COASTAL DUNE LAKES

A Management Plan for the Coastal Dune Lakes of Walton County; LAKEWATCH, 2008

Coastal dune lakes are generally found within two miles of the coast and are typically shallow and irregularly shaped. The lake-water is composed of both fresh and saltwater obtained from tributaries, groundwater seepage (in both directions, from uplands to the lakes and from the Gulf to the lakes), rainfall, and coastal storm surges. Most dune lakes around the world are called freshwater lakes with varying periods of saltwater intrusion. The lake-water is generally colored (e.g., tea or black colored) due to watershed contributions of dissolved organic matter. While these lakes are exposed to normal weather conditions just like any lake, Florida coastal dune lakes are also tremendously impacted by hurricane activity (i.e., storm frequency, strength, and duration).

The coastal dune lakes of Walton County are scientifically interesting because of their intermittent connection to the Gulf of Mexico. This periodic connection serves as control for flood-level waters (relative to structures and associated infrastructure) by opening a conduit to the Gulf. When a lake reaches a critical pre-flood level, breaching water forms an outlet (channel) through the dune system and empties the lake water into the Gulf. Depending on tides and weather conditions, saltwater and saltwater biota from the Gulf may fill the lake and any biota voids created by the lowered water level. The drainage of the lake and potential exchange with the Gulf continues until equilibrium is reached and the opening closes. When there is an exchange with the Gulf of Mexico, a brackish water-body is formed. Depending on the exchange, an estuarine ecosystem may be created. Each of Walton County’s coastal dune lakes, however, has individual outlet characteristics, with outlet openings varying in length, frequency and duration. These openings occur based on each lake’s critical water level, which is driven by climatic conditions (e.g., droughts and rain). As a result, some of the dune lakes can be completely freshwater, some brackish, and some salty, with varying degrees of salinity occurring between different lake stages. The changing condition of water chemistry in the coastal dune lakes makes them dynamic, biologically diverse ecosystems.