Red tides occur globally every year, and are the result of the explosive growth and accumulation of certain microscopic algae species, or dinoflagellates.

In August of 2014, Florida experienced its largest red tide since 2005. The phenomenon appeared as a great swathe of ominous, rust-colored water that measured 90 miles in length and 60 miles across, and extended for at least 100 feet into the water column. Red tides like this one occur globally every year. They result from the explosive growth and accumulation of certain microscopic algae species, or dinoflagellates. The term “red tide” is a colloquialism used in place of the more scientific term, Harmful Algal Bloom (HAB). It can be somewhat misleading given that these blooms are not always red, and have little to do with tidal activity.

Although there are some algal blooms that provide nutrition for species further up the food chain, HABs impact the marine environment and human communities negatively. Karenia brevis dinoflagellates cause Florida’s red tide. Like all other algal species involved in the creation of HABs, they produce toxins among the planet’s most potent. To date, scientists have recorded more than 50 species of toxic algae, and the negative impacts caused by each can be similarly diverse.

What happens to marine life during a red tide?

Some strains of harmful algae have a corrosive effect on marine creatures’ skin. Others secrete toxins capable of causing paralysis, amnesia or respiratory problems. A mass fish die-off is a common result of red tides. As a result of exposure to algal toxins, paralysis keeps fish from breathing. In Florida in 2004, the red tide caused the death of 107 dolphins, all of which were found during autopsy to have eaten contaminated fish. Because dinoflagellates are primary producers, HABs create a domino effect all the way up the food chain.

The manatee population in Florida suffers acutely from the effects of red tide. In 2013, 276 manatees died as a result of consuming seagrass coated with toxic algae. Unfortunately, those of us who eat seafood are also at risk from the effects of HABs. Shellfish in particular become toxic during a red tide, as they filter large quantities of contaminated algae and plankton. Although consuming affected shellfish can be fatal, the effects are more likely to include acute nausea, vomiting, diarrhea, tingling and numbness.

What do red tides cost?

Some strains of toxic algae, including Karenia brevis, can affect even those who don’t eat seafood. When the red tide hits the shore it can produce airborne toxins, which can cause respiratory and skin irritations. The negative impacts of HABs cause both tourists and fishing fleets to abandon those areas, causing much economic hardship in those communities. HABs cost the American economy an estimated staggering $82 million every year. The effects of red tides last long after the algal bloom begins to die off, too, because the process of decay can reduce oxygen levels in the water such that marine ecosystems become hypoxic and unable to support life. In these cases, marine life in the area either dies or must move elsewhere.

What causes red tides?

Despite their drastic impact, we know very little about what causes HABs. Scientists hypothesize that reduced salinity, warm surface temperatures and optimal light levels could all help create the perfect balance of conditions. Increased nutrients in the water, however, may be the biggest facilitating factor.

In some areas of the world, including China and Japan, agricultural and human waste dumped into the ocean introduces high levels of phosphates and nitrates into the water. These excess nutrients, combined with nitrogen absorbed into the ocean as a result of burning fossil fuels, could cause populations of single-celled organisms like dinoflagellates to grow exponentially. These regions could perhaps combat red tides by reducing the level of nutrient-rich waste deposited into the ocean.

In other areas, natural upwellings, brought inshore at certain times of the year by ocean currents, cause HABs. The Florida coast, for example, has records of Karenia brevis blooms dating back to the 1700s when early settlers wrote of sudden fish die-offs and poisonous fumes.

In 1958, Florida officials attempted to kill the toxic algae by spraying the bloom with a pesticide containing copper sulfate; predictably, this only succeeded in further polluting the already beleaguered coastline. South Korea has mitigated the effects of smaller HABs by spraying clay onto the bloom. This then carries the red tide particles with it down to the seafloor. However, scientists worry that there could be unforeseen consequences to benthic species as a result of this solution. For now, scientists are trying to develop methods to predict the HABs. If we can prepare for a toxic algal bloom, perhaps we can better deal with its effects.

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