March 6, 2000
Study suggests sewage in urban runoff may spur growth of harmful algal blooms
By Tim Stephens
Researchers at UC Sant Cruz and San Francisco State University have found that urea
from urban waste, generally ignored as a pollutant by environmental agencies, contributes
to growth of potentially toxic blooms of a common type of marine algae.
Assistant professor of ocean sciences Raphael Kudela and his coworker, William Cochlan
of SFSU's Romberg Tiburon Center for Environmental Studies, studied a major bloom
of single-celled organisms called dinoflagellates, which grow into potentially toxic
blankets of algae commonly known as red tides when excess nutrients cross their paths.
The researchers found that the dinoflagellates prefer urea, an organic nitrogen compound
found in urine and in agricultural and urban runoff, over inorganic nitrogen sources
such as ammonium and nitrate that occur naturally in the ocean. 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," Kudela said. "Our data suggests it was probably triggered
by the increased concentration of urea introduced to the ocean by urban runoff."
Kudela and Cochlan 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.
