Red tide hit hard on Anna Maria Island
Researchers at the University of South Florida published a new study on red tide, finding ocean circulation to blame for the 2018 red tide outbreak — considered the worst in more than a decade — and that land-based pollutants were not the cause.
Published in the Journal of Geophysical Research-Ocean, marine scientists at USF showed that ocean circulation played a controlling role in both the intensity of red tide and its widely impacted range on Florida’s Gulf of Mexico and Atlantic coasts.
The report states Tropical Storm Gordon, which churned the Gulf of Mexico in September of last year, played a major factor in helping red tide spread from the Florida panhandle all the way to Palm Beach County on the east coast.
Robert Weisberg, USF professor of physical oceanography, told the Bradenton Herald on Thursday the findings dispel “the myth that land-based fertilizers are to blame.”
However, Weisberg acknowledged pollutants may play a role in exasperating red tide conditions, but, “they are not the root cause. Of course, we don’t want to be polluting our waters but to blame red tide on land-based runoff is a stretch. But we don’t really even know that for sure, but we do know it doesn’t cause red tide.
“Now, what’s coming out of the Caloosahatchee River is a completely different problem. It’s not a red tide algae, but I’m not downplaying the need to be good environmental stewards.”
The red tide algae, Karenia brevis, causes respiratory issues in animals and was responsible for mass fish and marine wildlife kills in Manatee County waterways and other areas along the Gulf Coast during the outbreak. It was a prolonged event, first appearing in the winter of 2017 and eventually expanding and intensifying into Manatee County by August of last year.
The event did not end until February of this year. Red tide season typically begins in late summer.
Red tide experts at both Florida International University in Miami and Mote Marine Laboratory in Sarasota said because the report is so recently published, they could not yet comment on Weisberg’s findings. However, Stephannie Kettle, Mote’s public relations manager, said, “However, identifying the controls — in this case physical — on bloom initiation and expansion do lead us closer to being better able to predict bloom stages, and thus potentially targeting them for mitigation.”
Kettle said Mote already works closely with Weisberg and his team at the USF, “so our science staff look forward to communicating with them more about this report and their findings.”
Scientists are far from unanimous on what is most to blame for red tide outbreaks like the one that hit Florida last year.
In a 2007 study, Larry Brand, a University of Miami Rosenstiel School of Marine and Atmospheric Science phytoplankton ecologist, argued that increasing pollution has worsened red tide.
“It’s a natural thing. You can go back 500 years. What I argue is that it’s much worse today and that’s the result of human-generated nutrients,” he told the Miami Herald last year. “Obviously, it’s political. There’s no one source and obviously some sources are more important than others.”
A scientist at Mote Marine disputed Brand’s findings, saying his conclusions were based on records that included inconsistent sampling.
“One of the things we don’t know because there’s not measurements, or sufficient measurements of it, is how much, for example, nutrients flowing through the Caloosahatchee from Lake Okeechobee actually make it out to the coastal system,” Vince Lovko, an ecologist at Mote, told the Miami Herald. “It’s easy to make the assumption. ... That’s what the public has been assuming. But we can’t. That’s not how science works.”
How circulation impacts red tide
It would be easy to assume that if offshore waters contain a high level of nutrients then red tide outbreaks would be severe given that the organism feeds on nutrients to survive. However, it’s just the opposite, Weisberg said.
“Think about your lawn,” Weisberg said. “You don’t fertilize it in the spring because it won’t grow. Red tide works the same way because if nutrient levels are high offshore in the spring, other phytoplankton will grow much more rapidly than red tide and it won’t be able to bloom. If nutrients are depleted, red tide has mechanisms where it can obtain what it needs. So other plants will thrive when nutrients are high and red tide will thrive when the waters are nutrient depleted.”
Ocean circulation has a lot to do with that, Weisberg said. Water movements in the Gulf of Mexico are driven by several factors, including wind and interaction with the continental shelf. What the report describes as “upwelling” is when deeper water moves upward toward the surface.
The movement of the water determines where those nutrients will be the richest during key times when Karina brevis is, or is not present and where.
Since research has shown that red tide does not originate near shore but offshore in deeper waters, upwelling brings red tide to the surface and ocean circulation under the right conditions of depositing nutrients can cause red tide to expand to the levels reached during the last outbreak.
In this case, circulating water with low nutrient levels allowed for a massive bloom to develop and that same circulation pattern allowed the new bloom in the summer to merge with the lingering bloom that began in October of 2017.
Weisberg and his colleagues have been able to tie red tide outbreaks, or lack thereof, to ocean circulation in 20 of the last 25 years.
Will red tide return this summer?
The Florida Fish and Wildlife Conservation Commission, in collaboration with Florida State University researchers, deployed an underwater glider for almost a month, helping to pinpoint where the red tide outbreak originated, which was between 30 and 50 miles off the coast, from north Tampa Bay to Sarasota Bay.
As research continues into predicting red tide outbreaks, Weisberg’s study, “defines where and when we have to look,” he said. “In the past, most sampling occurred along the coast when someone complained about red tide. So FWC collects the water, looks under a microscope and most of our observations dating back to 1953 are event based like that.
“Not until recently have we been able to be more accurate with where it originates. It doesn’t originate onshore, it originates offshore where we can’t see it so we have been dealing with the end result of a bloom, not the initiation of a bloom. So this research determines where and under what conditions, so it better defines for us where we need to be looking in the future.”
Weisberg said the information may indeed be useful in predicting future blooms, but what to do about it remains a mystery.
“This is certainly a first step in mitigation efforts, but even thinking about the spatial area of of where red tide can develop is so large, it’s hard to imagine a simple way of treating that area,” he said. “Especially along the bottom where you can’t see it. It’s a very difficult problem and the other unknown with mitigation strategies is what are the consequences? We might suffer in other ways, so we have to be careful what we want to do.
“If we start killing red tide, what else are you going to kill? Not all strategies are possible and we have to be careful not to trade one environmental disaster with another.”
As for this summer, Weisberg said recent samplings show a very low presence of red tide, so there is no immediate concern.
However, “it is too early speculate on what future conditions may be.”
He said his team expects to have a better idea of the possible severity of the 2019 red tide season by mid-June.
Mark Young: 941-745-7041, @urbanmark2014