Agricultural Runoff Linked Directly to Algal Blooms
Katrina Outland, Science Journalist
Marine Biology and Writing, Hawaii Pacific University
outland@jyi.org
Satellite images show algal blooms (green areas) in the Sea of Cortez.
Image courtesy Stanford University.
8 December 2004 - Satellite data of the Sea of Cortez in
Mexico reveals the first direct link between coastal farming
and potentially harmful algae blooms, according to
researchers at Stanford University's School of Earth
Sciences. Lead author Mike Beman and co-authors Pamela
Matson and Kevin Arrigo presented their results at the
annual meeting of the American Geophysical Union in
December.
According to the researchers, algal blooms occur in water
where excess nutrients allow microscopic algae—called
phytoplankton—to grow and reproduce rapidly. Natural
algal blooms are often beneficial the food chain, but some
blooms contain phytoplankton species that produce toxins.
These toxic blooms, called brown or red tides, can poison
shellfish and other marine life. Algal blooms also increase the population size of bacteria that eat
phytoplankton. The bacteria use all of the oxygen in the surface waters and create “dead zones.”
Scientists have long suspected that fertilizer runoff from farms can lead to algal blooms, but they have
not found a direct link—until now. Beman and his colleagues analyzed satellite images of the Sea of
Cortez, a major coastal farming region, from 1998 to 2002. Using the NASA satellite SeaWiFS,
Beman’s team located green areas in the Sea of Cortez. Green water indicates the presence of
phytoplankton and algal blooms.
"I looked at five years of satellite data," said Beman. "There were roughly four irrigation events per
year, and right after each one, you'd see a bloom appear within a matter of days."
The artificially-induced algal blooms could have major effects on fishing industries in the area. For
example, the toxins from red tides can accumulate in shellfish, leading to outbreaks of paralytic
shellfish poisoning.
"The availability of high-resolution satellite data has opened up a whole new opportunity to look at the
importance of what's going on," said Matson. "This study shows that you have to pay attention to the
land-sea connections."
Journal of Young Investigators. 2004. Volume Eleven.
Copyright © 2004 by Katrina Outland and JYI. All rights reserved.