Red tide researchers develop two indices to offer objective way to gauge severity of events

Findings published in peer-review journal PLOS One

Red tide is evident offshore of this Florida community. Image courtesy of GCOOS

Researchers who analyzed red tide events along Florida’s west coast between 1953 and 2019 have developed two new indices to “offer a standardized and objective way to gauge how severe red tides are,” the Gulf of Mexico Coastal Ocean Observing System (GCOOS) has announced.

The indices should help decision makers better evaluate the risks of those events “and respond in ways that lessen the impacts on public health and local economies,” a news release points out.

The researchers recently published their findings in the peer-review journal PLOS One, the release notes.

Richard Stumpf. Image from the NOAA website

“Red tide blooms are described as hitting Southwest Florida nine of every 10 years,” said the study’s lead author, Richard Stumpf, an oceanographer with the National Oceanic and Atmospheric Administration’s National Centers for Coastal Ocean Science (NOAA-NCCOS), in the release. “But not all blooms are created equal. Some last longer,” he noted, while “some cause more respiratory irritation. Others affect a smaller area or produce less irritation.”

The study abstract explains, “Nearly all annual blooms of the toxic dinoflagellate Karenia brevis (Kbrevis) pose a serious threat to coastal Southwest Florida. These blooms discolor water, kill fish and marine mammals, contaminate shellfish, cause mild to severe respiratory irritation, and discourage tourism and recreational activities, leading to significant health and economic impacts in affected communities.”

Stumpf added in the release, “With these new indices, not only can we look historically at blooms using objective standards, we can also apply the index to current blooms to give the public a better idea of risk — similar to the way that the National Weather Service has developed the Heat Index, the Climate Extreme Index and other indices.”

During the development of the indices, the release says, the authors discovered the following:

  • A red tide bloom occurred somewhere on the Southwest Florida coast each year between 1994 and 2019, except for 2010.
  • In 2018, the Southwest Florida coast “had the most extensive red tide.” That year, the Respiratory Irritation Index in Sarasota and Manatee counties was also the highest on record.
  • “More frequent-than-usual offshore winds can suppress the respiratory impact of blooms.
  • “Wind direction matters for the amount of respiratory irritation. Years with severe blooms — 2006, 2012 and 2018 — caused noticeable respiratory irritation, while other years with blooms (2008, 2009, 2011, 2013, 2014) had almost no respiratory impacts because of more frequent than usual offshore winds.”
  • Sampling efforts before 1995 were not sufficient to allow for the comparison of bloom intensity.
  • Blooms most frequently occur between Sanibel Island and Tampa Bay between September and January.
  • “Red tides typically form in August and continue through the winter, with October and November being the months most frequently impacted.”

Kate Hubbard, who leads the Harmful Algal Bloom (HAB) Monitoring and Research Program for the Florida Fish and Wildlife Conservation Commission-Fish and Wildlife Research Institute (FWC-FWRI), noted in the release that the Bloom Severity Index and Respiratory Irritation Index are important tools for management agencies such as hers.

“Given the potential for respiratory impacts, having severity indexes we can use during and following blooms can help us better direct resources and evaluate potential mechanisms leading to variability over time,” she said in the release.

The most recent red tide event on the Southwest Florida coast began in the spring of 2021 and finally subsided late last fall.

This is Venice Inlet on Aug. 30, 2021, when red tide was present offshore. Image courtesy Florida Fish and Wildlife Conservation Commission

In early December 2021, FWC still was issuing twice-weekly updates on its sampling for the presence of Karenia brevis. For example, the Dec. 8 report said that the algae had been observed at low concentrations offshore of Pinellas, Hillsborough and Lee counties over the previous seven days. However, it added, no algae cells were found in samples collected from or offshore of Sarasota, Manatee, Charlotte, Collier or Monroe counties.

As of Dec. 17, 2021, FWC once again had reduced its reports to a weekly status.

In the four most recent updates prior to the publication of this issue of The Sarasota News Leader — on Jan. 21, Jan. 28, Feb. 4 and Feb. 11 — FWC noted that Karenia brevishad not been observed in any samples collected statewide over the previous week.

Details of the research and development of the indices

The introduction of the article in PLOS One explains that Karenia brevis was identified as the cause of “red tides” off the Southwest coast of Florida in the 1940s. “As a result of this awareness,” the study says, “more regular sampling of bloom events began in late 1953.”

This red tide severity index is shown in the article in PLOS One. Image courtesy GCOOS

The study further points out that Karenia brevis produces brevetoxins — PbTx — which are neurotoxins that accumulate in shellfish.” Neurotoxins, as the website Medscape explains, “are synthetic or naturally occurring substances that damage, destroy, or impair the functioning of the central and/or peripheral nervous system.” They also may cause metabolic imbalances that can produce secondary effects in the central nervous system, Medscape notes.

Thus, the presence of brevetoxins necessitates large-scale closures of shellfish beds, the study in PLOS One points out, as the “[c]onsumption of shellfish containing elevated PbTx concentrations causes neurotoxic shellfish poisoning (NSP).”

Further, unlike toxins from many other marine algae, the study continues, brevetoxins become aerosolized — released into the air — when Karenia brevis cells “break open in high turbulence (especially nearshore) and are subsequently transported onshore by winds. These aerosolized toxins cause respiratory irritation and pose a significant risk to beachgoers with chronic respiratory illnesses such as asthma,” the study adds. The toxins “have been measured at distances greater than 1.5 km from the shore during blooms,” it points out.

Karenia brevis cell counts for all of Florida were obtained from the HAB [Harmful Algal Bloom] Monitoring Database of the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Institute (FWC-FWRI),” the study continues. FWC-FWRI’s quality-controlled data set “was obtained as Excel spreadsheets on March 30, 2020,” the study notes.

This is the area that was the focus of the study. Image courtesy GCOOS

“The data set included sample date, longitude, latitude, water depth, observed Kbrevis cell counts … and the county where samples were taken, if relevant,” the study says. “Offshore sampling was highly variable in time and space, and heavily dependent on short-term research projects.” To ensure use of a consistent data set, the cell counts collected within about 5 km — 3 miles — of the coast were extracted through use of a software program, the study explains.

“Bays and estuaries were excluded by a straight line across their entrances,” it adds, “as bays are also inconsistently sampled, and do not directly impact Gulf beaches, where respiratory irritation is most commonly indicated.”

Subsequent analyses were conducted on a subset of the Southwest Coast of Florida — between 25.4° and 28.4° north, “which has had routine monitoring and where blooms typically occur annually,” the study says. “All water depths were included in the analysis, given the proximity to the shore.”

The Beach Conditions Reporting System (BCRS), maintained by Mote Marine Laboratory in Sarasota, “provided respiratory irritation reports, based on the intensity of coughing, a good indicator of brevetoxin aerosol impact,” the study explains. Volunteers routinely reported respiratory irritation conditions to the BCRS database at six beach sites in Sarasota County between September 2006 and January 2019, the study points out, along with two in Manatee County starting in January 2007. These sites were Lido Key Beach, Siesta Key Beach, Venice North Jetty, Nokomis Beach, Venice Beach, and Manasota Beach in Sarasota County, along with Manatee Beach and Coquina Beach in Manatee County.

This chart shows the relationship among the monthly respiratory irritation impact (RI) index, monthly bloom severity index (BSI), and winds.
The relationships are shown for algae cell counts higher than 50,000 cells for Sarasota and Manatee and counties. Color-coding represents the onshore wind frequency anomalies (OWFA) for each month, with positive (red) indicating onshore, and negative (blue) indicating offshore. Months when blooms were not observed were not shown. The size of the circles represents the wind speed for the month. The period of April through June 2007 was not included because respiratory irritation during those months was caused by the Bugaboo Scrub wildfire. The black dashed line shows the linear regression fit between the monthly RI and BSI. Image from the study in PLOS One

“Reports were made once or twice daily from the same site on the beach by trained sentinels,” the study notes. “These included report time, location, and corresponding respiratory irritation severity measure determined by the coughing frequency, namely the frequency of coughs and sneezes heard in 30 seconds … The respiratory measure falls into the following four categories,” the study adds: “none (i.e., no respiratory impact), slight, moderate, or high (severe) respiratory irritation.”

In its Conclusions section, the study also points out that both the bloom severity index (BSI) and the respiratory index (RI) have shown that while “some locations might be affected for several consecutive months during severe blooms, the entire coast is not constantly impacted from initiation to termination. The bloom severity and respiratory irritation indices can be updated and extended in real-time to inform stakeholders and to aid other studies on Kbrevis. These indices can help managers and decision makers evaluate the risks along the coast during a bloom,” but also in a year-to-year context, “and help them plan ways to better respond to and mitigate bloom impacts.”