Article on relationship between red tide blooms and nitrogen loads in watersheds, co-authored by director of Sarasota Bay Estuary Program, to be published in peer-reviewed journal

County Commissioner Smith points to opportunities to curtail red tide outbreaks, based on the research

David Tomasko. Image courtesy Sarasota Bay Estuary Program

During his report to his colleagues as part of their regular meeting on June 4, Sarasota County Commissioner Mark Smith announced that David Tomasko, executive director of the Sarasota Bay Estuary Program (SBEP), had been notified that a manuscript Tomasko had co-authored about the relationship between red tide blooms and nitrogen loads in watersheds had been accepted for publication in a peer-reviewed scientific journal.

In a May 14 report on the Sarasota Bay Estuary Program website, Tomasko wrote that the scientific journal in which the article will be published is Florida Scientist. The formal title of the paper, he added, is “An evaluation of the relationships between the duration of red tide (Karenia brevis) blooms and watershed nitrogen loads in Southwest Florida (USA).”

As a long-time leader of the Siesta Key Chamber of Commerce prior to his November 2022 election to the county board, Smith has talked with The Sarasota News Leader about his support for initiatives to reduce red tide events and research into killing red tide without harming other elements of the environment.

During his remarks as part of the June 4 County Commission meeting Smith pointed out, “What’s great about this” news regarding the peer-reviewed article is that it outlines measures that can be taken. He added that the commissioners should talk with leaders of the Chambers of Commerce in the county about a “concerted effort on how we contain and curtail the nutrients [in the watersheds] in order to diminish the duration of red tide in our area.”

Long-lasting red tide events have dealt significant blows to the county’s economy, as city and county officials have pointed out in the past.

Tomasko noted in his May 14 report on the SBEP website that his four co-authors are “Lenny Landau and Steve Suau (both highly talented and creative local engineers), Dr. Miles Medina (a brilliant statistician) and Jennifer Hecker, the Director of the Coastal and Heartland Estuary Program.”

Tomasko then explained, “A few years ago, there was a bit more controversy regarding what role — if any — humans have on red tides. While that may have been an appropriate view a few years ago, anyone who currently thinks that humans don’t play a role in red tide events either isn’t familiar with recent studies or is just being stubborn for some reason. Ten or twenty years ago, it was appropriate to be skeptical of such a link, but not over the past few years.”

For one example, Tomasko wrote, the SBEP’s Technical Library includes a paper about a “relationship between the intensification of red tide events and Caloosahatchee River loads, as well as evidence that a substantial amount of nitrogen loads out of the Caloosahatchee can be traced back to nitrogen loads coming into Lake [Okeechobee] from the north.”

Image from the South Florida Water Management District, via the Sarasota Bay Estuary Program

“Also in our technical library,” he continued, “is this paper, which showed a link between the red tide event in middle Tampa Bay in 2021 and nitrogen loads associated with the releases from the Piney Point facility back in 2021.

“So what was unusual about this recent study?” he wrote. “Well, we wanted to see if we could develop a robust, predictable and quantifiable [his emphasis] relationship between human activities and the duration of red tide events.”

At that point, Tomasko provided his first graphic, explaining, that the study area includes red dots representing more than “40,000 data points where folks from the Florida Fish and Wildlife Conservation Commission [FWC], Mote [Marine Laboratory in Sarasota] and others have sampled for red tide during the years of 2007 to 2022. We chose those more recent years,” Tomasko continued, “because we had a minimum of 1,000 sampling events each year (the lowest was the Covid year of 2020) and averaged well over 2,000 samples per year. Notice also the station locations on the land — places where both streamflow and water chemistry data were both collected. If you collect data on streamflow and nutrient concentrations,” he added, “you have a direct estimate of nutrient loads.

Image courtesy Sarasota Bay Estuary Program

“Next,” he noted, “we defined a red tide ‘event’ as one where levels of the red tide organism, Karenia brevis, exceeded two threshold values: 100,000 cells per liter, and 10,000 cells per liter. [FWC] considers values above 100,000 cells to be ‘high,’ he explained, “while values as low as 5,000 cells per liter can cause problems with breathing and fish kills. We considered an event to be a period during which daily maximum K. brevis counts in the region of interest exceeded those thresholds for at least a month (with some allowances to take into consideration reduced sampling during weekends, storms, etc.). Using that approach, we determined that there were 12 periods during which a low-threshold red tide event (10,000 cell count) occurred, and 11 periods during which a high-threshold red tide event (100,000 cell count) occurred …”

Tomasko inserted the second graphic at that point in his article:

Image courtesy Sarasota Bay Estuary Program

“The shortest event identified using this approach lasted about a month, while the longest one (2017 to 2019) lasted more than a year,” he wrote.

Nitrogen loads and hydraulic loads

Next, he continued, he and his co-authors pursued a retrospective analysis, “comparing the nitrogen load from the 30 days just before and after the initiation of each event to the duration of each event.” While the events ranged from 30 days to more than 300 days, he noted, “the nitrogen loads examined were always kept to a 60-day period, thus eliminating the potential for what we call a spurious conclusion.

“The hydraulic loads (amount of water) and nitrogen loads match up quite well because the five rivers we looked at didn’t have that big of a difference in nitrogen concentrations,” he pointed out. “As seen below [in the third graphic], the Caloosahatchee River loads about as much water and nitrogen as the other four systems combined (Peace and Myakka Rivers and Horse and Joshua Creeks). You can also see the effects of droughts in 2007, a very wet wet season in 2013, huge deliveries of water (and nitrogen) after Hurricane Irma in 2017, and a similar but smaller effect of 2022’s Hurricane Ian.”

Image courtesy Sarasota Bay Estuary Program

The final step, he wrote, entailed a “comparison between the duration of those 11 or 12 red tide events and the nitrogen load delivered during the initial 60-day period of each event. The figure below represents the relationships between the duration of red tide events (on the vertical axis) and the nitrogen load during the 60-day period for each of the 11 or 12 events,” he added of the fourth graphic:

Image courtesy Sarasota Bay Estuary Program

“Here’s a quick summary of what the results mean,” Tomasko added:

  • A statistically significant relationship exists between the nitrogen load delivered the month before a red tide event, added to the first month of the event, across all five rivers and the duration of these events.
  • However, he continued, “the relationship is driven by the Caloosahatchee River, as when you use it alone, the relationship becomes stronger, and without the Caloosahatchee, no relationship was found.”

Then he wrote with more emphasis, “Seventy-seven (77) percent of the variability in the duration of red tide events across this region of Southwest Florida is explained by variability in the nitrogen loads delivered to these coastal waters during the 30-days just before and then just after the start of each event. Further, the likelihood that this relationship is due to chance alone is less than one in a thousand.”

Tomasko added that he and his co-authors are excited about the study’s being accepted for publication in part “because it is another bit of information that links what we do here on the land and how long red tides are going to last in our coastal waters. This means, getting our act together on reducing nutrient loads not only helps us on a day-to-day basis, but it also means that we are likely helping reduce the amount of time during which we experience the choking air and fetid waters and economic hit associated with red tides.”

Moreover, he pointed out, the study “provides a relationship into which you can plug different load reduction scenarios. For example, we have estimates that suggest that nitrogen loads to Tampa Bay, Sarasota Bay, Lemon Bay, Charlotte Harbor and the Caloosahatchee River are about two to four times higher than they were in pre-development years. If we apply that percent change to a similar percent change during the red tide events studied here, we can conclude that humans have likely extended the duration of red tide events by weeks to months, depending on the event.”

He further explained, “We can also use this technique — albeit it needs to be refined further to do this appropriately — to determine what the impacts might be of various projects.”

Tomasko did point out, “This study should not be interpreted as saying that ONLY the Caloosahatchee River influences the duration of red tide events, because in science, the well-known adage is that ‘an absence of evidence does not constitute evidence of absence.’ But it does highlight the importance of the Caloosahatchee River as a source of nitrogen loads that can exacerbate red tide events, making them larger, more intense, and (as shown here) longer-lasting.”

He added that a 15-year-old report from the Florida Department of Environmental Protection (FDEP) “determined that nitrogen loads coming into the lower reaches of the Caloosahatchee River should be reduced by 23%. We are nowhere near achieving that goal and won’t be able to reach it with the projects currently under construction. Which means we need to do more, including working up into the central part of the state, enhancing our efforts to reestablish wet weather storage in areas above Lake [Okeechobee], in addition to efforts to build reservoirs downstream of the lake.

A graphic presented during the 2019 Sarasota County Water Quality Summit shows details about research as of that time into potential connections between the Lake Okeechobee discharges and red tide in Southwest Florida. Image courtesy Sarasota County

“What this paper concludes,” he wrote, “is that if we actually do enough to meet that 15-year-old 23% nitrogen load reduction goal for the Caloosahatchee River, we will not only help our coastal environment and economies during ‘normal’ conditions, but we’d also likely reduce the duration of future red tide events by a substantial amount of time as well.”

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