The Ecology of Florida Bay- Final

This discussion topic submitted by Daniel Phirman ( at 12:43 am on 6/10/00. Additions were last made on Friday, June 30, 2000.

The Ecology of Florida Bay

Florida Bay is located at the southern tip of Florida and is bordered on the southeast by the Florida Keys. The entire area south of Florida can be considered a part of the bay, but generally only the shallow area to the east, which covers about 1,000 square miles (2,200 km2), is referred to as Florida Bay. This area is an estuary, historically fed mostly by runoff from the Everglades and a number of nearby creeks. This water from the Everglades flows through marsh-like prairies, in wide, shallow sheets, known as sloughs. The Taylor Slough runs into the Florida Bay. The C-111 canal, built by the Army Corps of Engineers for flood control, now also feeds the area.

Estuaries are partially enclosed areas where freshwater runoff meets the ocean. In areas where this occurs, like Florida Bay, complex ecosystems that provide unique habitats are produced. This ecosystem is a gradual transition between freshwater areas and the open sea and allows for interactions between members of both areas. Estuaries are also typified by changing conditions, depending on the flow of freshwater. When freshwater flow is high, the salinity of the area will decrease and as flow decreases, the salinity increases. The change in water levels due to tides is also a factor in the variation of conditions and salinity in a particular estuary. In the Florida Bay, the tides have little effect on the water levels because of the carbonate mud banks that restrict water flow (Fourqurean and Robblee 1999). Therefore, the flow of freshwater from the Everglades area is the major determinant of the conditions in Florida Bay.

These varying conditions and a regular inflow of sediments that come along with the water help to make estuaries one of the most biologically productive systems on earth. (Sumich 1992) When rivers meet the sea, they usually widen and the flow decreases dramatically. This reduced flow forces the sediments including gravel, sand, and silt to settle out and be deposited according to size. Only the smallest clay particles and organic debris remain in the slow moving water. This deposition of materials allows the estuary to serve as a catch basin for materials flowing to the sea. The separation of the materials creates a range of habitats within a small area. The sediments are also rich in nutrients to make a large amount of energy available for the ecosystem.

The carbonate mud banks within Florida Bay lie atop a solid ridge of Pleistocene coralline limestone. This area began to flood with rising sea levels about 4500 years ago and the eastern part of the bay was the last to flood, around 1500 years ago. (Fourqurean and Robblee 1999) The mud then began to settle in the shallow waters to form the mud banks. The mud in these banks is of biogenic origin, with calcareous green algae, seagrass epiphytes, mollusks, and stony corals being important contributors. The changes occurring in these mud banks varies by location. In some areas of the bay, the mud banks are being eroded and deposited at equal rates, but they migrate. In areas where the rates are unequal, the banks are in either constructional or destructional zones.

Historically, the Florida Bay has served as a very productive estuary. It provides habitat for many plant and animal species including seagrasses, sponges, wading birds, fish, and many invertebrates. The bay serves as a home or nursery to 22 commercially or recreationally harvested species. While many of these species are harvested elsewhere, the bay serves as a nursery for many juvenile fish and larval invertebrates. Then as they develop, these organisms move out into the ocean to complete their life cycle. Economically important fish species including snook, tarpon, seatrout and mangrove snapper use the bay as a nursery. As adults, these fish are part of a huge guideboat fishing industry. Spiny lobsters use the bay as a nursery and it is the primary habitat for pink shrimp (Penaeus duorarum). Both are harvested commercially and the pink shrimp are also an important base in the bay’s food chain.

The ecology that has been established in the Florida Bay has begun to show signs of stress. In the summer of 1987, there was a large die-off of seagrass, with Thallassium testudinum (turtlegrass) being affected the most. These die-offs were followed by declining conditions including blooms of phytoplankton and algae along with sponge die-offs. These events were correlated with poor water clarity and a decrease in many animal populations. It is not certain, but it is hypothesized that this die-off is a result of increased salinity in the bay area. The increased salinity has been caused by the diversion of water from the Everglades to support cites in southern Florida. Because the bay gets less runoff, the salty ocean water intrudes farther inland. The diversion of this runoff has also altered the seasons in which the bay receives most of its freshwater runoff. While it is uncertain that this situation directly caused the seagrass die-offs to begin in 1987, the alteration of the historical water flows has a good chance of disrupting the Florida Bay ecosystem.

The solution to this problem is not far off. The Army Corps of Engineers has been put in charge of the South Florida Everglades Restoration Project, which is to restore much of the historical water flow to the Everglades. The result of this project will be an increase in the freshwater flow into the Florida Bay, which will lower the high salt levels. While this project will deal with one problem in the Florida Bay, it may cause some others. The increased water flow is expected to carry high levels of both nutrients and pollutants from the lands that will be flooded. These lands include many sugar farms. The runoff from these lands will carry pesticides and nutrients like phosphorus to the bay. The high levels of phosphorus may lead to large algal blooms in and beyond the bay, some of which could block crucial sunlight from coral reefs. The pesticides could contaminate or kill the many species that use the bay and are later harvested.

While the increased water flow to the Florida Bay brings hope of restoring the ecosystem, there are still issues to deal with. The Florida Bay will still need to be monitored for the effects that this change will have. The bay acts as a nursery that is very important both biologically and economically to the surrounding areas. If the conditions continue to decline in the Florida Bay area, then additional actions may need to be taken to improve the quality of the water flowing through the Everglades into the Florida Bay.

Dan Phirman
Tropical Marine Ecology
June 10, 2000

Boyer, Joseph N., James W. Fourqurean, and Ronald D. Jones. 1997. “Spatial Characterization of Water Quality in Florida Bay and Whitewater Bay by Multivariate Analyses: Zones of Similar Influence.” Estuaries 20(4).

Brown, Lester R., Christopher Flavin and Hilary French. 2000. State of the World 2000. W.W. Norton & Company, New York, NY.

Fourqurean, James W. and Michael B. Robblee. 1999. “Florida Bay: A History of Recent Ecological Changes.” Estuaries 22(2B).

Helvaarg, David. 1998. “Destruction to Reconstruction: Restoring the Everglades.” National Parks 72(3-4).

Hiaasen, Carl. 1995. “The Last Days of Florida Bay.” Sports Illustrated 83(12).

Stearns, Bob. 1993. “The Dead Zone.” Field and Stream 98(8).

Sumich, James L. 1992. An Introduction to the Biology of Marine Life, 5th Ed. W. C. Brown Publishers, Dubuque, IA.

Thayer, Gordon W., Allyn B. Powell and Donald E. Hoss. 1999. “Composition of Larval, Juvenile, and Small Adult Fishes Relative to Changes in Environmental Conditions in Florida Bay.” Estuaries 22(2B).

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