The red tide bloom off Sanibel and Captiva may be in decline based on lower counts as well as a shift in satellite imagery that shows it moving to the north and south of the islands.

The SCCF Marine Lab found the highest concentration of Karenia brevis over the past week to be 1.9 million cells per liter at Blind Pass Beach, with 1.5 million cells/liter at Tarpon Bay Road Beach.

Those counts of K. brevis — the organism that blooms into red tide — are down from high counts of 22 million cells/liter two weeks ago at two beaches on Sanibel.

“Our sampling in Pine Island Sound yesterday found the highest counts in Tarpon Bay with two readings over 250,000 cells per liter. The Redfish Pass area was also of concern, but overall, it seems to be subsiding on the Pine Island Sound side of the islands,” said SCCF Research Associate Mark Thompson.

Further south, they found low concentrations off Sanibel and background to no Karenia in San Carlos Bay, near the causeway.

Samples taken today on the Causeway and Sanibel beaches showed no Karenia cells. However, scientists and volunteers gathering samples still noticed respiratory irritation.

Satellite imagery also shows a significant shift in the bloom, indicating lower counts and less intensity as well as movement away from the islands.

Fish kills reported to SCCF have also decreased this week compared to last week. However, SCCF scientists caution that sometimes blooms diminish somewhat and then flare up again.

“The most recent satellite fluorescence shows smaller patches than we were seeing.
It may be declining, but it also may be regrouping,” said SCCF Research Scientist Rick Bartleson.

“Inshore and nearshore, concentrations can drop to zero, cells change shape, other species of phytoplankton start growing and blooming instead,” he explained. “Sometimes that happens right after cell counts skyrocket and fish die off en masse.”

Overall, SCCF scientists say conditions are improving.

Optimistically, they think the bloom has run its course for the time being but there are still patches offshore — one to the north off Captiva and one that is south toward Collier County.

Lee County Department of Health alerts remain active for many area beaches. Health alerts were added today for other locations. Respiratory irritation forecasts based on onshore winds are moderate today for Bowman’s Beach, Turner Beach, and Redfish Pass. Otherwise, the respiratory irritation forecast is low for Lee County beaches.

SCCF’s environmental policy team has been in communication with water managers as the Army Corps decides how to manage flows over the remainder of this dry season.

In this week’s Caloosahatchee Conditions Report, SCCF and partners made the following recommendation:

“We ask the USACE to reduce the recovery operations flow target at the S-79 structure to the lower range of the optimum flow envelope to reduce nutrient loading while protecting the salinity gradient of the Caloosahatchee Estuary,” said SCCF Policy Associate Allie Pecenka.

Sign up for SCCF’s Weekly Water Conditions Tracker to stay updated on the red tide conditions.