the amount of chlorophyll-
a
in the ocean (an index of algae biomass), along with several other
variables, over a five-year period. The data originated from a
satellite-borne instrument known as SeaWiFS (Sea-viewing Wide Field-of
view Sensor) that passes over the Gulf of California once every two
days, and measures chlorophyll by analyzing the color of the water. The
team discovered that immediately after each one-week irrigation event
in the Yaqui Valley, an enormous algal bloom would occur, covering 19
to 223 square miles and lasting for several days. As the researchers
report in
Nature's 10 March
issue, this is the first time a study has established a one-to-one
correspondence between an irrigation event and a massive algal bloom.
One of the next steps, says Beman, is to sample these blooms in order
to determine which species of algae are present. This will help clarify
the ecological impacts such blooms might have on the marine
environment, he says.
Beman also mentions that the research team, along with
collaborators, participates in a larger project working toward
sustainable agriculture in the Yaqui Valley to develop more precise
methods of fertilizer application on farms that would limit the impact
of these runoff events in the Gulf of California (see link below).
"This is a really nice piece of work," says Larry Harding, an
oceanographer at the University of Maryland Center for Environmental
Science who uses SeaWiFS remote sensing technology to study
phytoplankton blooms in the Chesapeake Bay. "This paper does exactly
the right thing in the way that it uses SeaWiFS data to extract a
response to coastal nutrient inputs from long-term patterns of
variability," he says.
- Erica Goldman
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