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A Region Under Stress-- Home
A Region Under Stress-- Introduction

Environmental Setting-- The Natural System
Watersheds and Coastal Waters

Environmental Setting-- The Altered System
Drainage and Development
Public Lands
Water Use
Water Budget

Water and Environmental Stress
Loss of Wetlands and Wetland Functions
Soil Subsidence
Degradation of Water Quality
Mercury Contamination
Effects on Estuaries, Bays, and Coral Reefs

Summary and Research Needs

Related Links

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U.S. Department of the Interior
U.S. Geological Survey
Circular 1134

The South Florida Environment - A Region Under Stress

Water and Environmental Stress

Effects on Estuaries, Bays, and Coral Reefs

Photo of the coast Photo of the coast

Changes in coastal estuaries are due, in part, to such factors as an overall reduction of the amounts of freshwater flow through the Everglades, the effects of constructing levees and canals near the coast to provide drainage and flood protection, the changes in the quality of runoff water, and the maintenance of lower ground-water levels along the southeastern coast (Smith and others, 1989; Halley and Hudson, 1993). Although the amount of flow reduction is disputed, the effects of drainage activities are observed in the encroachment of mangrove forests into northern Everglades National Park in recent years, the replacement of freshwater marshes by saltwater marshes, and the decline of coastal mangrove forests in areas that have been deprived of natural overland flow. Today, many of these coastal estuaries, especially along the southeastern coast and the central part of Florida Bay, frequently experience hypersaline conditions during the dry season (Davis and Ogden, 1994; McIvor and others, 1994).

Water management has resulted in more short-duration, high-volume water flow and less life-sustaining base flows to estuaries. Regulatory releases to control lake and ground-water levels according to prescribed flood-prevention formulae result in pulses of freshwater entering estuaries, causing rapid, drastic decreases in salinity that stress estuarine organisms. In addition, water flows have been diverted from one receiving basin to another, which changes the long-term salinity regimes in both systems. The combined effects of reduced freshwater inflow to Florida Bay as a result of man's alteration of the natural flow regime and the natural evaporation of water in this semiconfined embayment, have resulted in salinities that can be more than double those of the open ocean. Widespread chronic hypersalinity in the bay is extreme and frequently reaches 50 ppt over large areas, with known maximums of 70 ppt in small areas during severe drought (Tabb and others, 1962; Tabb, 1963). Biscayne Bay sometimes exhibits negative, or reverse, salinity gradients, with hypersaline conditions inshore. On the other hand, salinities in Manatee Bay have declined from 36 to 0 ppt in a matter of hours as a result of an abrupt release of freshwater.

Florida Bay has undergone changes during the last decade that are unprecedented within the period of recorded observation and that reflect a degradation of the ecosystem, in terms of its productivity of living resources, biodiversity, and stability (Bancroft, 1993; Boesch and others, 1993). Seagrasses have died in large areas (Robblee and others, 1991) and microscopic algae have bloomed with increasing frequency and intensity, thus turning the once clear waters a turbid green. Populations of water birds, forage fish, and juveniles of species of game fish seem to have been significantly reduced, catches of pink shrimp have declined, and many sponges have died, causing a potential threat to the catch of spiny lobsters. These and other ecological alterations in the Bay were recently summarized in a research plan by the Interagency Drafting Committee (unpublished, 1994). Because the freshwater flow through the Everglades into Florida Bay has been greatly reduced by consumptive use and watershed drainage (McIvor and others, 1994), much concern has been directed to flow reduction as the root cause of the deterioration of the bay ecosystem. However, other causes may be implicated in changes in the bay; for example, nutrient stimulation from internal sources (benthic sediment releases) and external sources (runoff and seepage from the land) (Lapointe and Clark, 1992) and natural influences, such as the diminished frequency of tropical storms in the vicinity of the bay (Boesch and others, 1993).

The amount and distribution of nutrients that flow into coastal waters of south Florida are extremely important to the health of the bays, estuaries, and reefs (Lapointe and others, 1990; Fourqurean and others, 1992; Lapointe and Clark, 1992). Denitrification in Everglades sediments is not an effective means of removing excess nitrogen that may be introduced as nitrate into coastal waters (Gordon and others, 1986). The flow of freshwater and nutrients from urban and agricultural lands, extensive mangrove and marsh wetlands, and phosphate-rich river waters of southwestern Florida may create a blend of water that contains adequate concentrations of nutrients to support algal blooms and enrichment in the western part of Florida Bay. In addition, if freshwater flow to the Atlantic Ocean is rediverted to the Gulf of Mexico and Florida Bay, as is being discussed as part of the Everglades restoration plan, then nutrient loading to the bay and gulf might substantially increase.

In much of Florida Bay, the Florida Keys, and the reefs, the quality of marine water is widely recognized as important, if not more important, than restoring freshwater flow to Florida Bay (Hallock and others, 1993). Continued residential development in the Florida Keys will intensify problems of increased stormwater runoff, septic tank and disposal well leachate, and nutrients and heavy metal contamination from marinas and live-aboard vessels within the Florida Keys area. The EPA and NOAA Florida Keys National Marine Sanctuary prepared a plan to deal with the continuing development and associated water- quality changes (U.S. National Oceanic and Atmospheric Administration, 1995). However, significant changes in water quality may have already taken place, as indicated by the seepage of sewage components in marine ground water offshore from the Keys (Shinn, 1993; Shinn and others, 1994).

Changes in Florida coral reefs (fig. 38) have received increasing attention during the last decade (Ward, 1990). Some damage to the reef corals by ships or divers, for example, may be easily related to human intervention (Hudson and Diaz, 1988; Talge 1992). Other changes, however, are slower and pose difficulties in determining their ultimate cause. For example, coral mortality is often the result of disease, such as "bleaching" that results when symbiotic algae are expelled from the coral tissue, thus causing the corals to lose their color (Brown and Ogden, 1993).

1976 photo of a coral reef community 1992 photo of a (changed) coral reef community
Figure 38. Long-term changes on a coral reef community are illustrated by photographs taken in 1976 and 1992 at the same location on a reef near Key Largo. The thick growth of staghorn coral (Acropora cervicornis) evident in 1976 had largely disappeared in 1992. This change may be natural or related to human activities. (E.A. Shinn, written commun. 1993.) Click on images to open larger picture (left - 28.3k, right - 28.7k).

In the last two decades, coral diversity and the amount of seafloor inhabited by coral has declined in the reef tract off the northern Florida Keys (Dustin and Halas, 1987; Porter and Meier, 1992). The decrease in general coral health has been variously attributed to global warming, nutrient increases, disease, and hypersaline water. These possible causes have not been fully documented. Some changes on the reefs are difficult to identify with local, regional, or global change. Some of the observed change may be related to widespread change throughout the Caribbean (Rodgers, 1985) or may be the result of more local causes. The U.S. Environmental Protection Agency is currently funding studies to monitor reef and hard-bottom community changes in the Florida Keys National Marine Sanctuary (Continental Shelf Associates, Inc., 1995).

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