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Red Tide: Cause, Effects, and Mitigation

February 26, 2019

The toxic red tide bloom that has plagued Florida since November of 2017 seems to finally have dissipated. Last week, the Florida Fish and Wildlife Conservation Commission reported that the red tide organism, Karenia brevis or K. brevis, is no longer present in water samples.

The outbreak was responsible for the deaths of hundreds of manatees and sea turtles and thousands of fish over the last 13 months, as well as the loss of tens of millions in tourist dollars. Unfortunately, scientists say that in the years ahead we are likely to lose many more.

According to the Fourth National Climate Assessment, a report produced by 300 scientists and 13 federal agencies, warming ocean temperatures will increase the intensity and frequency of red tide events.

What is Red Tide?

The ocean is full of microscopic plants called algae. When excess nutrients flow from inland areas into the ocean, it can cause the algae to multiply uncontrollably (causing a “bloom”). These events can last anywhere from a few weeks to more than a year.

Warm ocean surface temperatures, low salinity, high nutrient content, calm seas, and rain that is followed by sunny days can also contribute to these blooms, and it can be carried across long distances by wind, currents, storms, and ships.

Most algal blooms are beneficial to the environment, because algae is food for animals that live in the ocean. But a red tide is a harmful algae bloom (HAB), which happens when algae grow out of control and produce effects that are toxic or harmful to people and animals. The blooms are called “red tides” because, when dense enough, some species of algae (such as K. brevis) can give the ocean’s surface a deep red color.

Florida Red Tide

Red tide is deadly to marine life and can cause skin and respiratory problems in humans. (iStockphoto)

What Are the Effects of Red Tide?

K. brevis produces brevetoxin, which can affect the central nervous system of fish and other vertebrates, causing them to die. These toxins can also accumulate in shellfish and cause neurotoxic shellfish poisoning in people who eat them.

Commercially caught seafood (found in grocery stores and most restaurants) is generally safe to eat during a bloom because it is monitored for K. brevis, but caution is urged when eating recreationally caught fish. Cooking does not destroy red tide toxins.

Swimming is safe for most people during a red tide event, but it can cause skin and eye irritation. Swimming near dead fish is not recommended (or pleasant), as they can harbor harmful bacteria.

K. brevis cells can also be broken apart by waves, which releases the toxins into the air. This can cause respiratory ailments such as coughing, sneezing, and itchy, watery eyes — symptoms that generally disappear within a few hours of leaving the beach. However, people with severe respiratory conditions such as emphysema and asthma are advised to avoid the beach altogether during red tide.

‍How Can We Mitigate Red Tide?

Researchers are working on an antidote to red tide toxins, but slowing climate change is crucial to reducing the frequency and intensity of harmful algae blooms.

In the meantime, our best course of action is to closely monitor red tide outbreaks via water sampling and satellite imagery to mitigate its effects. The sooner red tide algae is detected, the sooner people in the area can receive warning not to eat infected shellfish or swim in contaminated waters.

“The key is observation,” said Robert Weisberg, a physical oceanography professor at the University of South Florida College of Marine Science, “but without observation we are flying blind.”

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