Research Article:
Water Quality & Effect on Food Chain in Florida Bay

What Types of Habitats Are Found in the Florida Bay Estuary?

The Florida Bay estuary contains two types of marine (saltwater) habitats: mangrove areas and inshore marine areas. The mangrove areas are swampy areas where tropical trees and shrubs (called mangroves) cover the shore and Keys. In the Florida Bay estuary, some of these mangrove areas occur in less saline (salty) areas than others. The more saline mangrove areas are found on the Keys in Florida Bay. These mangrove areas are covered with overhanging red mangroves and poison wood. Crocodiles are one of the apex predators in mangrove areas. Other animals at the top of the food chains include Atlantic bottlenose dolphins, West Indian manatees, and nurse and lemon sharks.

Inshore marine areas in the Florida Bay estuary are of two types. The first are grass-bottomed areas covered with turtle grass and shoal grass. Turtle grass is the most important plant in the food chain in these areas. It provides shelter and nutrients for juvenile shrimp, crabs, young marine fish, and loggerhead turtles. The second type of inshore marine area is the hard-bottom zone, where sponges grow and provide shelter for spiny lobsters.

What Affects Water Quality?

The ability of the water in the Florida Bay estuary to support life is affected by a number of influences, including salinity and mercury, nitrogen, and phosphorus levels.

When salinity increases, many plants and organisms can no longer survive. Most notably, when salinity increases in the inshore marine areas, turtle grass dies off. With this important base of the food chain gone, all organisms in the habitat suffer.

Mercury is a contaminant that is introduced into the Florida Bay estuary from agricultural fungicides used by growers of citrus fruits and sugar cane north of the Everglades. Even very small amounts of this poisonous substance can kill organisms in the environment.

High levels of nitrogen and phosphorus enter the Florida Bay estuary as rainwater washes fertilizer and mining tailings from nearby lands into the waters of the estuary. Ironically, the same nitrogen and phosphorus added to the soil to help food crops grow are carried in runoff into the estuary, where they encourage growth of phytoplankton, including algae. As the levels of these agricultural fertilizers increase in the water, the phytoplankton populations grow rapidly. The phytoplankton consume much of the dissolved oxygen in the water, reducing the amount of oxygen available for other organisms in the area. High levels of phytoplankton also block sunlight needed for seagrass to grow.

Why Are Dr. Hunt and His Team Studying Florida Bay?

Dr. Hunt and his team from the Florida Marine Research Institute are interested in the many organisms that live in the Florida Bay estuary. They are especially interested in studying forces (salinity, mercury, nitrogen, phosphorus) that affect the ability of water in the area to support life. Each member of Dr. Hunt's team is interested in a different organism found in the bay.

Two members of Dr. Hunt's team are interested in the phytoplankton and how it affects the life-sustaining ability of the water. To study the phytoplankton, they begin by flying over the Florida Bay areas in a low- flying airplane. Areas in which phytoplankton are thriving look purple to green from the air. Once areas of phytoplankton are located, the scientists sample the water with the help of volunteers-including teachers and students in Munroe County who are part of the Nature Conservancy "Baywatch" monitoring program. Baywatch participants measure dissolved oxygen, pH, temperature, salinity, turbidity, and some other factors, such as nitrogen and phosphorus levels, on a regular basis. These measurements provide researchers with ongoing information about the estuary's ability to support life.

Dr. Hunt is particularly interested in spiny lobsters that live in the hard-bottom sponge areas. Other members of his team are interested in sea turtles and young fish that live in the turtle grass-including barracuda, which move from the mangroves through the inshore marine habitats and out to the patch and barrier reefs. When the water is contaminated, barracuda carry the contaminants through the different food chains in the different habitats.

What Is the Focus of This Investigation?

As you've learned, turtle grass is the most important plant in the food chain in the inshore marine habitats of the Florida Bay estuary. It provides nutrients for many animals, including pink shrimp. In this Investigation, you will explore the life cycle of the pink shrimp to help you understand the effects of human activity (including increased salinity) on the turtle-grass food chains.

What Is the Life Cycle of the Pink Shrimp?

The life cycle of the pink shrimp begins when spawning (egg-laying) takes place in offshore waters between April and August. Most of the pink shrimp in the south Florida region travel to an area called the Dry Tortugas to lay their eggs. The Dry Tortugas are uninhabited islands located off the tip of the Florida Keys (Key West) in the Gulf of Mexico (about a four to six hour boat ride). Used in the past by pirates and by the Confederate army during the Civil War, these islands today are home to lemon sharks, shrimp, and other organisms.

During the spring and summer, tiny young shrimp cling to pieces of floating sea grass and float back from the Tortugas to Florida Bay, a nursery that provides food and shelter. If they successfully complete the long journey to Florida Bay, they stay in the bay feeding on organisms they find in the sea grass until they mature into adults. In the fall and winter, the adult shrimp return to the warmer waters of the Dry Tortugas to continue the cycle.

In recent years, much of the turtle grass in the inshore marine habitats has died. Scientists are still unsure what is causing this. They believe that one cause may be the increased salinity of the water in the estuary. The decrease in the amount of turtle grass means less food for the returning young shrimp. It also means less food for creatures higher up the food chain, such as the blue crab and juvenile fish.


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Revised: 17 Oct 1995