water quality blog

Rare Red Tide Makes Its Way to South Florida

Southwest Florida’s year-long red tide has spread to the east coast of South Florida, forcing local and state officials to close beaches and issue advisories right at the start of the busy winter tourist season. As Florida’s most populous region, the impacts on South Florida could be immense. Red tide also poses a significant threat to Biscayne Bay. Although red tides are notoriously difficult to predict, nutrient pollution and low levels of freshwater influx into Biscayne Bay create conditions known to allow the red tide causing algae, Karenia brevis, to bloom. Upcoming King Tides in October and November can make the outbreak even worse if more algae gets pushed closer to shore and into Miami’s protected bays and canals.


In early October, FWC reported elevated levels of K. brevis off Palm Beach County’s coast. Later that same week, elevated levels were confirmed on St. Lucie, Martin, Broward, and Miami-Dade Counties’ coasts as well. Red tides occur when the K. brevis interacts with favorable water conditions to cause the algae to “bloom.” There is no single identifiable cause of red tides, but water pollution, water salinity, water temperature, nutrient concentrations, winds, and currents all play contributing roles. Human-caused nutrient pollution (think fertilizer run-off from lawns and farms) plays a huge role in increasing the duration and severity of red tide events.  While K. brevis is common in the Gulf of Mexico, it does not naturally occur on Florida’s Atlantic coast. However, similar to concerns with oil pollution after the Deepwater Horizon spill in the Gulf, currents can drag significant concentrations of the algae around to the Florida Straits. Strong onshore winds can then push the algae closer to shore.


Red tides produce toxic chemicals that affect both marine organisms and humans. In the water, toxins produced by the bacteria result in large fish kills. Shellfish can also take up the toxins through filter feeding, making them unsafe to eat. When close to shore, winds can make the toxins airborne, causing health issues like burning eyes or difficulty breathing in sensitive individuals. They can even cause skin irritation for those swimming in affected waters. The best precautions to take are to avoid beaches and areas with even low levels of K. brevis.


To prevent future outbreaks, we can all take action now by changing behaviors and making planning decisions that consider environmental impacts. For example, using fertilizers without phosphorus will reduce the amount that may be carried into waterways by stormwater runoff. When doing lawn work, you should also avoid leaving grass clippings and leaves on streets or sidewalks where they can be washed into storm sewers and carried into our waterways. If you live on the water, plant native plant species along shorelines to absorb runoff contaminated with nutrients and organic matter before it can flow into the waterway. While a red tide will certainly occur again, we may be able to reduce its severity and frequency by educating ourselves and making sound environmental choices.




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