Sewage May Spur Growth Of Harmful Algal Blooms

The new findings, published in the current issue of Aquatic Microbial Ecology, suggest that urea in urban and agricultural runoff may play a greater role than previously thought in triggering or sustaining harmful algal blooms found growing off California's coastline.

"The particular bloom we looked at, which extended from the upper Baja peninsula in Mexico to the Monterey Bay, occurred after heavy urban runoff events in the southern California region," said Raphael Kudela, assistant professor of ocean sciences at the University of California, Santa Cruz. "Our data suggests it was probably triggered by the increased concentration of urea introduced to the ocean by urban runoff," Kudela said.

Kudela and coauthor William Cochlan of San Francisco State University's Romberg Tiburon Center for Environmental Studies examined the physiology and ecology of the bloom, which occurred in 1995 and was the largest and most widespread red tide found off California's coast since 1902. Though marine scientists usually monitor marine ecosystems for high concentrations of common inorganic nutrients known to spur harmful algal blooms, urea is generally ignored, the researchers said.

Previous studies have shown that urea can nourish the growth of dinoflagellates under laboratory conditions. The new study shows for the first time, however, that the naturally occurring red-tide dinoflagellate responsible for the 1995 bloom-known scientifically as Lingulodinium polyedrum-can use organic urea as a nutrient source and even prefers it over traditionally measured inorganic forms of nitrogen.

"Although urea as a source of pollution is generally ignored by state and federal environmental agencies, research shows that urea represents an average of one-third of the total nitrogen uptake supporting growth of phytoplankton in regions where red tides can occur," Cochlan said. "In some estuarine areas, such as the Chesapeake Bay, urea can represent 60 percent of the nitrogen uptake at certain times of the year."

Phytoplankton serve as the base of the marine food web, but unusually high levels of nutrients together with abundant sunlight can spur rapid growth, or blooms, of these single-celled plants, leading to dense patches of algae floating near the surface of the ocean that can double in size daily. While most blooms are not harmful, a small number of phytoplankton species can produce potent neurotoxins when they form into a bloom, sometimes poisoning or killing higher life forms such as zooplankton, shellfish, fish, birds, marine mammals, and even humans as the toxin is transferred up the food chain.

Although Lingulodinium polyedrum has been reported to produce yessotoxin, a compound related to the class of poisons that cause paralytic shellfish poisoning, the researchers found no evidence that the 1995 bloom was toxic. However, large algal blooms of any type pose an additional risk by lowering the available oxygen in the surrounding water when they decay, causing small marine animals, such as zooplankton and fish, to suffocate.

"Considering the role urea seems to play in spurring or sustaining growth of phytoplankton, including harmful algal blooms, this organic nitrogen source should be taken into consideration by environmental agencies that conduct bloom mitigation efforts," Cochlan said.

According to Kudela, these harmful bloom events are becoming more common off the California coastline. In addition to the dinoflagellate Lingulodinium polyedrum, the diatom species Pseudo-nitzschia australis also plagues California's coastal waters in deadly bloom form. Pseudo-nitzschia was recently identified as the culprit when more than 400 sea lions died and many more suffered from domoic acid poisoning on California's Central Coast in 1998. "However, there's no evidence that Pseudo-nitzschia responds to urea," Kudela noted.

The researchers emphasized that red-tide-causing phytoplankton species are driven to bloom by varying mechanisms and nutrients, so it's important to examine and understand each species individually. Toward that end, the researchers have been awarded a grant from the National Oceanic and Atmospheric Administration's Coastal Ocean Program to conduct a comprehensive field and lab study, along with several other groups on the West Coast, of the more deadly Pseudo-nitzschia.

U.C. Santa Cruz Contact: Tim Stephens, (831) 459-2495; stephens@cats.ucsc.edu.