Sawdust in the Water? Trichodesmium Algal Blooms
Boaters and beachgoers in southwest Florida pe-riodically observe what appears to be large mats of “sawdust” floating on the water’s surface. What they are seeing is not the remnants of someone’s woodwork-ing project, but a marine cyanobacteria (also known as blue green algae) called Trichodesmium. Trichodesmium naturally occurs in tropical and subtropical waters includ-ing the Gulf of Mexico. When environmental condi-tions are right, Trichodesmium cells rapidly repro-duce resulting in a bloom of the cyanobacteria that can be visible by onlookers.
According to the Florida Fish and Wildlife Research Institute, Trichodesmium cells form long chains, called trichomes. Trichomes then can gather into colonies called “puffs” or “tuffs,” and these colonies can aggregate at the surface of the water and form large mats that can extend for miles (also called “sailor’s sawdust”). The amount of Trichodesmium on the surface may vary with time of day, as this species is capable of migrating up and down in the water column. Blooms generally occur offshore in nutrient-poor waters, but currents and winds can push them near shore.
Trichodesmium blooms can take on a range of col-ors depending on stage of the bloom. Healthy blooms are typically brown in color, while blooms in initial decay may take on a green appearance due to accessory pigments leaching out and exposing the cyanobacteria’s chlorophyll. When the chlorophyll begins to deteriorate the blooms appear white in col-or. Trichodesmium blooms are also reported to have
a unique “sweet” smell when it decays and large blooms can turn the water red or pink when stressed cells leaks out water solu-ble, accessory pigments.
Unlike other algal blooms that occur in the region, Trichodesmium blooms are not related to coastal nutrient sources or pollu-tion. Most or all of the nutrients Trichodesmium requires are taken up di-rectly from the water.
Interestingly, the occur-rence of Trichodesmium blooms in Florida is thought to be connected to weather events on the other side of the planet. Blooms in the Gulf of Mex-ico tend to occur between May and September, which is also a time of high storm activity in the Sa-hara Desert in Africa. Iron-rich dust from these storms are transported across the Atlantic Ocean by wind currents, and deposited into the Gulf of Mexi-co. Trichodesmium cells contains enzymes that uti-lize this high concentration of deposited iron to con-vert nitrogen into useable forms. While Trichodesmium blooms can be aesthetically unappealing, it is not toxic and does not pose a health risk to humans. Ironically, Trichodesmium blooms are often seen prior to a Florida red tide bloom, which is a toxic algae that can result in nu-merous fish kills and human health issues.