"There has been an international effort to try to understand the productivity of the oceans and their potential vulnerability to nitrogen," said Pamela A. Matson, the dean of the School of Earth Sciences and co-author of the Nature study. "A map has been developed showing special regions in the world where nitrogen is low relative to other nutrients that phytoplankton need to grow, and the Gulf of California is one of those regions. Our study is the first to show that the addition of human-caused nutrients in these special areas causes extra blooms of phytoplankton."
Yaqui Valley agriculture
To assess the impact of agricultural runoff on the gulf, the Stanford scientists turned their attention to one of Mexico's most productive coastal farming regions-the Yaqui River Valley, which drains into the gulf.
"The Yaqui Valley agricultural area is 556,000 acres [225,000 hectares] of irrigated wheat," Matson said. "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 gulf. 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 Arrigo, associate professor of geophysics. "The greener the water, the more phytoplankton there are."
Stanford doctoral candidate J. Michael Beman carefully analyzed dozens of SeaWiFS