Red tide is a harmful algal bloom with a higher than average concentration of a plant-like organism called microscopic algae. In Florida, the species that causes the majority of red tides is called Karenia brevis (or K. brevis). Florida researchers call K. brevis “Florida red tide” to distinguish it from other species of algae.

Three components must be present for Florida red tide to bloom: Biology (the presence of the organism in the water), Correct Chemistry (nutrients that feed the blooms), and Right Physical Conditions (ideal conditions must exist for K. brevis to be concentrated and transported.

When K. brevis is present in high concentrations, Florida red tide discolors water with a red or brownish hue. Sometimes, the water may even appear normal. 

Red tide produces toxic chemicals and affects both marine life and human life. K. brevis produces brevetoxins that affect central nervous systems of fish and other invertebrates, leading to their death.

Wave action breaks open K. brevis cells and releases their toxins into the air, causing respiratory irritation. This is especially harmful to people who suffer from severe or chronic respiratory illnesses, like asthma, or emphysema. Red tide toxins also accumulate in shellfish like oysters and clams, which leads to Neurotoxic Shellfish Poisoning in people who consume these contaminated shellfish.