High amounts of both wastewater and stormwater flowing into bay result in low oxygen and high bacterial counts
Based on a second set of samples — collected on Aug. 15 — “it appears that our [Sarasota Bay] water quality is worse than what we found after [Hurricane] Ian,” David Tomasko, executive director of the Sarasota Bay Estuary Program, has reported.
He was referring to the conditions of bay segments following the downpours produced by Tropical Storm Debby during the first week of August.
The levels of the enterococcus bacteria were as much as five times higher than they were following Hurricane Ian’s strike on Southwest Florida in late September 2022, Tomasko wrote, while Little Sarasota Bay had practically no oxygen in the aftermath of this latest storm. Hypoxia is a condition that produces fish kills.
As Environmental Administrator Tom Higginbotham of the Florida Department of Health-Sarasota (DOH-Sarasota) has explained, “When [enterococcus] bacteria are found at high levels in recreational waters, there is a risk that some people may become ill.” He also has noted, “People, especially those who are very young, elderly or who have a weak immune system that swallow water while swimming can get stomach or intestinal illnesses. If water contacts a cut or sore, people can get infections or rashes.”
On Aug. 15, a group encompassing representatives of the Sarasota Bay Estuary Program (SBEP), Sarasota County Government and the Southwest Florida Water Management District (SWFWMD) finished its second sampling effort since Debby inundated the county. The latest efforts focused “on the water quality in the lower bay, where the combination of a high watershed-to-open water ratio and longer on-average residence times were expected to bring about the worst water quality,” Tomasko explained.
In addressing the findings, he wrote that, for example, the highest average level of enterococcus bacteria after Ian was recorded on Oct. 4, 2022 — a value of 400 colony forming units, or cfus, per 100 milliliters (ml) in Blackburn Bay. “After Debby,” he continued, “we’ve had values higher than 2,000 …”
The Class 3 Marine waters’ standard for enterococcus bacteria is 130 colony forming units per 100 ml, as recommended by the U.S. Environmental Protection Agency (EPA).
In a followup article on the SBEP website on Aug. 20, Tomasko included more details about the bacteria counts, about which he had received more information that day. He explained that the values are compared against the appropriate threshold value of 130/100 ml. “A lower standard of 70/100 ml is the upper threshold for bathing beaches, but none of [the] sites we sample are considered bathing beaches, which are separately monitored by the Florida Department of Health [in Sarasota County].” (See the related article in this issue.)
In a table he had included with the article, he noted, “Values that are higher than the Class 3 Marine standard are in red, and you can see that 16 of the 18 samples (9 locations sampled twice) exceeded that standard. However, the third row summarizes the change in bacteria levels between the two dates for each of the nine locations. Locations with an improvement in bacteria levels are in light blue, while those with higher values are in yellow. There are about as many areas with higher values (four) as lower values (five). However, there is some good news in Roberts Bay, where bacteria levels have declined by between 75 to over 99% from the first to the second sampling effort. The northernmost sample site in Little Sarasota Bay (closest to Roberts Bay) showed a 97% reduction in bacteria levels. Blackburn Bay remains problematic, as values continue to exceed Class 3 Marine standards, and levels have also increased from the first to the second sampling event.”
Further, Tomasko noted, “[A]fter Ian, we had hypoxia (dissolved oxygen [DO] levels below 2 [milligrams per liter] mg/L) only in Little Sarasota Bay, and only in the bottom waters. In contrast, [below] are the graphs from [the Aug. 15 sampling], showing plots of dissolved oxygen in each of the three locations within each of the three bays we sampled.”
He pointed out that “the vertical axis displays depths in meters (a meter is about 3 feet),” while the horizontal axis shows oxygen levels in the water, with the lower —worse — values on the left side of the graph. “The red line,” he wrote, “represents ‘hypoxia’ or levels of oxygen at 2 mg/L, which is a level below which many marine organisms cannot live.”
“In Roberts Bay (far left),” Tomasko continued, “we had hypoxic conditions only at one site (Roberts Bay South) at depths greater than 1.5 meters. In Blackburn Bay (far right) one site (Blackburn Bay North) was too shallow for us to look at values in the deeper waters, but we did see hypoxia at depths deeper than 0.5 meters.”
Then he noted, “In Little Sarasota Bay (in the middle) we not only saw hypoxia at depths deeper than 1 meter (at the two deeper sites) but we had almost a complete absence of oxygen. Values less than 0.5 mg/L are so low that you can have problems with the meters.
“Suffice it to say that we have very low levels of oxygen in waters deeper than about 1 meter in an area of Sarasota Bay from just south of Siesta Drive down to Venice Inlet,” Tomasko pointed out. “This condition is giving us fish kills for those species that are associated with the bottom. The first photo (below) shows a mixture of organic debris, and a few dead fish found in Blackburn Bay,” he continued.
“In the second photo (below),” Tomasko wrote, “there is a blue crab (Callinectes sapidus) swimming on the surface of the bay. Normally, these crabs are found on the bottom of the bay, but when there’s little to no oxygen in the water, they will swim to the surface to try and find enough oxygen to keep them alive.”
He also pointed out, “Some fish are not so lucky, such as [a] ray that we saw that was fairly decomposed, most likely after dying a few days earlier.”
Yet, in spite of the fish kills and high bacteria, Tomasko noted, “[W]e still saw people swimming in the bay.”
Why should anyone care that low oxygen has caused fish kills?, he asked. First, he wrote, “[O]ne of the reasons we are spending so much [taxpayer money] to improve our stormwater and wastewater infrastructure is to improve our water quality, to be able to maintain a healthy environment. Debby showed us the limits of our ability to protect our water quality.”
The next question, Tomasko continued, is, What caused these problems with oxygen?
“Oxygen levels have decreased in response to an increase in the load of organic material into the bay,” he explained. “Organic material brought into the bay will be consumed by bacteria, mostly, which use oxygen, just like you and I use oxygen. Those bacteria, as well as bacteria from sewage overflows, can test positive as ‘fecal’ bacteria, even if they have nothing to do with fecal material.”
Then Tomasko pointed out, “The state of Florida has three basic fecal indicator bacteria: fecal coliform, E. coli, and enterococci. These are all useful indicators, but none of them are specific to humans, or mammals or even animals as a source. You can, for example, get very high levels of ‘fecal coliform’ bacteria from decomposing grass clippings.”
Therefore, he continued, “If we had less organic material coming into the bay, we’d likely have lower levels of bacteria. And right now, our bay is functioning a bit like an underwater compost heap.”
Sewage overflows in context of water quantity
Tomasko then wrote, “This begs the question – is this all due to sewage overflows? Because we’ve had multiple tens of millions of gallons of such overflows from Bradenton down to Venice.
“[L]et’s keep in mind that none of those overflows are good for the bay — at all,” he pointed out. “But consider a utility has a reported overflow of 20 million gallons,” when it normally handles about 5 million gallons of sewage a day. “In such a situation,” he added, “it is more likely than not” that that most of the 20 million gallons “was bypassing the wastewater treatment plant, with perhaps 5 million gallons as raw sewage, with the remaining 15 million gallons likely from groundwater that infiltrated the decades-old collection pipes that transport sewage to the treatment plants.”
Alternatively, Tomasko suggested that readers think of the situation this way: “We have reported overflows of perhaps multiple tens of millions of gallons, much of it into the Manatee River, not directly into our watershed. But let’s say we had 50 million gallons of wastewater overflows that were directly discharged into our bay. And let’s, for the sake of argument, say that ALL of that was raw sewage, not sewage mixed with groundwater that had infiltrated into the collection system. Our salinities [on Aug. 15] ranged from as low [as] 5 practical salinity units (psu) up to 30 psu.”
Freshwater has 0 psu, he continued, while the Gulf of Mexico averages about 35 psu.
“Sarasota Bay is about 50 square miles in size,” he pointed out, “with an average water depth of about 6 to 12 feet,” though it is deeper in some areas and shallower in others. “Let’s use 6 feet as an average, for reasons that will hopefully become clear in a bit. At 6 feet of depth, 50 square miles would hold about 60 billion gallons of water. If we assume 50 million gallons of raw sewage came into a bay with 60 billion gallons of seawater, that would equal less than 0.1% of the volume of the bay — that is not nearly enough to drop the salinities as low as what we recorded.
“In contrast,” Tomasko wrote, “let’s assume we had ‘only’ 6 inches of rain across our 150 square mile watershed, and that only 25% made it into the bay. Both of those are underestimates but will work for this exercise. If we had 6 inches of rain on average, across a 150 square mile watershed, and just 25% made it into the bay, then that would amount to about 4 billion gallons of stormwater runoff. Four billion gallons of stormwater runoff into a bay of perhaps 60 billion gallons in volume better explains the change in salinity we recorded, especially in the lower bay (where the watershed is larger, and the open water is much smaller) and thus better explains the organic loads and bacteria and oxygen problems we are seeing in the bay.”
He added, “As an example of how much runoff came into the bay, check out this photo I took of the bay offshore of Cortez, on [Aug. 12]. In this photo, notice the plume of stormwater runoff, which spread out almost to the Intracoastal Waterway.”
Tomasko then wrote, “Wastewater overflows are serious, and we need to get them under control. And the best way to help our utilities folks is to make sure that we fund the types of projects that they are calling for. It’s not just wastewater treatment plants we need to upgrade, it’s the infrastructure that gets wastewater from our houses and businesses to those plants: collection pipes and lift stations included. As we’ve pointed out previously, wastewater overflows sometimes happen because people put ‘flushable’ (not dissolvable, by the way) wipes down their toilets, along with dumping oil and grease into our sinks.”
He also noted, “Wastewater is a stress to the bay … Wastewater overflows … plagued us between 2013 to 2019” — when more than 1 billion gallons of sewage overflows occurred.
Getting such overflows “under control is a big reason why our water quality has been improving recently. But storms like this? Wastewater overflows are one of many sources of the bacteria and low oxygen levels we’re now seeing in the bay. We need to control both —wastewater and stormwater — if we’re going to be able to protect our water quality and our living resources.”
He stressed, “For now, it is NOT a good idea to recreate in the bay – at all. Be careful out there, as we are. And hopefully, we’ll get to see improvements in our water quality over the next few weeks.”