Studies Bring New Data to Mystery of Red Tides


Red tides, the thick blankets of algae that kill marine life and turn coastal areas into scarlet scenes from Exodus, are growing more common, and the reason appears to be people.

Two recent studies show that some of the harmful algal blooms hanker for urine like that found in urban storm runoff. Those findings are making researchers rethink assumptions that the main food source is inorganic nitrogen like that found in nature and in runoff from farms.

“We can’t dismiss the fact that if you have a lot of urban runoff, you are going to pollute the coastal environment,” said William Cochlan, a San Francisco State University researcher who coauthored a study on red tides published in the February edition of Aquatic Microbial Ecology.


Cochlan and Raphael Kudella, an assistant professor of ocean sciences at UC Santa Cruz, examined single-celled organisms called dinoflagellates from a 1995 red tide that stretched more than 500 miles along the California coast. That was the biggest red tide seen off California since 1902.

They found that dinoflagellates ate urea, an organic nitrogen compound found in urine, more readily than inorganic forms of nitrogen such as ammonium and nitrate.

Nitrogen, whether organic or not, is food, Cochlan said.

“If you get grass, you’re also going to get weeds. Harmful algae blooms are those weeds,” he said.

Some of those “weeds,” like the 1995 bloom, had no apparent effect on the environment, other than driving away surfers and swimmers.

Other red tides--which aren’t always red and have nothing to do with tides--kill fish, birds and marine mammals, like a 1998 bloom in Monterey Bay that killed more than 50 sea lions. They also can make fish and seafood unsafe to eat, although federal monitoring programs are in place to keep tainted food off the market.

Researchers trying to learn how red tides form have overlooked organic nitrogen as a food source, Cochlan and Kudella said. That is a mistake, Kudella said, because urea may represent “as much as 50% of the nitrogen taken up in places like Chesapeake Bay.”


An examination of aquaculture ponds in Chesapeake Bay showed that dinoflagellates frequently multiplied into harmful algal blooms in water with high levels of urea, but not in water with low levels. That study was published in the December issue of Applied and Environmental Microbiology.

Because there are hundreds of different species of harmful algal blooms, urea is far from the only thing scientists need to examine, said Patricia M. Glibert, a University of Maryland professor who co-authored that study.

On top of the many ways that human activity can lead to red tides--sewage input, agricultural runoff, aquaculture--some red tides occur naturally, and scientists are still unclear on their exact development.

Environmental factors like predators, salinity and temperature also play a role in whether red tides form. Glibert said information about those many relationships is “something we really need to understand which organisms are going to bloom at which times.”

“We’re really in our infancy toward understanding this,” said Vera Trainer, marine biotoxins program manager for the Seattle-based Northwest Fisheries Science Center, a part of the National Marine Fisheries Service.

“It could be that some blooms are perhaps influenced by human development on the coast, but perhaps some species are not,” she said. Red tides occur even in relatively pristine areas of the Washington coast.


Documentation of red tides may begin with the biblical Book of Exodus, in which one of the plagues of Egypt was water that turned blood red and killed fish, said Peter Franks, a biological oceanographer at the Scripps Institution of Oceanography in La Jolla. Capt. George Vancouver found red tides when he explored the Northwest coast in the 1790s, he said.

Trainer said the rise in red tides is “potentially frightening,” especially given problems like the death of sea lions in Monterey Bay. “That’s large mammals, and that’s not so far from us,” she said.

“My personal opinion is that harmful algal blooms could possibly be an indicator of ecosystem changes--not necessarily their presence, but the magnitude of the events and the length of time present,” Trainer said. “It’s maybe an early warning of our environment being altered.”

Because temperature may play a role in red tides, global warming could be one reason for their rise, Trainer said, although she added that possibility is just starting to be studied.


On the Net: The Northwest Fisheries Science Center site on red tides:

Woods Hole Oceanographic Institution: