By Environmental Policy Staff

Situated at the intersection of the Caloosahatchee River and the Gulf, the Caloosahatchee Estuary is home to many ecologically important species that rely on specific estuarine conditions to survive, reproduce and thrive. One critical parameter of the system that determines the health of species like oysters, tape grass, and turtle grass is salinity. 

The Caloosahatchee Estuary relies on receiving a certain amount of freshwater flows from the watershed and Lake Okeechobee in order to maintain optimum salinities for these diverse species. Generally, if freshwater flows are too high, the estuary will become overly fresh, and if flows are too low, it will become overly saline. 

Salinity levels in the estuary become especially important as oyster spawning occurs each year in late spring. Oyster reproduction is most successful when salinities are between 10-25 parts per thousand (ppt). Oyster spawning season comes at the tail end of dry season, and salinity levels depend on both flows from Lake Okeechobee and watershed runoff from rain events, which fluctuate annually.

As the 2025 dry season nears an end, rainfall remains low, with the majority of flows to the estuary coming from scheduled lake releases. This year’s dry season has shown exceptionally low levels of precipitation, with Southwest Florida experiencing drought conditions, slightly above normal temperatures, and below normal rainfall. Necessary freshwater flows to the estuary have been maintained to date this season, with 14-day average flows remaining at or near the optimum level since late October 2024.

The optimum flow envelope for the health of estuarine species in the Caloosahatchee estuary is 750- 2,100 cubic feet per second (cfs) of freshwater, as measured at the S-79 Franklin Lock and Dam structure in Alva, FL. The flows are generated either from watershed runoff associated with rainfall or releases from Lake Okeechobee. As rainfall has been infrequent this dry season, lake releases have provided a steady stream of freshwater to maintain salinity levels throughout the winter. This careful balance remains dependent on decisions by water managers deciding how much water will be sent to our estuary.

In recent weeks, water managers have started lowering the Lake O flow schedule for the Caloosahatchee, dropping flows first from a target pulse of 2,100 cfs to 1,400 cfs, to 1,000 cfs, and most recently, 650 cfs. While this change is not expected to cause immediate harm to the estuary, it puts the estuary significantly closer to a precarious position maintaining species-specific salinity envelopes during this critical season.

Maintaining optimum flows is critical to ensure successful oyster spawning and protect all estuarine species reliant on certain salinities. The situation requires careful balancing, because although oyster spat requires salinity to be maintained, tape grass can suffer immediate mortality if salinity in the upper estuary rises above 15 ppt. Managing this complex system requires careful planning, stakeholder engagement, and quick decision-making.    

With only 650 cfs being received by the estuary through the S-79 structure, if rainfall remains low, the risk of our estuary becoming too saline is heightened. Maintaining the correct balance will require consistent monitoring of estuarine conditions while accounting for rainfall patterns to determine if a pulse schedule of 650 cfs is high enough to ensure the health of all species in the estuary.

Luckily, modeling from the South Florida Water Management District has suggested that the estuary will stay within the acceptable range for salinity. If actual conditions deviate from the model, it will be imperative to act quickly and decisively to protect the ecosystem. SCCF urges water managers to continually compare estuarine conditions to the previous modeling scenarios and maintain a flexible approach in their decision-making.

If rainfall does not supplement the estuary enough to maintain optimum salinity levels, it will be necessary to increase flows to the estuary to ensure a successful oyster spawn and overall estuarine health.

SCCF will continue monitoring conditions in the estuary and working with water managers to help protect our coastal ecosystems.

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