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Trichodesmium in Florida—2004

In late May 2004, an area of discolored water was reported off the coast of southwest Florida. By the first week of June, the discoloration was reported offshore and inshore as far north as Clearwater. Water samples were collected and were found to contain high concentrations of Trichodesmium sp.

Trichodesmium is a marine cyanobacterium found worldwide in surface waters of tropical and subtropical oceans, including the Gulf of Mexico. Reports of Trichodesmium blooms date back to the 1700s in the ship logs of Captain Cook, who wrote extensively about the large, brown blooms that resembled sandbars. Sailors sometimes refer to Trichodesmium as sea sawdust because it forms colonies that can be quite large (up to 1 cm) and visible to the naked eye. Small blooms resemble sawdust floating on the water surface; larger blooms can look like oil slicks or slightly foamy pollution and can be so far-reaching that they are visible from space. Trichodesmium migrates up and down in the water column, so the amount seen on the surface of the water may vary with time of day. Trichodesmium blooms occur every year in the gulf, generally offshore in oligotrophic waters, but they can occasionally reach nearshore areas when currents and winds push an established offshore bloom to inshore areas.

Trichodesmium blooms are not related to coastal nutrient sources or pollution. Most or all of the phosphorus that Trichodesmium requires for nutrition is taken up directly from the water, and most of the nitrogen it requires is derived from nitrogen gas (N₂) dissolved in the water. High-biomass Trichodesmium blooms in the Gulf of Mexico tend to occur between May and September, a time of high storm activity in the Sahara Desert in Africa. Dust from these storms contains high concentrations of iron. Nitrogenase, the specialized enzyme Trichodesmium uses to convert N₂ to a more usable form, has a high iron requirement. The iron-rich Saharan dust is blown into the atmosphere, transported across the Atlantic Ocean by wind currents, and deposited into the Gulf of Mexico. In general, Trichodesmium is not a good food source for zooplankton or fish; it is actively grazed on by only a few specialized zooplankton species.

Trichodesmium blooms have a unique "sweet" smell when they decay, and large blooms can turn the water red or pink when stressed cells leach out water-soluble accessory pigments (called phycoerythrins). At various times in their development, Trichodesmium blooms can also appear brown (healthy bloom), green (blooms in initial decay state after accessory pigments have leached out, making the chlorophyll a visible in cells), or white (after chlorophyll a decays). Trichodesmium blooms in the gulf are not harmful to humans, just aesthetically displeasing.