March 10, 2005
Runoff May Harm Marine Life in Mexico
SAN FRANCISCO (AP) -- Agricultural runoff is triggering massive algae blooms that could harm marine life in the Gulf of California, one of Mexico's most important fishing regions, according to a study published Thursday.
Stanford University researchers found a direct link between fertilizer runoff from Mexico's Yaqui River Valley and sudden bursts of marine algae in the 700-mile-long gulf, also known as the Sea of Cortez, which separates the Baja California peninsula from mainland Mexico. Their study, based on an analysis of satellite photos, will be published in Thursday's issue of the journal Nature.
"We saw these really big blooms following every irrigation event in the valley," said Michael Beman, the study's lead author and a doctoral student in Stanford's Department of Geological and Environmental Sciences. "It demonstrates that certain areas of the ocean are more vulnerable to agricultural runoff than previously thought."
Scientists have long believed that large-scale coastal farming produces algae blooms that can disrupt marine ecosystems at sea, but Beman said the Stanford study presents the clearest direct evidence of the phenomenon.
Algal blooms, the rapid growth of marine algae known as phytoplankton, are natural events that can benefit marine life that feed on the microscopic algae.
But some types of phytoplankton can generate destructive blooms, known as red or brown tides. These can poison fish and mollusks, and spark disease outbreaks that can shut down fisheries.
Excess algae growth also sucks oxygen from the water, creating dead zones where few organisms can survive. Scientists believe fertilizer runoff from the Mississippi River helps explain a 5,700-square-mile dead zone that appears every summer in the Gulf of Mexico off the coasts of Louisiana and Texas.
Algae blooms are fed by nitrogen, phosphorous and other nutrients naturally present in ocean water, but scientists believe they also can be fueled artificially when tons of nutrient-rich fertilizer runs into rivers that flow to the sea.
The Stanford researchers suspected that algae blooms in the Gulf of California were linked to runoff from farm fields in the Yaqui River Valley, a 556,000-acre wheat-producing area that is one of Mexico's most productive farming regions. The river drains into the gulf from the mainland side.
To test their hypothesis, they analyzed images taken between 1998 and 2002 from a NASA satellite that can detect phytoplankton near the sea surface. The researchers found that a massive algae bloom, covering up to 223 square miles in the gulf, was recorded within days of substantial runoff in the Yaqui River Valley. Water is released from a reservoir about four times a year to irrigate the valley's fields.
Beman attributed the size and speed of the blooms to the gulf's relative lack of nitrogen and predicted similar experiences in the Bay of Bengal, the South China Sea and other regions that could be vulnerable to sudden infusions of nitrogen. He said Stanford researchers are working with Yaqui Valley farmers to reduce agricultural runoff.
The researchers warned that algae blooms could endanger the Gulf of California's shellfish and shrimp industries. Other marine life, including fish, squid, whales and the world's smallest porpoise, the vaquita marina, also could be harmed, they said.
The Natural Resources Defense Council said the Stanford study should prompt changes that could lead to less pollution flowing into the gulf.
Farmers should apply less fertilizer, use natural fertilizer and install filters to reduce runoff, said Jonathan Kaplan, who heads the environmental group's sustainable agriculture project.
"We need to do a better job of promoting more sustainable practices and encouraging producers to adopt them," Kaplan said. "It's hard to change old habits. We need more leadership from our universities, government agencies and agriculture industry organizations to help producers understand there are better ways out there."
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