What are Estuaries?

Estuaries are special. They occur in areas where freshwater meets and mixes with salty ocean waters. The term estuaries, according to general usage, refers to protected, nearshore waters, such as bays and lagoons.

Survival of plants and animals in estuaries requires special adaptations. Estuaries are dynamic systems where waters are alternately salty and fresh. The ebb and flow of tides may leave some plants and animals, such as seagrasses and oysters, temporarily high and dry. Shallow estuarine water temperatures can range from freezing to more than 100 degrees F during the course of a year.

Estuarine organisms are naturally adapted to withstand these ranges in salinity, tides, and temperatures. They must, however, have a balanced flow of fresh and saltwater. This balance can be upset if: 1) there is too much freshwater, as when causeways are constructed impeding the free flow of tides, or if; 2) there is too little freshwater, as in the diversion or damming of a river. Estuarine-dependent marine life may die if the precarious balance of fresh and saltwater is not maintained.

Why are Estuaries Special?

"The cradle of the ocean" is a most appropriate title for estuaries. More than 70 percent of Florida's recreationally and commercially important fishes, crustaceans, and shellfish spend part of their lives in estuaries, usually when they are young. Many fishes and crustaceans migrate offshore to spawn or breed. The eggs develop into larvae (immature forms) that are transported into estuaries by tides and currents. The shallow water, salt marshes, seagrasses, and mangrove roots provide excellent hiding places from larger, open-water predators. Some species grow in estuaries for a short time; others remain there for life.

Shrimp, for example, spawn offshore. The larvae then move toward inshore waters, changing form by molting as they progress through various stages of development. As young shrimp, they burrow into the sea floor at the mouth of the estuary as the tide ebbs, then ride into the estuary on the incoming tide. If successful in reaching the estuary after this hazardous journey from the sea, the young shrimp find seagrasses and algae to conceal them from predators. Because many larger animals cannot survive in the lower salinity of the estuary, the young have the added protection of a "salt barrier". Once the shrimp approach maturity, they leave the estuary for the sea to spawn, and the cycle begins anew.

Estuaries are among the most productive ecosystems in nature. Rivers and streams drain into estuaries, bringing in nutrients from uplands. Plants use these nutrients, along with the sun's energy, carbon dioxide, and water, to manufacture food. Among the most important plant forms that contribute to estuaries are microscopic algae called phytoplankton. Other plant forms include marsh grasses, mangroves, seagrasses, and macroalgae. When these larger plants die, they are broken down into detritus and are colonized by microbes (bacteria, fungi, and other organisms). During decomposition, detritus becomes smaller and smaller and the nutrients and small particles become food for thousands of organisms. Larger animals feed directly on these tiny particles or on smaller animals that fed on detritus.

As long as nutrient-rich freshwater flows and tides interact without human interference, our estuaries will remain productive. Snook, trout, mullet, jack, grouper, redfish, silver perch, spot, catfish, sheepshead, spiny lobster, shrimp, crabs, oysters, and clams are examples of the diverse marine animals dependent upon healthy estuaries. Estuaries also provide breeding and nesting areas, or rookeries, for many coastal birds, including several endangered species, such as brown pelicans. Estuaries' role as the ocean's nurseries cannot be overemphasized.

Florida’s Estuaries

Florida is undergoing tremendous growth. Development pressure is impacting marine fisheries habitat components important in maintaining viable commercial and recreational fisheries.

Florida Marine Research Institute scientists are locating and calculating the acreage of existing estuarine habitat components such as salt marshes, mangroves, and seagrasses.

Information used to map and monitor Florida's coast is available from LANDSAT satellite and other satellite information sources. The scientists are also noting trends in habitat change by analyzing aerial photographs from the 1940's, 1950's, and 1980's. Results of the habitat trend analyses have shown substantial losses of fisheries habitat throughout Florida. One study area on the east coast included the Indian River from-Sebastian Inlet south to the St. Lucie Inlet. Over a forty year period, an 86% decline in the availability of mangrove habitat to fisheries was documented, in addition to a 30% loss of seagrass acreage. Tampa Bay, in southwest Florida, has experienced an 81% loss of seagrasses and a 44% loss of mangrove and salt marsh acreage over a  100-year period.

Estuarine habitat loss is a serious problem in Florida's coastal zone. It is difficult to put a price tag on estuaries. They are one of our greatest natural resources. This resource, however, can be destroyed. The coast's appeal if very evident; 78% of Florida's estimated 14 million residents live in the coastal zone. Dredge and fill operations for waterfront homesites and seawall construction destroy mangrove shoreline and underwater grassbeds. Though these activities may temporarily enhance real estate value, ultimately they may decrease long-term value as the natural amenities disappear, the water becomes foul, and wildlife leaves. These activities often eliminate habitat and feeding areas for young fish, shrimp, and crabs. Without estuaries many important fisheries will disappear.

Estuaries are special. Help protect them.

This page was derived from a brochure developed by the Department of Environmental Protection, Florida Marine Research Institute under a grant from the NOAA, Office of Ocean and Coastal Resource Management.