Red Tide Resource Hub

A red tide, or harmful algal bloom, is a higher-than-normal concentration of a microscopic alga (plant-like organism). In marine (saltwater) environments along Florida’s west coast and the elsewhere in the Gulf of Mexico, the species that causes red tides is Karenia brevis, often abbreviated as K. brevis.

No, red tides were documented in the southern Gulf of Mexico as far back as the 1700s and along Florida’s Gulf coast in the 1840s. Fish kills near Tampa Bay were even mentioned in the records of Spanish explorers.

The Florida red tide alga, Karenia brevis, needs the following components to form a bloom. The first is biology — the organism must be present in the water and it must out-compete other phytoplankton. The second is the correct chemistry — this includes the appropriate temperature, salinity, and nutrients that it needs to grow and multiply. The third component is the right physical conditions to concentrate and transport K. brevis. The fourth component is ecology – the presence or absence of other life forms, such as other marine algae that may encourage or inhibit K. brevis blooms.

Red tides can last as little as a few weeks or longer than a year. They can even subside and then reoccur. The duration of a bloom in nearshore Florida waters depends on physical and biological conditions that influence its growth and persistence, including sunlight, nutrients and salinity, as well as the speed and direction of wind and water currents.

Many red tides produce toxic chemicals that can affect both marine organisms and humans. The red tide organism in Florida, Karenia brevis, produces brevetoxins that can affect the central nervous system of fish and other vertebrates, causing these animals to die. Wave action can break open K. brevis cells and release these toxins into the air, leading to respiratory irritation. For people with severe or chronic respiratory conditions, such as emphysema or asthma, red tide can cause serious illness. The red tide toxins can also accumulate in molluscan filter-feeders such as oysters and clams, which can lead to Neurotoxic Shellfish Poisoning in people who consume contaminated shellfish.

Red tides in Florida develop 10-40 miles offshore, away from man-made nutrient sources. In contrast to the many red tide species that are fueled by nutrient pollution associated with urban or agricultural runoff, there is no direct link between nutrient pollution and the frequency or initiation of red tides caused by Karenia brevis. Red tides occurred in Florida long before human settlement, and severe red tides were observed in the mid-1900s before the state’s coastlines were heavily developed. However, once red tides are transported inshore, they are capable of using man-made nutrients for their growth.

Yes, the scientific data available so far suggest that it is possible for nutrients flowing from land to sea — including natural AND human-contributed nutrients carried by storm water runoff and the input of rivers — to serve as additional “food” for growth of Karenia brevis red tide blooms that have moved to shore. However, the process is very complex.  K. brevis can use at least 13 different sources of nutrients, including multiple forms of both nitrogen (N) and phosphorus (P), and some of those sources include human-contributed nutrients.

Control of red tide in Florida is not a simple issue. The harmful effects of a red tide are caused by toxins released by the organism. Potential controls must not only kill the red tide organism but also eliminate the toxins from the water. To date, this has not been possible; however, researchers are identifying ways to reduce shellfish toxicity. In addition, any control strategy must not harm the environment. In the 1950s, U.S. Fish and Wildlife Service and State of Florida scientists used copper sulfate to attempt to eliminate a red tide in coastal Florida waters. Although the copper sulfate killed some of the red tide cells, it led to the release of toxins that, along with the copper sulfate, had negative effects on other marine organisms. Controls must also be practical. Red tides vary greatly in size – expanding as far as 10,000 square miles – and can be present from the surface of the water to the seafloor. Presently, there is no practical and acceptable way to control or kill red tide blooms.