Florida Bay: A Unique Everglades Estuary

Florida Bay lies at the southernmost tip of the Florida peninsula and bordered to the south by the Florida Keys (Figure 1). To many, Florida Bay is a recreational fishing hotspot and one of the reasons why the Florida Keys are the fishing capital of the world. Most of Florida Bay is contained within the bounds of Everglades National Park, which is a World Heritage Site, an International Biosphere Reserve, and a Wetland of International Importance.
Florida Bay is an estuary, meaning it is where the river meets the sea. In this case, it is where the Everglades (also known as the River of Grass) meets the Gulf of Mexico and Atlantic Ocean. As an estuary, Florida Bay is sensitive to the quantity of fresh water it receives from the Everglades. Due to changes in water management over the past century and the disconnection of Lake Okeechobee — the historic headwater supply — from the Everglades, Florida Bay receives much less freshwater inflow from the Everglades than it did historically.

The salinity of water in Florida Bay often ranges from fresh to brackish to marine at different times of the year as freshwater from the Everglades mixes with ocean water. However, Florida Bay is not a typical estuary like the Chesapeake Bay, where the river channel gives way to the sea and salinity increases from one end to the other. Florida Bay is more like an ice cube tray — a series of shallow basins or depressions separated from one another by shallow, seagrass-covered mud banks (Figure 2).

This complex ice-cube-tray shape affects the exchange of water across Florida Bay (Figure 2). At one end of the ice cube tray, freshwater flows in from Taylor Slough. Freshwater from Shark River Slough mixing with saltwater from the Gulf of Mexico fills the other end. In the middle of Florida Bay there are times when not enough freshwater enters from the Everglades and high evaporation of bay water results in the accumulation of salt. This often leads to salinity levels higher than ocean water, or hypersalinity.

Without the headwater supply of freshwater from Lake Okeechobee to the Everglades, mild hypersalinity has become a common problem in Florida Bay. However, during drought years, extreme hypersalinity can be detrimental to all aspects of the ecology of Florida Bay including seagrass, mangroves, numerous species of fish and shellfish, manatees, crocodiles, and Roseate spoonbills. This is exactly what we experienced in the summer of 2015, when extreme hypersalinity from a localized drought and a lack of freshwater inflow led to a massive area of seagrass die-off (nearly 80 square miles) in Florida Bay.

Figure 1: Map of South Florida going from the southern Everglades to the Florida Keys showing the present distribution of freshwater inflow to Florida Bay from Shark River Slough and Taylor Slough (red arrows). The C-111 spreader canal project will restore the volume, timing, and distribution of flows from Taylor Slough into Florida Bay (as illustrated with blue arrows). This will result in decreased salinity over a much larger area of Florida Bay.

Figure 2: (TOP) Illustration showing the weakening of tidal exchange across the different basins of Florida Bay from west to east. (BOTTOM) These basins are bordered by shallow seagrass-covered mud banks that allow limited inter-basin exchange of water during high water events (e.g., high tides and storm surges). However, the basins are more isolated from one another much of the time, resulting in reduced circulation and potentially hypersaline conditions in the dry season.

Figure 3: A simplified water balance for Florida Bay in the historic Everglades ecosystem (when connected to Lake Okeechobee) and the current Everglades. In Florida Bay, rainfall and evaporation are roughly in balance during an average year. Freshwater inflows from the Everglades, therefore, are essential in maintaining a positive balance of freshwater coming into the bay and maintaining healthy salinity levels. Presently, with a drought and without the flow of freshwater from Lake Okeechobee, evaporation and lack of rainfall leads to extreme hypersalinity and seagrass die-off.