Rain of foreign dust fuels red tides

For years, scientists have puzzled over the episodic emergence of toxic algal blooms in Florida’s coastal waters. One type, referred to as red tides, takes its name from the rosy cast that the algae impart to the surface of affected waters. A new study now establishes that the periodic arrival of plumes of iron-rich African soil can jump-start these red tides.

Huge storms often scoop up soil and propel it long distances, even across oceans (see “Dust, the Thermostat” in this issue: Dust, the Thermostat). Parched regions of Saharan Africa serve as a major source of such dust plumes crossing the Atlantic. Indeed, much of the red soil common throughout the Caribbean arrived there on winds from Africa.

But not all of this foreign soil rains out on land. Much of it falls into the water. Theorists projected that if this soil arrives at the right time in summer, the resulting iron enrichment of receiving waters should fertilize the growth of blue-green algae known as Trichodesmium. The algae use the iron to produce an enzyme that fixes nitrogen; that is, these algae convert nitrogen gas–which is not biologically usable–into the dissolved organic nitrogen compounds on which they and other aquatic life depend.

In the September Limnology and Oceanography, biogeochemist Jason M. Lenes of the University of South Florida in St. Petersburg and his colleagues report measurements that confirm the proposed connections between the arrival of African-dust clouds, the iron enrichment of local waters, and the subsequent 100-fold growth in local Trichodesmium populations. They also show that this algal bloom coincided with a three- to four-fold enrichment in fixed nitrogen in local waters.

Other algae can convert this fixed nitrogen into urea and ammonium. Both are prime nutrients for the red-tide alga Karenia brevis, Lenes notes. No one now knows which local organisms are producers of those nutrients. However, he says, if all of the fixed nitrogen that Trichodesmium produced were converted into these nutrients, there would have been enough present to fuel a red-tide algal bloom. In fact, his team showed that the 3-month-long sequence of events preceding a major red tide in October 1999 would easily account for the toxic bloom.

People who eat shellfish contaminated with a toxin produced in red tides can become paralyzed or even die. The scientists’ goal, Lenes says, is to uncover indicators–before harm occurs–of where and when red tides will emerge.

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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