We have a lot of reasons to thank algae. Running the gamut from microscopic single-celled organisms to 200-foot long stretches of kelp, these simple plants are at the base of the marine food web and are responsible for producing around 50 percent of the planet’s oxygen.

But they have a dark side, too, and you can see it every summer when Florida’s Gulf Coast is stricken with a red tide, or what scientists prefer to call a harmful algal bloom (HAB).

HABs happen when colonies of algae undergo a population explosion, resulting in deleterious effects on people, fish, shellfish, marine mammals and birds. These blooms of harmful microscopic plankton — specifically, a subgroup known as dinoflagellates — happen along ocean coasts and are spurred on by a number of factors. Warm surface temperatures, high nutrient content, low salinity and calm seas create the perfect environment for these blooms to thrive. Sunny weather that follows summer rains creates particularly fertile conditions for red tides. HABs can occur in most coastal areas of the United States, courtesy of the following three dinoflagellates:

  • Alexandrium fundyense: Causes red tides along the Northeast coast, ranging from the Canadian Maritimes to southern New England
  • Alexandrium catenella: Causes red tides along the West Coast from California to Alaska
  • Karonia brevis: Causes red tides in the Gulf of Mexico along the west Florida coast
These specific HABs are often referred to as red tides because, not surprisingly, they can turn the water red. But the term is a little slippery. Some nontoxic species can turn the water a reddish-brown; some toxic plankton may be abundant enough to be harmful, but they aren’t so plentiful as to tint the water.

Red tides happen across the planet, taxing marine ecosystems from Scandinavia and Japan to the Caribbean and the South Pacific. The first documented case of red tide was in the fall of 1947 along the Gulf Coast, when residents of Venice, Florida, noted thousands of dead fish and a “stinging gas” that punctuated the air. While that was the first time the phenomenon was recorded by scientists, Florida residents had been reporting similar events since the mid-1800s.

HABs raise a red flag because they have an impact on human health and marine ecosystems, but they can have far-reaching effects on regional economies as well — tourism and fishing, in particular. The toxins produced by these harmful algae not only discourage swimming and make the air hard to breathe, but they kill fish and make shellfish dangerous to eat.

Red tide dangers

At times, red tides are bad enough to shut down the shellfish harvest. (Photo: Kenton Blagbrough/Shutterstock)

In 2012, Texas endured an exuberant red tide that lead to the collapse of its local oyster industry. The Gulf’s algae, K. brevis, produces a neurotoxin called brevetoxin that accumulates in exposed shellfish and leads to neurotoxic shellfish poisoning, a type of food poisoning that brings with it severe gastrointestinal stress and neurologic symptoms, like tingling fingers or toes. Brevetoxin is very reactive and attaches to other molecules, making them hard to detect during shellfish health testing. When combined with a lipid, brevetoxins can accumulate in internal organs and become more potent on nerve cells, causing even more risks for shellfish consumers.

Human health problems that come from eating brevetoxin-tainted shellfish are well documented, notes the Centers for Disease Control and Prevention (CDC), but scientists don’t know very much about how other forms of exposure to brevetoxin may affect us. “Anecdotal evidence suggests that people who swim among brevetoxins or inhale brevetoxins dispersed in the air may experience irritation of the eyes, nose, and throat, as well as coughing, wheezing, and shortness of breath,” says the CDC. “Additional evidence suggests that people with existing respiratory illness, such as asthma, may experience these symptoms more severely.”

Aside from fish and shellfish, other species are hard-hit by red tide as well. In 2013, 276 manatees died in southwest Florida due to red tide — marking an increase of 30 percent over the former highest mortality tally for these gentle giants. There is a lively debate about whether red tides are getting worse, or if it’s just a perspective shift as awareness and monitoring are increased. Some people, like Rob Magnien from the National Oceanic and Atmospheric Administration (NOAA), says an actual change has occurred. “Most people believe it’s not just the ability to detect [harmful blooms],” Magnien, who is the chairman of a United Nations panel on harmful algal blooms, told E&E News Service. “There are true increases in frequency and severity of blooms.”

Audubon Magazine lists potential culprits such as ships that inadvertently bring in microbes and the increased runoff of nutrient-rich ingredients like fertilizer and sewage into the sea. Climate change may also be a contributing factor, they note. Scientists suggest that changing agricultural practices are likely a primary cause. “We’re flooding the ocean with fertilizer,” says William Sunda, a phytoplankton ecologist with NOAA. The fertilizer provides a feast for dinoflagellates; fertilizers are created and used to make crops and lawns flourish – why wouldn't the same hold true for plankton?

Because HABs travel whimsically due to winds and tides, pinpointing a red tide at any given moment is challenging. But researchers at the National Ocean Service have been working for years on improving detection and forecasting of HABs. In the meantime, if you live in an area that experiences red tides, be sure to heed local warnings during algal blooms ... and if you notice red-tinged seas and "stinging gas" in the air, know that the dinoflagellates are getting rowdy and it's time to step away from the shore.

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