Researchers discover direct link between agricultural runoff and massive algal blooms in the sea

Scientists have found the first direct evidence linking large-scale coastal farming to massive blooms of marine algae that are potentially harmful to ocean life and fisheries.
Researchers from Stanford University's School of Earth Sciences made the discovery by analyzing satellite images of Mexico's Sea of Cortez, also known as the Gulf of California—a narrow, 700-mile-long stretch of the Pacific Ocean that separates the Mexican mainland from the Baja California Peninsula. Immortalized in the 1941 book Sea of Cortez, by writer John Steinbeck and marine biologist Edward Ricketts, the region remains a hotspot of marine biodiversity and one of Mexico's most important commercial fishing centers.

The results of the Stanford study will be presented at the annual meeting of the American Geophysical Union (AGU) in San Francisco on Dec. 13.

Algal blooms

Algal blooms occur naturally when cold-water upwellings bring from the seafloor to the surface nutrients that stimulate the rapid reproduction and growth of microscopic algae, also known as phytoplankton. These events often benefit marine ecosystems by generating tons of algae that are consumed by larger organisms.

But several phytoplankton species produce harmful blooms, known as red or brown tides, which release toxins in the water that can poison mollusks and fish. Excessively large blooms can also overwhelm a marine ecosystem by creating oxygen-depleted "dead zones" in the ocean. Scientists have long suspected that many harmful blooms are fueled by fertilizer runoff from farming operations, which in many regions pour tons of excess nitrogen and other nutrients into rivers that eventually flow into coastal waters. However, some agricultural industry groups contend that there is not enough evidence to link farm runoff to red tides or dead zones.

Satellite imagery

To assess the impact of agriculture on marine algae, Stanford scientists turned their attention to one of Mexico's most productive coastal farming regions—the Yaqui River Valley, which drains into the Sea of Cortez.

"The Yaqui Valley agricultural area is 556,000 acres [225,000 hectares] of irrigated wheat," said Pamela A. Matson, the dean of Stanford's School of Earth Sciences and co-author of the AGU study. "The entire valley is irrigated and fertilized in very short windows of time during a six-month cycle. The excess water from irrigation runs off through streams and channels into the estuaries, and then out to sea."

Matson and her colleagues wondered if each fertilization and irrigation event would trigger a noticeable phytoplankton bloom near the mouth of the Yaqui River, which is located on the mainland side of the Sea of Cortez. To find out, the researchers analyzed a series of images from an orbiting NASA satellite called SeaWiFS, which is equipped with special light-sensitive instruments that can detect phytoplankton floating near the surface of the sea. "These instruments measure the level of greenness in the water," explained Kevin R. Arrigo, an associate professor of geophysics at Stanford and co-author of the AGU paper. "The greener the water, the more phytoplankton there are."

Dramatic results

Stanford doctoral candidate Mike Beman carefully analyzed dozens of SeaWiFS images taken over the Sea of Cortez from 1998 through 2002. The results were dramatic.

"I looked at five years of satellite data," said Beman, lead author of the study. "There were roughly four irrigation events per year, and right after each one, you'd see a bloom appear within a matter of days."

Each bloom was enormous, he said, covering from 19 to 223 square miles (50 to 577 square kilometers) of the Sea of Cortez and lasting several days. "Sometimes eddies actually pulled the plumes across the gulf, from the mainland side all the way to the Baja Peninsula," Beman added.

"Mike found that immediately following each one-week window in which much of the valley was irrigated, there was a response in the ocean off the coast of the Yaqui Valley," Matson explained.

"We were quite surprised," Arrigo added, noting that the AGU paper marks the first time that scientists have documented a "one-to-one correspondence between an irrigation event and a massive algal bloom."

Red tides and dead zones

According to the researchers, artificially induced algal blooms could have major impacts on recreational and commercial fishing, major industries in the Sea of Cortez. Red tides, for example, can cause outbreaks of life-threatening diseases, such as paralytic shellfish poisoning, which can shut down mussel and clam harvesting for long periods of time.

Another concern is hypoxia, or oxygen depletion, which is caused by excessive algae growth. As the algal mass sinks, it is consumed by bacteria, which use up most of the oxygen in the water as they multiply. The result is an oxygen-depleted dead zone at the bottom of the sea where few creatures can survive. A massive dead zone appears every summer in the Gulf of Mexico off the coast of Louisiana and Texas. Scientists believe that agricultural runoff from the Mississippi River plays a pivotal role in creating this annual dead zone, which measured 8,500 square miles (22,000 square kilometers) in 2002—an area bigger than the state of Massachusetts.

"In the Sea of Cortez, there's the possibility that hypoxia could occur at a local scale, which could be devastating to the shrimp and shellfish industries," Matson said. "Shrimp fisheries are very important economically, and they're already under a lot of stress from overfishing and aquaculture. It is possible that agricultural runoff could cause additional stress if it does lead to toxic blooms or hypoxia." She and her colleagues plan to conduct follow-up studies to assess the ecological impact of Yaqui Valley runoff events.

"The availability of high-resolution satellite data has opened up a whole new opportunity to look at the importance of what's going on on land in the sea," Matson added. "This study shows that you have to pay attention to the land-sea connections."

Source: Stanford University

Related stories:

'Deadly dozen' reports diseases worsened by climate change
Health experts from the Wildlife Conservation Society today released a report that lists 12 pathogens that could spread into new regions as a result of climate change, with potential impacts to both human and wildlife health and global economies. Called The Deadly Dozen: Wildlife Diseases in the Age of Climate Change, the new report provides examples of diseases that could spread as a result of changes in temperatures and precipitation levels. The best defense, according to the report's authors, is a good offense in the form of wildlife monitoring to detect how these diseases are moving so health professionals can learn and prepare to mitigate their impact.
Study shows continued spread of 'dead zones'
A global study led by Professor Robert Diaz of the Virginia Institute of Marine Science, College of William and Mary, shows that the number of "dead zones"—areas of seafloor with too little oxygen for most marine life—has increased by a third between 1995 and 2007.
A recipe for saving the world's oceans from an extinction crisis
Jeremy Jackson, senior scientist emeritus of the Smithsonian Tropical Research Institute and the Scripps Institution of Oceanography, asserts in the Aug. 12 issue of the journal Proceedings of the National Academy of Sciences, that the following steps, if taken immediately, could reverse the demise of the oceans: Establish marine reserves, enforce fishing regulations, implement aquaculture, remove subsidies on fertilizer use, muster human ingenuity to limit fossil fuel consumption, buy time by establishing local conservation measures.
Large dead zones predicted for Gulf, Chesapeake Bay
Record-setting "dead zones" in the Gulf of Mexico and Chesapeake Bay appear likely this summer, according to new forecasts from a University of Michigan researcher.
Summer Storms Could Mean More Dead Zones
(PhysOrg.com) -- It's summertime and people are flocking to the coasts around the country. But when summer storms arrive, it's not only beach-goers who are affected; the rains can also have an impact on living creatures far below the ocean surface.
Domoic acid from toxic algal blooms may cause seizures in California sea lions
Scientists, reporting in the current issue of the online journal Marine Drugs, state that an increase of epileptic seizures and behavioral abnormalities in California sea lions can result from low-dose exposure to domoic acid as a fetus. The findings follow an analysis earlier this year led by Frances Gulland of the California Marine Mammal Center that showed this brain disturbance to be a newly recognized chronic disease.
Scientists discover stinging truths about jellyfish blooms in the Bering Sea
A new study helps explain a cyclic increase and decrease of jellyfish populations, which transformed parts of the Bering Sea--one of the U.S.'s most productive fisheries--into veritable jellytoriums during the 1990s.
Scripps Oceanography Research pegs ID of red tide killer
Researchers at Scripps Institution of Oceanography at UC San Diego have identified a potential “red tide killer.” Red tides and related phenomena in which microscopic algae accumulate rapidly in dense concentrations have been on the rise in recent years, causing hundreds of millions of dollars in worldwide losses to fisheries and beach tourism activities. Despite their wide-ranging impacts, such phenomena, more broadly referred to as “harmful algal blooms,” remain unpredictable in not only where they appear, but how long they persist.

News discussion:

Space & Earth science news