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Monitoring Freshwater Inflows to Caloosahatchee Estuary

May 15, 2023
Spawning Grounds 1800

By Isabella McDonnell, SCCF Research Assistant

The Caloosahatchee estuary contains a multitude of life and is a nursery for a variety of species. Oysters, fish, and other marine creatures begin their lives in estuaries, where there is food, shelter, and a gradient of salinity that supports a variety of ecosystems.

Freshwater inflows can dramatically alter the salinity of estuaries impacting the health, survival, growth, and reproduction of marine life that utilize the estuary during part of — or all of — their life cycle. As we approach the wet season, SCCF Marine Laboratory researchers, including Research Associate Leah Reidenbach, are tracking the amount of freshwater entering the Caloosahatchee estuary system, specifically Lake Okeechobee outputs.

“Currently, the combined flows from Lake Okeechobee and the Caloosahatchee watershed are conducive to salinity ranges that are optimal for oyster spat development in the mid-estuary,” explained Reidenbach.

Steady flows from the Lake of about 2,000 cubic feet per second (cfs) throughout the dry season have promoted a zone of salinity from the Midpoint Bridge to the Iona area of approximately 10 – 20 practical salinity units (psu) which is ideal for oyster spat, she adds.

Spawning and changes in salinity

Millions of fish, oysters, and other marine creatures begin their lives after a spawning event. Spawning and the success of eggs and larvae afterward can be highly impacted by changes in salinity.

Sedentary animals like oysters and clams reproduce by releasing their gametes into the water when environmental cues indicate conditions are optimal, when sperm fertilizes the eggs in the water column, the estuary can be filled with the beginnings of hundreds of individuals.

When the eggs develop, the young of many species start their lives in the “nursery” system of the estuaries. Among the oyster reefs and seagrass beds, small eggs can be hidden from predators and young animals have a multitude of food sources and shelters.

Seasonal fluxes in salinity impact the base of the food web, phytoplankton, zooplankton, and algae. Increases in freshwater increase nutrient availability and decrease benthic primary production due to decreased light penetration with murkier water.

Dramatic decreases in salinity can decrease the abundance of estuarine residents and marine species that use estuaries as nurseries, evoking stress responses or emigration, resulting in a decrease in biodiversity.

“Our biggest concern with the lake being too high for this time of year is that flows will be too high once the wet season begins, which also corresponds with oyster spawning season that runs April to November,” said Reidenbach.

If flows increase to over 3,000 cfs for an extended period during the wet season, there could be a detriment to oyster recruitment and growth.

More freshwater inputs

Increased human population in coastal areas has led to greater freshwater inputs into the estuary. Hard surfaces like roads, driveways, and foundations lead to greater runoff instead of absorption, draining fresh water and pollutants to the coasts. This is multiplied by those living in the Lake Okeechobee Watershed. The water in Lake Okeechobee is released into the Caloosahatechee estuary depending on the level of the lake.

Oysters are an incredibly important part of our ecosystem, as they filter water and provide shelter and food for other animals. Oyster reefs also protect against erosion and serve as a natural breakwater during storms.

Increased freshwater inflow to the estuary can increase predation and decrease food sources. Many marine organisms are affected by these changes. SCCF research is informing advocacy efforts to urge water managers and policymakers to protect the Caloosahatchee estuary from harsh changes in salinity that could negatively impact local marine life.


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