Red tide on the rise

An explosion in the population of single-celled marine micro-organisms cause so-called tides of many different colours.

A surfer in Santa Cruz, California, rides a wave teeming with marine algae, a so-called red tide.

On the big screen, the sea runs red when a shark attacks a swimmer on a summer's day. In real life, however, when the sea turns red - or blue, green or brown, for that matter - the cause is not usually a rampaging great white, but creatures so small they can only be seen under very powerful microscopes. Red tides are caused by growth in the populations of a variety of single-celled marine organisms, among them dinoflagellates, which propel themselves with two hair-like projections called flagellae. Other culprits include diatoms, algae encased in silica cell walls that are the most common forms of phytoplankton in the sea.

Spikes in the numbers of blue-green algae or cyanobacteria, organisms with the internal membranes required for photosynthesis, also contribute to the phenomenon. While they may be tiny, these organisms can prove as deadly to the marine life around them as a hungry shark is to swimmers. Red tides can produce toxins that kill shellfish and fish, along with animals further up the food chain such as birds, dolphins, dugongs and manatees.

Fish can suffocate if their gills become clogged the overabundant algae, and mass death can result if oxygen levels drop when the red tide organisms die out and decay. Red tides - more accurately known as algal blooms, since they are not really a tidal phenomenon - often happen at around this time of year off the coasts of Oman. In some years tens of tonnes of dead fish have washed up on Omani shores.

At Muscat and the Batinah region of Oman, the scores of dead fish that have collected on the beach have all accumulated toxins, making them unsafe. The Middle East's shores are not alone in experiencing red tides. Off the west coast of Florida in the Gulf of Mexico the tides are caused by a species of dinoflagellate called Karenia brevis. It is here that Dr Gabriel Vargo, an emeritus associate professor at the University of South Florida, has studied the phenomenon for more than a generation.

"In our particular area they've occurred annually for the past 30 years, although we know from scientists around the world that they can happen just about everywhere, from North and South America to Europe, Asia and even tropical areas, such as at the Pacific islands," he says. Just as red tides, which are also seen in freshwater environments, come in a variety of colours, they have a number of causes.

Commonly, they result from seasonal upwelling from the sea floor. This brings nutrients such as nitrates and phosphates to the water's surface, which allows formerly low populations of the single-celled organisms to multiply. The microscopic creatures that cause red tides are always present in the oceans, but only when their population density reaches a certain critical mass, about one million cells per litre, does the water become discoloured.

In many areas of the world, including Hong Kong, Florida, the south coast of England and Oman, red tides appear in late summer or autumn. "Many red tides start when the summer heat or increased sunlight causes an increase in the temperature of the surface layer of the water," Dr Vargo says. "That forms a thermal barrier to movement down deep into the water column, and nutrients in the surface water are used perhaps more efficiently by some species than others. That species becomes the dominant organism, and if it produces a toxin you can have problems."

Increased levels of nitrates and phosphates in the water from pollution have also contributed to the spread of red tides. Common sources of such pollutants are fertilisers used in agriculture and gardening, and runoff from roads polluted with nitrates from car exhausts. Typically, waters may remain discoloured for two to three months, but a few years ago in Florida Dr Vargo and his colleagues observed a red tide that persisted into the following winter, spring and summer, lasting 14 months.

"We thought physical mixing, such as you get with storms and waves, would disperse them, but that's not necessarily it," according to Dr Vargo. "It seems current transport is a major reason. The red tides get moved out of an area by currents and they probably get diluted. They don't really die off, they just move away and are dispersed." Another subject little understood is how the micro-organisms that form red tides can produce such deadly toxins for fish.

"It's an excellent question and it might win you a Nobel prize if you found the answer," Dr Vargo says with a laugh. The likely explanation, he says, is that the toxins are a waste product of cellular metabolism and are produced all the time. It is only when the micro-organisms become present in such high concentrations that toxin levels become high enough to kill off other creatures. "When the population of the red-tide organism is high enough - about 250,000 cells per litre - you start to have fish kills," Dr Vargo says.

Red tides are not just hazardous to marine life: humans can also suffer through respiratory problems and skin irritation swimming in affected areas. And the risks are not limited to people on the beach or in the water. "In many cases, you wouldn't be able to ingest enough cells to become sick, but in some cases, depending on the species, the toxin can be released into the water and become aerosolised," Dr Vargo says.

"Waves breaking and bubbles, they inject particles into the air that contain toxins. You can then breathe them in. In those instances, you might be half a mile from the beach and have a reaction to the toxins." Just as scientists are unsure why the micro-organisms produce toxins, they are not certain whether red tides are becoming more common. There are more reports of them than in the past, but Dr Vargo says this could be due to increased monitoring of the world's seas. Overall, however, given that eutrophication has caused red tides in areas where they were not observed previously, scientists speculate that they are more common and, when they do occur, more severe.

Aside from cleaning up the oceans there have been effective ways to reduce the outbreaks. Coastal areas of China, Japan and Korea, where fish is a main staple, have all succesfully suppressed the tides using clay to protect their fishing industries. Clay particles attach to the micro-organisms and sink to the sea bed, killing the tiny organisms. Tests have also taken place in the United States of simmilar procedures, where eliminating red tides would prevent losses to fishermen and ensure that beaches popular with tourists can remain open.

As scientists study different methods to prevent the tides, perhaps red seas will be relegated to the Hollywood studios where they were made famous. @email:[e-mail]