By Dave Tomasko, director of the Sarasota Bay Estuary Progam
I recently received notification that a manuscript my co-authors and I produced was accepted for publication in the upcoming issue of the peer-reviewed scientific journal “Florida Scientist.” The title of the paper is “An evaluation of the relationships between the duration of red tide (Karenia brevis) blooms and watershed nitrogen loads in Southwest Florida (USA).” My four co-authors include Lenny Landau and Steve Suau (both highly talented and creative local engineers), Miles Medina (a brilliant statistician) and Jennifer Hecker, the director of the Coastal and Heartland Estuary Program.
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 example, the SBEP’s Technical Library includes this paper, which showed a relationship between the intensification of red tide events and Caloosahatchee River nutrient discharges, 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 O from the north.
Also in our technical library 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? Well, we wanted to see if we could develop a robust, predictable and quantifiable relationship between human activities and the duration of red tide events.
We started with a study area that extended from Lake Okeechobee and the northern sections of its watershed, then added watersheds for the Caloosahatchee, Myakka and Peace rivers, plus Horse and Joshua creeks. Offshore, we had thousands of samples testing for red tide from the mouth of Tampa Bay south to Estero Bay. All together, there were over 40,000 data points between 2007 and 2022. We chose the more recent years 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 yearly. The stations on land – places where both streamflow and water chemistry data were both collected — gave us a direct estimate of nutrient loads.
Next, 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,” 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 those ranges exceeded that threshold 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.
The shortest event identified using this approach lasted about a month, while the longest one (2017 to 2019) lasted more than a year.
The next thing we did was 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 in time between 30 to over 300 days, the nitrogen loads examined were always kept to a 60-day period, thus eliminating the potential for what could be called 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. The Caloosahatchee River loads about as much water and nitrogen as the other four systems combined. We also can track the drought of 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.
For the final step, we compared the duration of those 11 or 12 red tide events and the nitrogen load delivered during the initial 60-day period of each event. We discovered:
1) there is a statistically significant relationship 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,
2) however, the relationship is driven by the Caloosahatchee River, as when it stands alone, the relationship becomes stronger, and without the Caloosahatchee, no relationship was found.
3) creating plots tracking those values showed very clear statistically significant correlations between nitrogen loadings and the duration of red tide events. In the Caloosahatchee River, 77% 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.”
So why are we so excited about this study 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, fetid waters and economic hit associated with red tides.
Also, the value of this study is that it 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. 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. There are several projects that are involved with Everglades Restoration that have the potential to decrease nitrogen loads down the Caloosahatchee River – but what is the likelihood of them being big enough to make a measurable difference in the duration of red tide events? This study lays the groundwork for getting that information.
This study should not be interpreted as saying that ONLY the Caloosahatchee River influences the duration of red tide events. 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.
There is a 15-year-old report from the Florida Department of Environmental Protection (FDEP) that 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. This means we need to do more, including working to control nitrogen loadings in the central part of the state and enhancing our efforts to reestablish wet weather storage in areas above Lake O, in addition to efforts to build reservoirs downstream of the lake.
What this paper concludes 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.
How much more motivation do we need?