Study finds horse-associated bacterial DNA in Palma Sola Bay samples; no human sewage detected

The Sarasota Bay Estuary Program presented a source‑identification study Sept. 10 showing detectable—but unquantified—genetic signatures linked to horses at three of four sampling sites in Palma Sola Bay, while tests returned no detectable human (sewage) signal.

Dave Temasco, director of the Sarasota Bay Estuary Program, told the Bradenton City Council the team used DNA‑based methods that can apportion indicator bacteria to likely sources such as humans, birds and horses. Temasco said the lab results available from the first sampling event showed horse‑associated DNA at three sites and a bird signal at the bathing beach site. When a source was detected, Temasco said, the abundance of that source in the total bacterial mix was too low to quantify.

The study followed public concern about elevated enterococci counts on the north side of the Palma Sola Causeway, where chartered horseback rides and some private horses enter the bay. Temasco said the region’s overall water‑quality indicators have improved in recent years: nitrogen concentrations and chlorophyll have trended down, and seagrass coverage has increased (about 40 acres gained from 2022 to 2024). He emphasized that Palma Sola Bay’s overall report‑card color is in the “good” range but that physical damage and localized bacterial exceedances can still occur.

Temasco described two recreational thresholds for bacteria used in Florida: the more stringent bathing‑beach criterion (enterococci, geometric mean or equivalent excursion metric of 70 per the protocols used in local weekly monitoring) and a broader “Class III” (swimmable/fishable) metric (about 130). He said Department of Health (DOH) weekly samples at the designated bathing beach on the bay’s south side rarely exceed the bathing‑beach threshold (about 8% of samples in the cited period were “poor”), while weekly sampling at a north‑side site run by a nongovernmental sampler showed more frequent exceedances (a higher share flagged “poor”).

Temasco said the program has undertaken three sampling events for source tracking but had results back from only the first event because the lab analysis is expensive. That first event found horse DNA at three sites but only at “detected, not quantifiable” levels—meaning presence was established but relative contribution to total bacteria could not be estimated. Importantly, Temasco said, no human signal was identified in the first set of samples, which reduces the likelihood that sewage leaks are the cause of the detected enterococci at those sites.

Mayor Jean Brown and council members discussed next steps. Brown said the commercial horse operators agreed voluntarily to keep horses off the beach from Sept. 8 through Oct. 6 to allow targeted sampling while the animals are absent. Temasco and staff proposed intensive sampling at 1, 2 and 3 weeks after the exclusion period begins and repeat sampling after horses return to assess any change in bacterial measures and the presence/absence of horse‑associated DNA.

Horse tour operator representative Carmen Hansen of Seaponies disputed an online narrative that blames horses without scientific proof and submitted recent local monitoring numbers showing Palma Sola values often lower than other local beaches on particular dates. She told the council her business is being portrayed unfairly and warned against singling out horses without full evidence.

Temasco told the council a plausible transport mechanism is flotation of horse fecal material: unlike human waste, he said, horse feces tend to float and can be transported by tides and wind. He recommended a controlled exclusion test and additional source‑tracking sampling to determine whether horses are a meaningful contributor—and if so, how large that contribution is—before imposing regulatory action.

Council members and staff discussed other management options mentioned in the presentation, including targeted beach raking, better public education and enforcement of cleanup by individual horse owners. Temasco noted other sources of indicator bacteria—decomposing vegetation, grass clippings and bird congregations—that can produce positive tests for the same indicator bacteria without representing sewage contamination.

Temasco said the program and the city have budgeted for the follow‑up work and will return to the council with results when subsequent lab data arrive. The city emphasized the study’s preliminary nature and said no regulatory action would be taken without clearer quantitative evidence.