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Harmful Algal Bloom

Motion of the ocean: How ocean circulation can trigger an intense red tide

Robert H. Weisberg, Yonggang Liu, Chad Lembke, Chuanmin Hu, Katherine Hubbard, Mathew Garrett. The Coastal Ocean Circulation Influence on the 2018 West Florida Shelf K. brevis Red Tide BloomJournal of Geophysical Research: Oceans, 2019; DOI: 10.1029/2018JC014887

Imagine shorelines covered with masses of decaying fish, dead whale sharks and manatees floating in the water, signs warning people not to swim, and a ghastly stench (not just rotting flesh but poisonous toxins), causing people to cough and gag as their lungs began to tingle. This was the shoreline of Florida from September 2017 to January 2019 as an intense red tide bloom overtook both the east and west coasts of the Sunshine State. Researchers at the University of South Florida and the Fish and Wildlife Research Institute, led by Dr. Robert Weisberg, set out to discover the origin of the red tide ravaging the coast. What they found is that ocean circulation may be at the center of triggering intense blooms of toxic algae.

A red tide in La Jolla, California. Intense blooms can be spotted from the high concentration of dinoflagellates in the water, giving the patches their characteristic red color. Photo taken in 2005 by P. Alejandor Diaz and found on Wikimedia Commons.

The Perpetrator

The red tide is the handiwork of a dinoflagellate called Karenia brevis. If looking at K. brevis under a microscope, you would see a small, one-celled alga with two whip-like tails. It generates food through photosynthesis, like plants and other algae. However, K. brevis has one other characteristic that makes it so deadly: it releases toxins that kill fish and can make humans incredibly sick. It is this final characteristic that makes it such a menace to the Florida coast and to tourism. A year with a bad bloom hurts fishing, boating, swimming, and the Florida economy, and while it is not unusual for a red tide to hit Florida, the most recent one was particularly hard on the state.

Up until this point, how K. brevis manages to become such an all-encompassing bloom has been a bit of a mystery. In order for phytoplankton to grow, they need nutrients like nitrogen. These nutrients are in limited supply in the ocean: once the nitrogen is used up, phytoplankton can’t continue to grow, so whichever species of phytoplankton can obtain the nutrients first are generally the ones that get to bloom. That’s the central challenge for K. brevis; dinoflagellates grow slower than other plankton like diatoms, so if nutrients are made available through upwelling (when nutrient-rich, deep ocean water is pushed to the surface) diatoms grab them and bloom before K. brevis has a chance. So if K. brevis is at a disadvantage biologically, how come massive blooms of this dinoflagellate are such a menace?

The perpetrator of red tide, Karenia brevis. Notice the two whip-like appendages, called flagella, which give the name to this kind of organism, dinoflagellate. Photo by the Florida Fish and Wildlife Conservation Commission, found on Wikimedia Commons.

Offshore Origins

In order to understand what was going on, Weisberg and his team released a glider from August 24, 2018 to September 17, 2018. The glider took water quality measurements, primarily temperature, salinity, chlorophyll (the chemical used for photosynthesis which can be used to measure phytoplankton concentrations), and dissolved oxygen. By comparing these water quality measurements with models of the way the ocean flows around Florida, the team thinks they have a compelling argument for the origin of K. brevis.

Dr. Weisberg and his team suggest that the red tide has to originate somewhere else and be transported to the coast. While upwelling in coastal waters brings nutrients that feed diatoms, K. brevis has an advantage in nutrient-poor waters. Off the coast, there wouldn’t be much nitrogen, but there also wouldn’t be a lot of silicate, an essential nutrient diatoms need to make their outer glass-like skeleton. Without silicate, it doesn’t matter how much nitrogen there is; the diatoms won’t be able to grow. This allows K. brevis to eat up the little nitrogen available before other phytoplankton.

Once a bloom starts, K. brevis can ensure that there are enough nutrients by releasing their toxins, killing fish, and then thriving on the nutrients released from their decaying corpses. This is how an organism that seems biologically prone to losing can take over Floridian beaches. On a normal day, there might be as many as 1000 cells of K. brevis in a liter of seawater; according to Dr. Rick Bartleson, in 2018, there were around 10 million cells per liter.

A fish kill caused by toxins released by K. brevis in Sarasota Bay, Florida. Photo by Leo Procise.

Getting to the Coast

So how do these cells get to the coast? Ocean circulation. Dr. Weisberg and his team propose that when K. brevis blooms start offshore, upwelling transports the cells to the western coast of Florida. However, if this upwelling happens too early in the bloom, it will also bring nutrients to diatoms on the coast, and K. brevis will be outcompeted. Done at the right time though, a well-established red tide can be brought to the western coast of Florida.

In the 2018 bloom, however, the red tide spread farther. The researchers believe the bloom made it to the Panhandle with help from Tropical Storm Gordon and that continued upwelling introduced K. brevis cells into the Gulf Stream, transporting the red tide up the east coast. The 2018 bloom in particular was so ghastly because there were two years in a row, 2017 and 2018, that ocean circulation created ideal conditions for red tide. As a result, the red tide in 2018 added to the already existing bloom from 2017. Together, these blooms caused a historic red tide that killed countless sea creatures, sickened locals, and sucker-punched Florida’s tourism industry.

While the exact origin of K. brevis is still unknown, determining what conditions allow a red tide to form is an excellent step in predicting future blooms. Even less understood is what causes the red tide to eventually disappear. The end of a particularly bad bloom in 2005 started, ironically, with the destructive force that was Hurricane Katrina, but why? In many ways this phenomenon is still mysterious, but the secrets of red tide are linked to the motion of the ocean. We will have to watch where it goes.


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