Study of Island Lakes Reveals Poor Quality

by SC Reporter Emilie Alfino

Mark Thompson, Research Associate at the Sanibel-Captiva Conservation Foundation gave a presentation of the 2024 SCCF Community Lakes and Groundwater Monitoring Report. The full report is available at www.sanibelcleanwater.org.

The study covered 50 lake sites, 26 groundwater monitoring wells, and six lake groundwater study sites.

Only one site received a B grade; 12 received a C grade, and 37 – or 74 percent – received a grade of D or F. Just six sites were trending better – and, Thompson stressed, that is not even taking into account the increased salinity of the water.

Thompson said vegetation is going to be the way to improve the water quality. As far as salinity, those lakes just need rain, he said. “That’s the only way they’re going to get fresh,” he said. Shallow lakes can get fresh within three years, as can sloughs, but deep lakes take much longer. Lakes were almost fresh two years post-Ian but are now “bad” again, Thompson said, after the storms of 2024.

Vegetation removes (it’s called evapotranspiration) and stores water. Trees remove nutrients and keep the soil porous for better infiltration. In turn, more infiltration means less runoff and more microbes to remove nutrients. Woody vegetation resists surge overwash and reduces upland to wetland conversion on barrier islands. Hurricanes and clearing can destroy and reduce vegetation capability of removing water from the landscape. Ian and post- Ian clearing destroyed and damaged vegetation water-moving capability. Less evapotranspiration + more rain = higher water table and standing water.

Over the past two years, the multiple storm events have had varying degrees of impact to the island and its natural resources. There is concern for what the negative implications of these events might be as it relates to wildlife, native plant communities, and water quality. Natural Resources Department staff initiated these efforts to investigate conditions in formerly freshwater systems, tracking their progression over time and what that might mean for the island’s ecology.

Pre-storm water quality conditions are well documented through the City’s established Sanibel Communities for Clean Water Program (SCCW). To document post-storm conditions, the City’s contractor sampled a sub-set of the sites included in the program. This included 50 of the 78 lakes that are typically sampled as part of the SCCW Program.

This past year’s study also included the installation of groundwater monitoring well sites to evaluate the relationship between groundwater and surface water.