Stay in the know about wildlife, water quality, and ecosystems on Sanibel and Captiva Islands and in Southwest Florida

SCCF Scientist Explains Value of Water Retention

September 22, 2021

Sanibel has preserved 70 percent of its wildlands and protected the wildlife that lives here, with the interior wetlands serving as the heart of barrier island habitats. Though Sanibel’s wetlands have been dredged and shaped by manmade intervention, they now allow a natural community to thrive there. Managing stormwater runoff in the rainy season is a nuanced process in Florida and on Sanibel Island, where SCCF and the City of Sanibel are working together to monitor the nutrient load of water released from the Sanibel Slough into Tarpon Bay and Pine Island Sound. 

The dominant element of the island’s modified wetland system is the Sanibel Slough, also known as the Sanibel River. “When it rains, the slough fills and stores water in our shallow aquifer. The wetlands and aquifer provide life to the island throughout the long dry season,” says SCCF Research Associate Mark Thompson.  

The ability of the island to hold and store water is an asset, which the City of Sanibel has acknowledged within its codes. Codes require developed land to contain a significant amount of rainwater onsite and ordinances encourage natural vegetation that absorbs stormwater runoff and removes nutrients. “The idea is to hold that water on the land, in the wetlands, and in the aquifer so it functions like a natural island ecosystem,” says Thompson.

The Sanibel Slough level has risen during this wet season as it does every year, and Sanibel’s shallow aquifer is high and near the ground surface. The Sanibel Slough was modified with weir structures to provide flood control when needed. In a typical year, the water level in the slough rises and the City of Sanibel opens the weirs to lower water levels and the water flows directly into the East End canals and/or Tarpon Bay. Pictured above are the flood control gates at the Tarpon Bay weir, with the gates open in the photo below.

The Sanibel Slough has higher nutrient concentrations than a healthy system due to lawn fertilizer and wastewater. The slough was classified by the state as impaired (polluted) due to excess nutrients in 2017. When nutrient-laden water is released to the bay, it contributes to a host of nutrient enrichment problems which affect seagrass, oysters, fish, and the entire ecosystem. 

“In 2020, the waters of the bay became classified as impaired due to nutrient enrichment—just like the Sanibel Slough. The release of water from Sanibel’s wetland system should not be taken lightly,” says Thompson. “Every inch of water released from Sanibel Slough contains 20 pounds of nitrogen and two pounds of phosphorus.”

In 2015, SCCF, with funds provided by the City of Sanibel, installed flow monitoring equipment at the two weirs. Today, when water is released from the Sanibel Slough, the SCCF Marine Lab calculates how much water, nitrogen, and phosphorus (loads) are released. 

“This helps us to understand relationships between the water releases and regional events such as red tide, algal blooms, fish kills, seagrass losses, and oyster mortality. In 2020, we calculated more than two billion gallons of water were released from the two weirs combined,” Thompson said. Within this released water was 46,700 pounds (23.4 tons) of nitrogen and 2,341 pounds (1.2 tons) of phosphorus.

He urges understanding during the rainy season.

“Be patient when you see standing water in your yard or in a nearby swale. It is simply part of living in harmony with Sanibel and the surrounding wetlands,” he says. “Eventually, the water is filtered by plants and soil as it percolates through the soil and is stored in the aquifer for the dry times ahead.” 

In most parts of South Florida, wetlands have been drained and there is no longer aquifer recharge. Living on Sanibel means living in a more natural system, where large volumes of water in the rainy summer months don’t run off lands quickly during storms, like it does in other places. 

Sanibel is a unique community that was founded based upon the idea that all stakeholders—residents, business owners, city officials, and other entities, including SCCF—will seek to understand and accommodate nature and natural events. Sanibel’s vision statement says, “Sanibel is and shall remain a barrier island sanctuary, one in which a diverse population lives in harmony with the island’s wildlife and natural habitats.”  

Opening the flood gates is not a solution to a problem; it is a symptom of a problem, which is being passed downstream to Pine Island Sound. The city and SCCF are working to minimize the number of times those flood gates are opened.




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