It has been an up-and-down few years on Sanibel in many ways. For one thing, the salinity of Sanibel’s freshwater habitats has been a rollercoaster over the past two and a half years.

The Sanibel Slough, which is often called the Sanibel River, is the freshwater system located in central Sanibel which is connected to San Carlos Bay via the East Basin and Tarpon Bay via the West Basin through a system of two weirs.

“This central wetland system benefits and protects the island by providing habitat to wildlife, replenishing fresh water to our groundwater aquifers, and by storing precipitation which would normally just pool and runoff causing flooding,” said SCCF Marine Lab Research Associate Mark Thompson. “It also improves water quality, prevents saltwater intrusion through groundwater, and provides low salt conditions needed for residential vegetation plantings.”

A New Era in Saltwater Intrusion

In previous years, water releases from the Slough, through the weir system were few and saltwater surges over the weirs and into the Slough were almost unheard of. However, increasingly frequent and intense tropical storms, have ushered in new times on Sanibel. 

During Hurricane Ian in 2022, the saltwater surge from the Gulf of Mexico spread over much of Sanibel and displaced freshwater. Most of Sanibel’s lakes and wetlands instantly became saltwater habitats with salinities from 15 to 30 practical salinity units (PSU).

A salinity above 10 PSU is considered to be a “marine” habitat. Along with this change came a massive die-off of freshwater fish, reptiles, amphibians, invertebrates, and freshwater-dependent vegetation.

“The food supply for many creatures higher up on the food chain such as mammals, birds, and alligators disappeared – so they left or died,” said Thompson. “In our lakes, some displaced saltwater fish survived for a while until poor water quality conditions caused fish kills.”

Above Average Rainfall Revived Freshwater Habitats

The dry season at the beginning of 2024 thankfully supplied an unusual amount of precipitation to Sanibel and patches of freshwater began to appear again. Precipitation continued at an increased rate during the 2024 wet season.

Data from the SCCF Marine Lab weather station shows precipitation in 2024 will be nearly 20 inches greater than the historical average range (39-45 inches). By July of 2024, the large volume of precipitation resulted in normalization of salinity in the Sanibel Slough and the average salinity of Sanibel lakes dropped from a post-Ian high of over 20 PSU to 9 PSU.  Freshwater habitat was returning along with associated flora and fauna.

Three Surge Events Since August

In August, Tropical Storm Debby passed nearby Sanibel and brought with it a localized saltwater surge event that covered many of the freshwater habitats near Sanibel’s Gulf of Mexico shoreline.

Several formerly freshwater lakes (pre-Ian) such as those at Casa Ybel resort, the Jordan Marsh, and Bailey’s Pond had their salinity significantly reduced during the 2024 precipitation but reverted again to near seawater strength after Debby.

The East Basin of Sanibel Slough was also near being freshwater before Debby and the surge from Debby brought the saltiness back up to half seawater strength — killing much the freshwater flora and fauna which was beginning to be established again after Ian.

The West Basin of Sanibel Slough was not affected in the same way during Debby due to the surge being more localized to the East Basin.

In September, Hurricane Helene passed within 150 miles of Sanibel and brought with it a more significant surge than Debby. Both the east and west basins were impacted with salinity increasing to marine water levels.

“The October arrival of the Hurricane Milton surge put an exclamation mark on converting the Sanibel Slough basins into marine systems once again,” said Thompson.

The salinity of both the East and West Basins of Sanibel Slough are now over 20 PSU which means they are about 2/3 the saltiness of seawater. A survey of local lakes after Helene and Milton has shown that they are also about the same salinity as the Slough, which is about the same condition they were in after Ian.

“In fact, lakes on the Sanctuary Golf Course have actually become more salty after Milton than they were after Ian, suggesting the surge from Helene and Milton affected a larger area of the island than Ian,” he said.

To be able to irrigate a lawn without harming it, the salinity must be below 2 PSU. Some native plants such as cabbage palm and spartina grass are able to survive periodic episodes of higher salinity conditions.

“They can separate freshwater from salty water for their survival. But when the freshwater above ground and the aquifer below are both salty, it will take fresh precipitation to allow recovery, and this may take several years as we learned after Ian,” said Thompson.