Low oxygen levels reported as cause of Siesta Key’s Heron Lagoon fish kill

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By Rachel Brown Hackney

A November fish kill in Siesta Key’s Heron Lagoon appeared to have been caused by a low level of oxygen in the water, Sarasota County staff told SNL.

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 that Heron Lagoon connects 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 SNL 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.”

Andrew Nilssen — Lynn Nilssen’s husband — submitted a Pollution Concern Form to the county, which he shared with SNL.

“Water turned dark brown/’chocolate’ quickly starting Friday 11/15,” he wrote. “Lots of dead fish/fish struggling to breathe. No human respiratory irritation (so not red tide). Entire lagoon [affected]. Highly unusual.”

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

Melody Kilborn, Southwest Region public information director for the Florida Fish and Wildlife Conservation Commission (FWC), told SNL that testing of an FWC water sample taken from the lagoon on Nov. 21 showed the red tide algae “was not detected …”

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