Aquatic Field Investigation

Background Information &
Research Article


What do the aquatic sites in your neighborhood have to do with aquatic sites in southern Florida? The rivers, creeks, or marshes of your local area might seem to you to be totally different from the Florida Everglades, with its freshwater and saltwater habitats including endless prairies of waving sawgrass, dark Cypress swamps, mangrove estuaries and vast saltwater marshlands! Each of these, like your own freshwater or saltwater habitat, is home to a special group of animals and plants. For example, the sawgrass freshwater marshes are home to American alligators and white-tailed deer; cypress swamps are home to the rarely seen Florida panther. The alligator's cousin, the American crocodile, can be found basking along the mangrove estuaries in Florida Bay, and hundreds of birds make their home along the vast saltwater marshlands.

But your own aquatic habitats are similar to those in Florida (and others all over the world) because all of these aquatic sites share some common characteristics: they are all composed chiefly of water, and they all contain life forms which live in water. Most of these life forms are less tolerant of changes in temperature and salinity, or of chemical pollutants, than are most terrestrial (land-based) organisms. To understand more about them and how temperature, salinity, pollutants, and other physical data affect these habitats, scientists monitor and measure these characteristics.

In this Investigation, you will build and use some of the tools that scientists use to measure characteristics such as salinity, pollutant levels, and temperature. You will identify and visit an aquatic site near your school and conduct Experiments that will rival those of Dr. Ballard and his team of scientists in southern Florida.

Types of Aquatic Environments

The two main types of aquatic environments that we're studying are freshwater and saltwater habitats. The saltwater systems that we're studying are shallow; they consist of inshore marine habitats, such as seagrass beds (or tidal zones) and estuaries; mangrove shorelines; coral reefs; and salt marshes.

Freshwater habitats consist of lakes, rivers, and wetlands. Wetlands are areas where the soil is saturated with water for long periods of time. Wetlands are some of the most fertile ecological areas on the planet, and are home to one- third of the world's endangered species. Yet, as you might guess, plant growth in such an environment is limited to species that can withstand having their roots submerged in water for long periods of time.

Some of the most familiar wetlands are swamps, marshes, and bogs. Do these all sound pretty much the same? Let's take a quick look at the characteristics of eight different types of saltwater and freshwater habitats:

Freshwater marshes are areas of soft, wet, low-lying land characterized by grassy plants. Freshwater marshes often form a transition zone between freshwater and land. In the United Kingdom, these marshes may be called "fens."

Saltwater marshes are coastal wetlands that are sometimes called tidal marshes because they occur in the zone between low and high tides. Saltwater marshes contain a variety of plants including rushes (tufted marsh plants with hollow stems), sedges (tufted marsh plants with solid stems), and grasses.

Bogs are areas that have wet, spongy, acidic soil composed chiefly of sphagnum moss and peat. In the United Kingdom, bogs may be called moors or heaths.

Swamps are seasonally flooded low-lying lands. They have more woody plants (such as trees and shrubs) than marshes do, and they have better drainage than bogs do. One type of swamp-the cypress swamp, such as Big Cypress Swamp in Florida and Okeefenokee Swamp in Georgia-exists in a brackish to fresh water environment in the subtropics.

Streams are areas in which water flows in a channel or bed. Small rivers and large brooks are considered streams.

Mangrove shorelines are areas along ocean coastlines where mangrove plants grow. The mangrove plant, which is very sensitive to extreme temperature fluctuations, obtains freshwater from saltwater. These areas are mainly composed of the roots from the mangrove plants. which build up thicker and thicker, absorbing sand and silt to form land around themselves, like an underwater forest. As the roots decompose, they add to the soil.

Estuaries, one type of inshore marine habitat, are areas where freshwater meets salty ocean water. Rivers and streams drain into estuaries, bringing nutrients. Microscopic phytoplankton thrive in estuaries and serve as food for a variety of other estuary animals.

Coral reefs are colonies of coral polyps joined together to form specialized habitats that provide shelter, food, and breeding sites for many plants and animals. They are found in shallow ocean areas with warm, clear water.

Measuring the Ability of Aquatic Environments to Support Life

The diversity of the Earth's species is a measure of the richness and well-being of life on the planet. Similarly, the diversity of animals at any given aquatic site is an indicator of the environmental well-being of that site. It is difficult, however, to know how many kinds of organisms exist in any area such as a stream or swamp, and still harder to count every organism. But scientists have devised tools to help them sample the population of aquatic environments to get an idea of the levels of diversity. Once they have collected a sample of animals, scientists use a biotic index, a scale based on the types of organisms living in the environment, to measure the "health" of the environment. Habitats that are diverse or support more different kinds of organisms, are generally thought by scientists to be "healthier" habitats than those which support very few kinds of organisms.

Scientists also use an abiotic index, or scale, to measure physical characteristics such as temperature, salinity, pH, turbidity, and dissolved oxygen levels. Temperature is measured using a thermometer and salinity is measured indirectly with a hydrometer.

A hydrometer is a device that measures density, or the ability of an object to float or sink in a liquid. When the hydrometer floats higher up in the liquid, the liquid is said to be more dense then when it floats lower down. A more dense liquid usually has a higher salinity or salt content.

Dissolved oxygen levels are used by scientists to determine how much oxygen is available in the habitat for all the different kinds of organisms that live there. All living organisms need oxygen to survive. When the amount of oxygen in the water is high, the habitat usually has the ability to support many different kinds of life. When the dissolved oxygen is low, only a few special kinds of life are able to live.

Scientists use the physical measurements of pH and turbidity to determine the presence or absence of pollutants, including chemicals and particles. The pH measurement describes the concentration of hydrogen molecules in a chemical. Each chemical has a specific pH "fingerprint," rated from 1 to 14, and scientists can uncover mystery chemicals in a liquid such as water using this test. The test measures the concentration of hydrogen molecules (grams per liter) in the liquid. When the concentration is low, the liquid is said to be an acid. An example of an acid is vinegar. When the concentration of hydrogen molecules (grams/liter) of the liquid is high, the liquid is said to be a base. An example of a common base is dishwashing soap. When the concentration of hydrogen is neither high nor low, the liquid is said to be neutral. Distilled water-the bottled water used to clean contact lenses-is neutral.

Turbidity is a measurement of the clarity of water. The more turbid the water, the more suspended particles are present and the less transparent the water is. A turbid habitat may make it difficult for aquatic plants to make food using the sun. A Secchi disc, which consists of a black and white circle, is the most common tool used to measure turbidity.

In this investigation, you will take part in a baseline study to scientifically describe a local aquatic site. You will use several types of tools and sampling techniques to measure biotic and abiotic characteristics-which, when taken together, can help you determine the ability of water at your site to support life. So when your teacher tells you it's time, pack up your collection nets, pH kits, dissolved oxygen tests, hydrometers, corers, and Secchi disks!

Try this quick game to test your knowledge


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Gene Carl Feldman (gene@seawifs.gsfc.nasa.gov) (301) 286-9428
Todd Carlo Viola, JASON Foundation for Education (todd@jason.org)
Revised: 14 Nov 1995