Unlike a stunningly scarlet sunset, a red tide does not inspire aesthetic appreciation. A more likely response is alarm. Red tides occur when surface-dwelling phytoplankton experience a sudden population explosion in response to abundant nutrients. The high density of organisms, sometimes called an algal bloom, results in ocean waters changing colour, ranging from bright green to deep red or brown. Shifting wind and water patterns can trigger this growth, as can pollution or any localized addition of nutrients. In many coastal regions, red tides may occur with fairly regular frequency.

Many different organisms are responsible for red tides, and some are potentially toxic. Florida, normally a popular tourist destination, experiences numerous red tides. Research is ongoing, searching to better understand the causes of these sometimes devastating events. Florida blooms involve Karenia brevis, a common species of motile photosynthetic dinoflagellate that produces neurotoxins (called brevetoxins) potent enough to kill fish, birds, and mammals. The tiny red tide organisms concentrate up the food chain, with filter feeders accumulating particularly high levels of toxins. Shellfish are of particular concern for the human population.

Brevetoxins can also have airborne effects in sea spray and may even cause problems after skin contact. These health hazards are minor compared with full-fledged Neurotoxic Shellfish Poisoning. To avoid problems, local government authorities conduct regular testing programs and apply harvesting bans when red tides reach a set concentration level. Luckily, some premium seafood types, including lobsters, crabs and shrimp, remain unaffected, as the toxins do not accumulate in edible body parts.

In Florida, red tides can last for up to five months, although even longer events have been observed. These phenomena can have wide-ranging impacts, from beach and fishing closures to public health issues. The result can be economically costly, with the US National Oceanic and Atmospheric Administration estimating a loss of $75 million each year. Ongoing research is aimed at understanding the triggers for red tides, in an attempt to predict and monitor their effects. A study just published concludes the outflow of the Mississippi River, carrying a rich burden of nutrients from surrounding agricultural areas, is a key factor. Seasonally shifting winds carry nutrients east to Florida or west to Louisiana and Texas. Given this pattern, red tides are unlikely to be prevented but may be better predicted and their effects mitigated.

The addition of critical nutrients into water may come from agricultural runoff, industrial byproducts, and even untreated human sewage. There are some naturally occurring causes, but many red tide events are triggered by human sources. Increased regulation, treatment and improved practices may reduce the size and frequency of red tides, but they are part of the aquatic environment.