Saharan dust feeds Gulf phytoplankton
The last weekend in June, a Saharan dust plume of historic proportions blanketed the southeastern United States and the Gulf of Mexico. A smaller plume followed close on its heels and continued to hover over parts of the Gulf coast and the Caribbean as of last week.
“These plumes bring high temperatures and hazy skies to Florida, but they can also have a surprising and equally important effect on the Gulf of Mexico’s phytoplankton community,” Sanibel-Captiva Conservation Foundation Marine Lab Research Assistant Kevin Jones said.
Saharan dust plumes are an annual summer occurrence, but the one that recently reached the United States was among the largest on record. They begin as sandstorms in west and central Africa.
Fine particles are carried high into the atmosphere, where they are swept westward over the Atlantic by wind currents known as the African Easterly Jet. As they travel across the Atlantic, the dust clouds can actually suppress the formation of hurricanes and tropical storms. Upon reaching North America, some of the dust settles in the Gulf, where it provides an important ingredient to the offshore waters – iron.
“This influx of iron is a vital nutrient source to a small cyanobacterium called Trichodesmium,” Jones said. “Cyanobacteria, or blue-green algae, are photosynthetic bacteria. They, along with other types of phytoplankton, form the basis of the marine food web.”
Trichodesmium, sometimes called “sea sawdust,” is found in open water throughout the world. It blooms each summer in the Gulf and forms brown patches at the water’s surface that may look like floating oil or sawdust. Unlike red tide, the blooms are not believed to be hazardous to humans or marine life. Typically found offshore in nutrient-poor waters, the plankton has a special adaptation for the difficult environment where it thrives.
Trichodesmium is one of the relatively few organisms capable of nitrogen fixation, or converting atmospheric nitrogen into a usable form. This ability gives Trichodesmium an important advantage, since nitrogen is a vital nutrient for phytoplankton.
Most phytoplankton utilize various forms of dissolved nitrogen from the surrounding water, but these compounds are usually scarce in Trichodesmium’s environment. This is where the iron from the Saharan dust plumes comes in.
Iron ions are a critical component of the special enzymes that allow Trichodesmium to carry out nitrogen fixation. Blooms of Trichodesmium often occur after a dust plume as the influx of iron allows them to draw nitrogen from the atmosphere, and the nitrogen in turn can become a nutrient source for other phytoplankton.
“Marine lab staff have observed dense patches of Trichodesmium west of Sanibel and Captiva recently,” SCCF Marine Lab Director Dr. Eric Milbrandt said on July 8. “While these blooms are not harmful on their own, there is some evidence to suggest that they may play a role in the initiation of toxin-producing red tide blooms.”
Red tide connection remains unclear (subhead)
Nitrogen is a necessary nutrient for Karenia brevis – the red tide algae – and small-scale experiments in the laboratory have shown that K. brevis will readily scavenge the nitrogen produced by Trichodesmium.
However, the extent to which this phenomenon occurs in the wild and its potential impact on red tide blooms is not yet well understood. The sources of nutrients, including iron to the Gulf of Mexico, include coastal runoff, ground water (submarine springs), atmospheric deposition (dust) and upwelling from the deep ocean.
When determining the causes of red tide bloom formation, these sources can contribute differently in different bloom events. Today, there are multiple competing hypotheses for the causes of red tide blooms but most scientists agree that the extent and duration of a bloom event are made worse by runoff from large rivers in Southwest Florida.
“Much more research is needed to explore the relationship between these two algae, but reducing coastal nutrient pollution remains the most important consideration in mitigating red tide blooms,” Milbrandt said.