What is Red Tide?

Red tide is almost an annual event on the west coast of Florida. This year’s red tide is worse than usual, but I confess, I wasn’t overly concerned until it migrated around to Florida’s east coast and came to my beach. Scratchy-throated visitors are coughing and wheezing. Fish are dying. But what exactly is going on?

Red tide is not always red and has nothing to do with tides. It is more aptly described as a “harmful algal bloom.” It occurs when a particular species of algae, which may be quite harmless in low numbers, starts reproducing with abandon. This happens when nutrient-rich deep water wells up to the surface or is mixed up in storms, and the normally scarce nitrogen and phosphorus become plentiful and enable the algae to bloom out of control. Harmful algal blooms occur worldwide and are not new. The first red tide documented in Florida dates back to 1840.

The culprit behind Florida’s red tide is a species called Karenia brevis, a single-celled photosynthetic dinoflagellate with two tail-like flagellum that send it spinning through the water. It has reddish-brown photosynthetic pigments that really do turn the water red in high enough concentrations. K. brevis was fondly named for world renowned researcher, Dr. Karen Steidinger. This species causes red tides from Mexico to Florida, but it usually stays in the Gulf of Mexico. The blooms show up in late summer and early fall and can last from a few weeks to a year. Florida’s red tides develop offshore and travel inshore by currents and winds. They are affected by salinity (there’s no red tide in fresh water), temperature and nutrients. The blooms aren’t caused by manmade pollution, though nutrients from agricultural runoff, lawn fertilizers, and sewage can allow K. brevis to keep blooming longer than it otherwise would.

Karenia brevis produces powerful neurotoxins called brevetoxins. In heavy surf the toxins become airborne and people on the beach breathe them in, which causes them to wheeze and cough. Some people break out in rashes when they swim in water with red tide. Eating filter-feeding mollusks, clams, oysters, mussels, that concentrate brevetoxins in their tissues is also dangerous. When contaminated shellfish are eaten by people, it causes neurotoxic shellfish poisoning, nausea, vomiting and more. Scallops are safe. Scallops filter K. brevis from the water like oysters do, but we only eat the muscle on a scallop and the neurotoxin accumulates in the gut. For the same reason, eating fish is safe; eat the fillet, throw out the guts.

Karenia brevis causes injury and death in many animals. Eating contaminated shellfish kills ducks that feed on them. Manatees that eat enough seagrass covered with K. brevis develop respiratory paralysis and can die. Bottlenose dolphins can die from eating herbivorous fish that have eaten K. brevis. Thousands and thousands of fish die from the brevetoxins or from the lack of oxygen that results when all that algae dies and decomposes.

Researchers are looking for answers. They know where, when and how many K. brevis cells per liter there are in red tide affected waters. The Florida Fish and Wildlife Conservation Commission publishes online status maps that can help us decide between a trip to the beach or not. Other answers, like how to make red tide go away, are more elusive. We can’t do much about red tide except avoid the beach during outbreaks.

There are other problems, like ocean plastic, that we can do something about. When the red tide has passed, grab a bag from the BlueTube at your beach. Pick up trash. Throw it away. Don’t forget to add clean, used plastic bags so others can help get rid of ocean plastic too.