"Over 20 percent of the estuarine wetlands in the lower 48 States are
located in Florida. The northwest Florida coast alone has 120,000 acres
of nearly-undisturbed coastal wetlands. This area is experiencing rapid
population growth, however, as people seek the benefits of living near
the coast. We do not know the current state of these wetlands, nor do
we yet understand the geological processes that will cause them to
Coastal wetlands provide important functions for human activities.
Wetlands filter water to remove nutrients, contaminants, and sediment, thereby improving coastal water quality. They support fisheries, including shrimp, oysters, and blue crabs. They also supply an abundant quantity of food and a protected nursery habitat required by many species of fish. The ability of marshes to trap silt helps to maintain coastal water clarity for the benefit of seagrasses. In turn, these seagrasses help dampen waves, thereby stabilizing the wetland. Wetlands also provide protection for inland houses from coastal storms by acting as a buffer against flooding. Florida's northwestern coast has a high frequency of hurricanes (14 in the past 100 years). Some have produced storm surges of up to 6 feet, making flooding a serious problem.
A variety of factors, both natural and manmade, can place stress on the wetlands.
Since 1939, sea level change has risen an average of 1.2 mm per year at Cedar Key, Florida. Fluctuations of 5.1 centimeters appear on top of this change. This may alter vegetation patterns by increasing flooding and soil salinity.
Rainfall, development, and changes in sea level can alter wet-lands, or marshes. Sea level has been increasing slowly along the northwestern coast of Florida for several thousand years. The marshes must build upward or move landward to survive; those that cannot move are drowned. Sea level has risen about 60 millimeters (2.4 inches) in the past 50 years, and cores taken from marshes show accretion at a comparable rate. This rise may result from changes in global climate. Oscillations in sea level occur on shorter cycles, with some changes of 100 millimeters (4 inches) over just a few years. Changes in sea level may also increase salinity along the coast, causing stress in species that are less tolerant of high salinity and frequent flooding. Drought conditions can cause a reduction in the flushing of salts from soils that are infrequently flooded, which in severe cases produces salt barrens. Flow from the Floridan aquifer through numerous springs also supports less salt-tolerant vegetation. Changes in flow most likely occur as use of the aquifer increases; a direct result of population growth. The most direct human impact, that of construction, dredging, and filling in marshes, can be controlled and is less severe than in the past. Signs of stress in wetland habitats of the northwestern coast of Florida have begun to appear, but environmental scientists do not yet understand the processes that cause this stress.
Evidence of stress is found in many areas including dying Sabal palms and freeze-killed mangrove trees. Unlike most marsh coasts in the United States, this coast is subtropical; it has a limited supply of mud and so much of the marsh becomes established around exposures of limestone bedrock. Fresh water that supports brackish vegetation comes mostly from springs and spring-fed rivers and streams such as Crystal River, St. Marks River, and even the larger Suwannee River. Portions of this coast have experienced a population growth rate of 250 percent since 1970. Most of the growth is along the watersheds and upland margins of the salt marshes, where the detrimental effects of human activity may be greatest.
Changes in flooding and salinity levels may contribute to the decline
of coastal hammocks. Sabal palm (Sabal palmetto), Florida's State
tree, is currently experiencing high levels of morbidity along the Gulf
U.S. Geological Survey (USGS) scientists use satellite images to map changes in the characteristics of the Florida wetlands.
Satellite images provide a regional view of change. Areas of change can be mapped, and precursors of erosion and loss are seen in decline of vegetation that can be quantified using satellite data. These images can help identify areas requiring closer monitoring of geological and physical factors. Understanding how these factors may be influenced by human activity will allow the best use and most appropriate management of these coastal wetlands. USGS studies improve understanding of the geological and physical processes that control changes in the wetlands.
Right: The normalized-difference vegetation index indicates spatial patterns in the green canopy. Green represents increased green canopy from 1986 to 1991, and yellow and red indicate decreased cover. Green areas along the coast quantify the recovery from the impact of Hurricane Elena in September 1985. Red areas inland show indication of reduced green cover; along the upper portion, many areas have been converted to barrens.
USGS scientists began a five-year geological study of this fragile environment to describe the unusual characteristics of this area and to document erosion. The study involves determining rates of sediment transport and erosion, as well as monitoring sea-level fluctuations, the effects of salinity variations, and changes caused by tropical and winter storms. This research is designed to provide baseline data and information, and to describe existing problems, potential problems, and areas where mitigation efforts may be successful.
For more information please contact:
Dr. Richard P. Stumpf, USGS Center for Coastal Geology, 600 4th Street South, St. Petersburg, FL 33701
Phone: (813) 893-3100