Salinities of inland waterbodies continue to shift on Sanibel, showing how dynamic barrier islands can be. Thanks to heavy rains in August, some native frogs and toads will be able to reproduce and disperse across the island again in refreshed, shallow pools that had become brackish.

 “Over the last three years, with the changing salinities of our freshwater areas, we have seen plant community shifts and altered wildlife occurrences and distributions due to multiple storm surge events from hurricanes,” said Wildlife & Habitat Management Director Chris Lechowicz.

Noticeable changes to the flora and fauna on the island were documented relatively quickly after the events and are still being recorded today.

“The salinity shifts in our freshwater bodies, due to saltwater intrusion from the surges, have caused the most significant changes in our ecosystems,” he added.

Shallow, ephemeral pools across the island recovered to tolerable salinities — less than or equal to 6 practical salinity units (psu) — for most forms of freshwater-dependent wildlife this year due to the rains in the early part of the wet season.

The torrential rainstorm that occurred on Aug. 10, dumped nearly 12 inches of rain on the island in just over 13 hours.

“Rain levels like that do not happen very often. This event certainly improved our previously freshwater areas by flooding out brackish pools with rainwater,” noted Lechowicz.

This change will allow various species, such as marsh crayfish (Procambrus alleni), southern leopard frogs (Lithobates sphenocephalus), and Florida softshell turtles (Apalone ferox), that have been struggling to find usable habitats to once again use these areas.

“Native frogs and toads will be able to reproduce and disperse across the island again in these areas,” he added. “Long periods of time between surge events will allow wildlife populations to increase and establish altered food webs and population structures.”

Deeper waterbodies still salty

However, the more permanent and deeper waterbodies, such as the Sanibel Slough, Devitt Pond, and the Gulf Ridge Marsh, were holding higher salinities (≥ 12 psu) since the surge events of Hurricanes Helene and Milton. They changed very little with the beginning of the rainy season. 

Many of these areas experienced high vegetation loss — primarily buttonwood trees (Conocarpus erectus) and other hardwoods whose root systems sat in highly brackish water too long.

Many developments from the 1970s and 1980s on Sanibel dug lakes from wetland areas within the permitted development site and used the fill to raise the homesites above the original elevations.

This resulted in deep, man-made lakes around which the homes were sited.

The storm surge events from the past three years exchanged the freshwater that originally filled these lakes with salty water from the Gulf and Bay.

“These lakes also typically have a very small watershed, resulting in a very slow freshwater recharge rate. Places like Murex Lakes, Palm Lake, The Dunes, and more still have very salty lakes even though we had large rain events in mid-August,” said Research Associate Mark Thompson, the SCCF Marine Lab’s freshwater expert.

The average salinity for lakes on Sanibel is greater than the average salinity for wetlands and ponds.

“It may take years until these large-volume lakes can support freshwater vegetation and wildlife,” he observed.