Sarasota Bay Estuary Program director reports on initial results and discusses ‘1,000-year’ storm events
An Aug. 8 sampling of water at nine stations between Venice Inlet and the Ringling Causeway in Sarasota found that the bottom of Sarasota Bay had much higher salinity levels, except at the site by the mouth of Philippi Creek, David Tomasko, executive director of the Sarasota Bay Estuary Program SBEP, reported on Aug. 9 on the organization’s website.
The initiative was undertaken by SBEP representatives and staff of the Southwest Florida Water Management District (SWFWMD), after Sarasota County staff made an official request of SWFWMD, Tomasko noted.
“As a reminder,” he wrote, “freshwater has a salinity value of about 0 practical salinity units (psu) while the Gulf of Mexico has salinity values of about 35 psu.” As seen in a chart below, he continued, “the surface waters in our lower bay segments were not pure freshwater (although the location of Roberts Bay Central — located by Philippi Creek — was close to freshwater …”
The bottom row in the table, he explained, “quantifies what we call ‘salinity stratification’ or the difference in salinity values between the top and bottom. Values higher than about 10 psu or so represent conditions where there is a substantial enough difference in salinity that the bottom waters are functionally separate from surface layers. These conditions were strongest in northern Roberts Bay and central and southern Blackburn Bay,” he pointed out.
“Somewhat different from what we saw after Hurricane Ian, salinity stratification was not as strong in Little Sarasota Bay, on average,” Tomasko added.
During an April 2023 presentation to the Sarasota County Commission, focused on the potential restoration of a tidal connection between Little Sarasota Bay and the Gulf of Mexico, Tomasko explained that in cases of salinity stratification and bottom water hypoxia, an influx of freshwater rests above the saltier water, and the saltier water is unable to get oxygen from the atmosphere or photosynthesis. “Very poor habitat quality” results, he added.
A week after Hurricane Ian’s strike on Southwest Florida in late September 2022, he told the commissioners, he worked with researchers to collect water samples from Little Sarasota Bay, Roberts Bay and Blackburn Bay. The samples showed the biggest problems with stratification and bottom water hypoxia were in Little Sarasota Bay, Tomasko pointed out.
That situation “lasted at least two weeks,” he added.
“When the bottom waters of the bay are ‘isolated’ from the surface waters,” he wrote in his Aug. 9 report, and “oxygen levels get low enough to kill small fish and crabs and worms and starfish (things that can’t ‘swim away’ very effectively) [and] then the decomposition of those organisms that die on the bottom of the bay will further reduce oxygen levels as bacteria decompose all those newly dead animals on the bottom of the bay,” he continued.
In an Aug. 14 update on the SBEP website, Tomasko noted, “Earlier today, I was contacted by someone who saw blue crabs swimming to the surface in Little Sarasota Bay, which likely indicates that the bottom of the bay’s levels of dissolved oxygen are crashing.”
The next table displays the results from the sampling for dissolved oxygen (DO), Tomasko added in the Aug. 9 report.
“Levels of DO below about 2 mg/L [milligrams per liter] are considered ‘hypoxic,’ which means that most organisms cannot long withstand such values,” he explained. “The results shown here do not indicate a problem with DO levels in the surface waters of any of the locations that were sampled.
“However,” he pointed out, “problematic or nearly problematic DO levels were found in the bottom layer of water from southern Roberts Bay down into northern Little Sarasota Bay, and also in central Blackburn Bay. Some of the areas with the greatest salinity stratification (e.g., northern Roberts Bay)” did not have DO levels that low, he added. “So, these results are already a bit different from what we found after Ian came through.”
Yet, Tomasko noted, “it’s early still. It could be that water quality continues to decline over the next few days to weeks. Why? Well, it may take a while for bacteria to respond to the big flush of stormwater runoff, dead fish, and sewage overflows that just came into the bay. Which could mean that next week’s sampling effort could give us worse results than what we have this week.”
He pointed out that “our region has been impacted by untold millions of gallons of sewage overflows associated with this rain event [produced by Tropical Storm Debby], along with a big slug of urban stormwater runoff, as seen in the photo below”:
Tomasko also pointed out that “lab parameters” were not among the data he provided in the Aug. 9 report. Those include the nutrient content (nitrogen is the primary food of the red tide algae), “the amount of algae in the water column, and, very importantly, the amount of bacteria in the water column.”
Bacterial levels exceed safe values for entering the water
In the Aug. 14 update, Tomasko reported, “Well, we just got our bacteria levels back … Nine locations were sampled out in the middle of the bay, from Ringling Causeway south to Venice Inlet (the same stations sampled after Hurricane Ian came through). We compared our results against the Class 3 Marine waters’ standard for the fecal indicator bacteria ‘Enterococci’ standard of [130 colony forming units per 100 milliliters], and each site exceeded those values.”
He added, “Roberts Bay had the highest values, by far, as shown below”:
“These values are from the middle of the bay,” Tomasko explained, “not the shoreline. Which means they are likely lower than values closer to the shore, as well as values in our numerous creeks.”
Therefore, Tomasko warned people not to go into the water, “at least until we have evidence of improving water quality.”
In an Aug. 5 email that Tomasko sent to members of the SBEP Board of Directors and other local government leaders in the Cities of Sarasota and Bradenton, and Sarasota and Manatee counties, Tomasko wrote that while he did not have “ ‘official’ records yet … parts of our watershed may have received as much if not more rain as we got with Hurricane Ian in 2022, or the high-intensity rain event we had in June,” referring to “Invest 90L.”
However, he continued, “Debby brought about a degree of wind and storm surge that was broadly similar to what happened with last year’s Tropical Storm Idalia. So, Tropical Storm Debby seems to have brought us a combination of wind, rain, and storm surge that we haven’t seen in a few years.”
Sarasota and Manatee counties “had a great amount of street flooding,” for example, he noted. “All of the grass clippings, yard waste, dog poop, brake dust (your brakes wear out over time), and tire residue (your tires wear out as well) across a big chunk of our watershed just got washed into [Sarasota Bay]. That urban stormwater runoff mixed with the materials from the many wrecked boats out in the bay and also with highly turbid water from the Gulf of Mexico.”
More water sampling planned in coming weeks
In his Aug. 9 update, Tomasko wrote that more sampling was planned this week, with another round set for two weeks after that.
“After Ian,” he added, “we sampled these same stations one, two, and four weeks after the storm came through. … After Ian, our water quality at two weeks was still bad (low bacteria, high nutrients, high algae, high bacteria) but the water had recovered to pre-storm levels by four weeks. Hopefully, the same timeline works out for our post-Debby sampling,” he wrote.
Tomasko explained that it has been SBEP’s practice in recent years to collaborate with SBEP’s partners to “get out onto the water as soon as possible” to analyze the effects that a storm has had on the water bodies that make up Sarasota Bay, Tomasko explained.
“[W]e’ve found that the best role for the SBEP and its partners is to work with our local stakeholders to document water quality impacts, thus allowing local governments to focus their resources on the physical impacts to our neighborhoods. Under such circumstances, one might wonder why we’d even be concerned about water quality, when we have so many people dealing with flooded properties, etc.” he wrote.
‘Once-in-a-thousand years’ events
Tomasko also included in his Aug. 9 report a suggestion for readers to think about the phrase, “one-in-a-thousand year event,” in discussions about rainfall.
“Ever wonder where those categories come from?” he asked. “[T]he amount of rainfall that folks design infrastructure for is based on a statistical analysis that is itself based on datasets that go back 50 to about 100 years. That’s a lot of data! But we don’t have 1,000 years of rainfall data. Instead, folks take the data we have, and figure out how ‘rare’ an event must be, based on whether it has happened before. Something quite rare (or never recorded) in the historical record is given a lower probability of happening in the future. That’s how you get rainfall values that are considered rare enough to be called ‘thousand-year storms.’ ”
The graph below, from the National Oceanic and Atmospheric Administration’s Precipitation Frequency Data Server “displays expected amounts of rainfall at a latitude and longitude within our watershed,” Tomasko pointed out. “It shows, for example, that about 12 inches of rain in 24 hours is only expected to occur about once in a hundred years. Well, we had about that much rain, or more, twice in the last two years, after Ian and Debby.”
“Over the past few years,” he continued, “we’ve had individual rain events in the range of 100-year events at least twice now (this hurricane season is not over yet). We also had, on June 10th of this year, what appears to have been the highest hourly rainfall amount recorded over the past 50 years.” Referencing the “Invest 90L” downpours in Sarasota, he noted that it seems that nine “of the top 10 highest hourly rain events have happened over the past 25 years, and only one over the prior 25 years.”
Then Tomasko explained, “Our air is warmer now than what we experienced during most of the 20th century … And we know that warmer air can hold more water vapor. When I flew small planes, I could calculate the height of the cloud layer by knowing the ambient air temperature and dew point alone, based on the same principle. So maybe we shouldn’t be so surprised if our recent and likely future rainfall amounts result in previously ‘rare’ events occurring on a ‘not so rare’ basis.
On its website, the SBEP explains that it is one of the 28 National Estuary Programs, each of which is “charged with developing and implementing a Comprehensive Conservation and Management Plan (CCMP) … [That] establishes priorities for activities, research, and management of the estuary. The CCMP serves as a blueprint to guide future decisions and actions and addresses a wide range of environmental protection issues and opportunities including water quality, habitat, wildlife, and public access to Bay resources.”