Low oxygen levels appear to be cause of Heron Lagoon fish kill, Sarasota County staff reports

Still, residents getting water samples tested to determine whether sewage leak could be culprit

This is a still from a video Sarasota County staff took of Heron Lagoon during the fish kill incident the week of Nov. 18. Image courtesy Sarasota County

A fish kill last week in Siesta Key’s Heron Lagoon appears to have been caused by a low level of oxygen in the water, Sarasota County staff has told The Sarasota News Leader.

In a Nov. 26 email, county Media Relations Officer Brianne Grant wrote that staff members of the county’s Air and Water Quality Division — part of the Environmental Protection Division — have been to the lagoon on two separate occasions to conduct inspections. “In both instances,” she added, “no illicit discharges were observed at the time of inspections:

  • “On Nov. 15, staff noted a portion of the lagoon was discolored and fish were observed at the surface piping for air.
  • “On Nov. 21, staff took dissolved oxygen (DO) readings. The DO level measured in the lagoon was 1.36 mg/l [milligrams per liter] and multiple dead fish were observed.”

“(For reference,” Grant continued, “as DO levels in water drop below 5.0 mg/l, aquatic life can be placed under stress. The lower the DO concentration, the greater the stress. Aquatic DO levels that remain below 1-2 mg/l for an extended period of time may result in large fish kills.)”

Grant further explained to the News Leader that Heron Lagoonconnects to the Intracoastal Waterway by two 48-inch pipes. Those were inspected on Nov. 21, Grant added, and were “found to be open and flowing at full capacity.”Additionally, Grant wrote that when a Sarasota County Public Utilities Department field crew visited the area, “The lift station and surrounding system was found to be in working order and no spills have been reported.”

Reporting extra details from the Air and Water Quality Division, Grant continued, “It is more than likely that the lagoon had an algae bloom in recent weeks. The cold snap last week and several overcast days may have contributed to the algae dying off rapidly. When algae dies off, it sinks to the bottom of the water column and begins to decay. The process by which algae decays reduces the levels of dissolved oxygen in the water. Lower levels of dissolved oxygen may lead to fish kills.”

On Nov. 21, a Siesta resident, Lynn Nilssen, contacted the News Leader about the situation, writing in an email that she and her husband “have experienced a horrible fish die off in the lagoon. It started about a week ago and is not getting better. We have lived here for 13 years and never experienced anything like this. It is not characteristic of red tide and the lagoon is closed to any other body of water except it exchanges water through a drain pipe with the bay. We suspect sewage or some other contaminant.”

They also took water samples, she pointed out, which they delivered to a local lab for testing.

County Media Relations Officer Grant told the News Leader in a Nov. 25 email that the county’s Contact Center estimated it had received slightly more than 12 calls about the fish kill, plus two or three emails. Additionally, Grant noted the next day, the county’s Air and Quality Division received six reports of the situation.

Andrew Nilssen — Lynn Nilssen’s husband — was one of those who submitted a Pollution Concern Form to the county, which he shared with the News Leader.

This is the top of a county Pollution Control Form. Image courtesy Sarasota County

After noting the nature of the incident, Andy Nilssen wrote, “Water turned dark brown/’chocolate’ quickly starting Friday 11/15.”

He added, “Lots of dead fish/fish struggling to breathe. No human respiratory irritation (so not red tide). Entire lagoon [affected]. Highly unusual.”

This is the FWC map showing Heron Lagoon on Nov. 21 and the absence of the red tide algae. Image courtesy FWC

As of Nov. 21, Nilssen continued, the situation was “not getting better.”

In response to a News Leader inquiry this week, Melody Kilborn, Southwest Region public information director for the Florida Fish and Wildlife Conservation Commission (FWC), wrote in an email that results for an FWC sample taken from the lagoon on Nov. 21 was made available to the public through FWC’s daily update map. She attached the image.

The red tide algae, Karenia brevis, “was not detected in this sample,” Kilborn confirmed.

Awaiting further testing

During a Nov. 26 telephone interview, Lynn Nilssen made it clear that she and her husband did not suspect red tide from the outset, because the lagoon showed no telltale signs of the algae’s impacts.

She also pointed out that she had spoken to neighbors who had lived along the lagoon far longer than she and her husband. “They’ve never seen anything like that,” she added of the fish kill.

Told of county staff’s analysis indicating that low oxygen levels appeared to be the cause, she acknowledged that “such an upset in the balance of nature” could be the culprit.

After contacting a representative of the nonprofit Suncoast Waterkeeper for a recommendation about a lab that could test for substances in the water, she pointed out, she took samples from the lagoon to Benchmark EnviroAnalytical Inc. in North Port. Staff there could test for nitrogen and phosphorus levels, she added.

“I spoke to the owner yesterday,” she told the News Leader on Nov. 26. Three days into the testing, she said, he seemed to agree with the Nilssens about the likelihood that a sewage leak caused the fish kill. “I won’t know until Friday,” she added, as that was when he expected the final results to be ready.

However, it turned out that the lab produced the results more quickly. Late in the afternoon of Nov. 27, Nilssen emailed the News Leader to report that she had received the results that day and had spoken with a person at the lab. “[It] appears from the samples that I took last week there was no evidence … at the time of a sewage spill/leak,” she wrote.

A brief history of the lagoon

An aerial map shows Heron Lagoon on Siesta Key. Image from Google Maps

A 1974 Report on the Hydrography and Biology of Two Man-Made Canal Systems on Siesta Key — the Grand Canal and Heron Lagoon — undertaken by the Division of Natural Sciences at New College of Florida, pointed out that before 1921, the southern end of Siesta Key “had a very different appearance from the present. … What is now Heron Lagoon was the northern half of Little Sarasota Pass. The Pass opened into Little Sarasota Bay in the vicinity of Bird Keys.”

At that time, the report noted, Midnight Pass did not exist.

“Little Sarasota Pass cut north between the southern tail of Siesta Key on the east and the northernmost stretch of Casey Key, an area then known as Treasure Island,’ on the west, and opened into the Gulf of Mexico through what is now known as Sanderling Beach,” the report explained.

A hurricane in 1921 “broke through Casey Key at the southern end of Little Sarasota Pass and formed ‘Musketeer’s Pass,’” which later was known as Midnight Pass, the report said. (The Sarasota County Commission gave property owners permission to close Midnight Pass in the early 1980s, because erosion was threatening their homes.)

“In 1926 another hurricane dumped masses of sand and shell into the center of Little Sarasota Pass where there were once two small mangrove islands, and the Pass was cut in half.”

The 1921 hurricane had “practically filled” the opening of Little Sarasota Pass to the Gulf, the report continued. The second hurricane “succeeded in closing it completely. The result was a landlocked Heron Lagoon nearly two miles long.”

Cleaning up the dead fish

After the incident began, some Heron Lagoon residents reportedly expressed the view that the county should clean up the dead fish. County Media Relations Officer Grant wrote the News Leaderin a Nov. 22 email that staff with the county’s Parks, Recreation and Natural Resources Department (PRNR) “continues to monitor public beaches and coastal parks for accumulated marine debris per the board approved Beach Cleaning Policy.” Grant noted that the Heron Lagoonis a private water body.

The County Commission adopted the beach cleaning policy in May 1995, she pointed out, and then amended it in 1997 and again in 2013.

“Attachment C; Special Conditions Beach Cleaning Policy reads that accumulation [of dead fish and/or natural debris] must reach an estimated volume to fill one 5-yard truck per two-mile continuous section of beach that is accessible to motorized equipment or vehicles, if two tidal cycles have not removed [the materials] naturally,” Grant added.

This is the county’s beach-cleaning policy. Image courtesy Sarasota County