Bacteria Levels After Idalia

Letter from David Tomasko, Ph.D, Executive Director, Sarasota Bay Estuary Program to Commissioners and City Staff:


My last email update spoke about the impacts of Idalia on our bay and watershed.  In contrast to Irma and Ian, we were on the strong side (it passed to the west of us) of this storm, in terms of storm surge.  Irma and Ian brough us lots of rain, but the prevailing winds (which move counter-clockwise) mostly pulled water out of the bay – remember the pictures of manatees stranded on seagrass meadows in Sarasota Bay after Irma (see attached)? On the other hand, Idalia brought us water levels (aka storm surge) that was about 3.5’ higher than the water was expected to be, based on tide stage and moon phase.  Lucky for us, the storm surge was starting to go back down just a few hours before the spring high tide that was on its way, which could have added another foot on top of that event.  At the long-term gage in St. Pete, the storm surge peaked at 5.9’ above MLLW, at about 10 AM on Wednesday, August 30.  In our region, MLLW is about 2 feet lower than Mean High Water, which means that this storm brought us a surge that was about 4 feet higher than the average high tide, and about 5 feet higher than the average sea level.  Not huge, but not inconsequential, as water covered much of Lido Key, it covered most of Cortez and Longboat Key Village, and came close to coming over the top of both Manatee Avenue and Cortez Road.  Lots of damage to roads on Casey Key, and severe damage to the road on Manasota Key.

When that surge of water drained back into the bay, it brought with it the grass clippings and dog poop and yard debris that is found across our watershed.  Simply put, the water quality took a hit afterwards, but seems to be recovering.  After storms such as this, it is most important to recognize that it might not be safe to be on or in the water, due to high levels of pathogens.  That was the case after Hurricane Charley in 2004, and after Ian in, in 2022.  But what about after Idalia?

In our region, the beaches themselves are sampled by FDOH, and after initial calls to stay out of the water, the conditions on our beaches have been safe for swimming for some time now.  For the bay itself, we’ve just received bacteria results from September 5th, less than a week after Idalia’s landfall.  The bacteria data comes from Suncoast Water Keepers, which collects the samples and gives them to Benchmark Laboratories, which is a great lab that meets all regulatory guidance for sampling procedures. 

For Sarasota County, seven bay sites were sampled on September 5th, and only one site – Bayfront Park / Marina Jack exceeded guidance criteria.  For Manatee County, all four sites sampled on September 5th had low levels of bacteria, including the north side of Palma Sola Causeway.  Salinities were high across the bay, which reflects the lower amounts of rainfall we received (compared to Ian). 

After Ian, the lower part of the bay took between 2 and 4 weeks for bacteria levels to drop down to safe levels.  After Idalia, it looks like bacteria levels were back into the good range in less than a week.  This might be related to the fact that the saltier conditions of the bay led to more rapid die-off of pathogens than occurred after contaminants were washed in from.  Not sure if that’s the case or not, but for whatever reason, it appears that the bay’s health is likely to recover more quickly after Idalia than was the case after Irma and Ian.

Thanks – again – to Suncoast Water Keepers for not only collecting the bacteria data, but for also sharing it in such a timely manner.

Have a good weekend and see you at our Policy Board meeting next Friday!


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