Red Tide

Phytoplankton, primarily diatoms and dinoflagellates, are floating single-celled algae that are present throughout the marine and freshwater ecosystems and are a significant foundational component of food chains and webs. When conditions are right and these microscopic algae reproduce in great numbers, it can give rise to a phenomenon known as red tide. The common name, red tide, refers to the color of the water, with the hue determined by the species of microalgae. This reproductive activity is called a bloom. Algal blooms can be red, brown, blue, blue-green, or yellow. Blooms are not limited to the coastal waters of the United States and have been documented worldwide. If an algal bloom occurs involving one of the few toxic species of dinoflagellate, then the event is classified as a harmful algal bloom (HAB). The microscopic marine algae most often associated with red tides is Karenia brevis. K. brevis produces toxins called brevetoxins, which are responsible for mass death of fish, birds, and other marine organisms and can cause severe neurologic and gastrointestinal symptoms in humans exposed to it. Red tides, as a natural phenomenon, have been documented since early times. The Gulf of Mexico states Texas and Florida have committed resources to research and monitor red tide because of the negative impact a bloom can have to these states’ economies. Understanding red tide as health, environmental, and economic concerns became a national focus by the 1990s and a global concern in the early 2000s.

Possibly one of the earliest descriptions of red tide can be found in Exodus as one of the first of 10 plagues of Egypt: “all the waters in the river were turned to blood. The fish that were in the river died, the river stank …” Another early published record of red water in the ocean is that of Charles Darwin, who, in 1832, while aboard the H.M.S. Beagle, sailing off the coast of Chile, examined water from muddy bands through which the Beagle had passed. He observed the water to have a pale reddish tint and to contain swarming minute “aminalculae.” In 1858, a researcher ascribed the cause of various colors of seawater near Bombay to a dinoflagellate, and by 1868 scientists understood the cause of discoloration in the ocean waters to be microscopic algae. Nunez Ortega produced a historical record in 1879 that provide the earliest verifiable record of red tide in the Gulf of Mexico. In the mid-1940s, a red tide event began, causing a large fish-kill along the western coast of Florida. This HAB was estimated to have killed millions of fish that washed ashore on beaches, with the dead amounting to 100 pounds per linear foot of shoreline. Studies indicated the dinoflagellate involved to be a Gymnodinium sp., now called Karenia. Ample evidence indicates that algal blooms of all kinds have been widespread throughout time and at multiyear intervals.

In the last 50 years, considerable research has focused on HAB events off the coasts of Texas [Page 801]and Florida. K. brevis blooms produce strong chemical toxins that are harmful to marine animals and humans, causing public health concerns. Brevetoxins can become concentrated in the shellfish tissues, which, when consumed by people, can cause neurotoxic shellfish poisoning, a kind of food poisoning with severe gastrointestinal symptoms and neurologic reactions. Breathing in air near red tides or swimming in red tide waters affects human by causing shortness of breath; irritation of the nose, eyes, and throat; and coughing. People with existing respiratory impairment may experience symptoms to a greater degree. In coastal Texas and Florida, recreational and commercial fisheries and tourism are major industries that can be negatively impacted by a red tide bloom. For these areas and others where red tide can be an environmental and health emergency—as well as an economic disaster—it becomes important to understand the causes of blooms; to evaluate the blooms from environmental, health, and economic perspectives; and to plan and prepare for the impact of red tide. Both Florida and Texas lead the Gulf of Mexico states in funding research for, educating the public about, and monitoring HABs. The economic damage caused by HABs to coastal areas that rely on tourism, recreation, and seafood harvesting was estimated at $1 billion in losses over the last several decades.

...