Freshwater Life

The animals and plants that live in freshwater are called aquatic life. The water that they live in is fresh, which means that it is less salty than the ocean. The terrestrial (land) environment that surrounds the freshwater environment has a large impact on the animals and plants that live there. Some factors that influence the freshwater environment include climate, soil composition, and the terrestrial animals and plants in the area.

Just as on land, aquatic plants require carbon dioxide, nutrients (substances such as phosphate and nitrogen needed for growth) and light for photosynthesis, the process where plants make their food from sunlight, water, and carbon dioxide. Aquatic animals need to breathe in oxygen and consume food. The physical conditions surrounding the body of water or wetland (lands that are covered in water often enough so that it controls the development of the soil) control the availability of these resources. For example, the concentrations of nutrients, oxygen, and carbon dioxide in the water depend on how much air gets into the water and on the chemical composition of the land nearby. The sediments (particles of sand, gravel, and silt) in the water influence how much light reaches the bottom of the lake or river. The temperature of the water affects how quickly animals and plants grow. The characteristics of the bottom of the body of water (sand, mud, rocks) and the speed of the currents (horizontal movement of water) control what kinds of plants and animals can live and reproduce in an area.

In general, freshwater environments are divided into two major categories: lentic waters and lotic waters. Lentic waters are those that are moving, as in rivers and streams. Lotic waters are those that are stationary, as in lakes and ponds. Sometimes, however, rivers and streams flow into lakes and ponds and the two different habitats merge together. Some wetlands may also contain many characteristics of freshwater environments.

Life in rivers and streams

Rivers and streams are characterized by several physical features. They are generally comprised of freshwater that flows in one direction. The flow of water is most often from an area of high altitude (like a mountain range) to an area of low altitude (like an ocean). Usually, the water flows quickly initially and slows as it moves downstream. Streams often join rivers, so there is more water at the end of a river than at the beginning. As rivers flow, they erode (wear away) rocks and pick up sediments, making rivers often murkier at the end. Because rivers and streams change so much from their beginnings to their ends, there are many different types of habitats for animals. As a result, the number of animal species that live in rivers and streams is greater than the number of species that live in lakes and ponds.

Plant life in rivers and streams A major challenge facing plants that live in rivers and streams is staying in place, especially in swift currents. Plants have several different techniques to overcome the drag (the pull) of the water. Diatoms are a type of algae. Algae are marine organisms that range in size from microscopic phytoplankton to giant kelp and that contain chlorophyll, the same pigment used by land plants to perform photosynthesis. Diatoms avoid currents by using their small size. They grow in a single layer on the surfaces of rocks. Because of the friction between the rock surface and the water, the water flow slows nearly to a stop within about a tenth of an inch (one-quarter of a centimeter) from the rock's surface. This region is called the boundary layer, and it provides the diatoms with protection from the forces of the current that would otherwise drag them downstream.

Typical large river plants include algae, mosses, and liverworts. These plants overcome the drag of the water by using special adaptations to grip rocks. Large algae often attach themselves to rocks with root-like structures called holdfasts. In addition, plants often anchor themselves in nooks between rocks or where waters pool, to avoid the drag of the river water.

River plants that live within the currents have developed techniques to withstand the forces of the water. These forces would quickly snap any plant with rigid stems or leaves. As a result, plants that live in rivers are very flexible so that they can easily bend and move with the currents.

Animal life in rivers and streams Animals that live in rivers and streams also face the challenge of staying where they are. Many animals have hooks and suckers that they use to attach themselves to rocks. Blackfly larvae that live in streams in the northern United States and Southern Canada have suction cups that they use to stick to rocks in streams. Mayfly larvae have hooks that they use to fasten themselves to the algae growing on rocks.

Other animals have streamlined shapes that minimize drag by presenting little resistance to water. Trout, which are extremely common in oxygen-rich fast-flowing waters, are shaped like torpedoes. Limpets are flattened molluscs that cling to the surfaces of rocks. Their flat shape decreases the currents' drag on them.

Diadromous Fish

Diadromous fish are fish that live in two different aquatic habitats. During one part of their lives, they live in freshwater and during another part of their lives they live in saltwater. In order to move from saltwater to freshwater (or freshwater to saltwater), these unique fish must undergo drastic changes that affect the way that their gills collect oxygen from the water. Only about 1% of all fish in the world are diadromous. Two types of diadromous fish are anadromous and catadromous.

Anadromous fish are fish that spend the majority of their lives as saltwater fish and then migrate (move periodically or seasonally) into freshwater to spawn. Many species of salmon, striped bass, sturgeon, and steelhead are all anadromous. In most of these species, the eggs are laid in freshwater and after they hatch the juveniles migrate into the oceans. In most of the salmon species, the adults migrate back to the rivers where they were born, where they spawn and then die. Other anadromous fish migrate back to freshwater to spawn several different times during their lives, returning to the ocean in between spawnings.

Catadromous fish are fish that live most of the their adult lives in freshwater and then migrate to saltwater to spawn. In the United States, the only species of fish that is catadromous is the American eel. It lives in rivers all along the east coast of the United States. When it is ready to spawn it migrates thousands of miles (kilometers) to the Sargasso Sea, near Bermuda. Once it leaves the freshwater, it does not eat, and so after it spawns its energy reserves are used up and it dies.

Animals that live in streams and rivers have developed interesting ways of gathering food in the fast-flowing waters. Snails, limpets, and caddis fly larvae scrape algae from rocks using special mouthparts. Many different insect larvae, as well as freshwater clams, filter the water for small bits of food. They have specialized mouthparts that look like brushes or combs that they use to strain the water and extract the edible plankton (animals and plants that float with currents) that float into their reach.

Rivers and streams are homes to a large number of fish. Perch, smallmouth bass, largemouth bass, bullhead, carp, pike, and sunfish prefer the parts of rivers where waters slow. These fish tend to be large, visual predators (animal that hunts another animal for food) that hunt in pools for smaller fish and invertebrates (animals without a backbone). Sculpins and darters prefer the faster moving sections of the river where waters are highly oxygenated. They use the swift current to bring food to them rather than hunting for their prey. Trout are also found in these faster moving parts of the river.

Life in lakes and ponds

Large lakes are often divided into zones. The near-shore area is called the littoral zone. This is the part of the lake that is shallow enough for aquatic plants to grow. The limnetic zone, also called the epilimnion, is the surface water of the lake away from the shore. (The prefix epi means "on the surface" and the root word limn means "lake.") It extends down as deep as sunlight penetrates. The majority of the plant life in this zone is phytoplankton (microscopic plants that float in currents). The deep part of the lake is called the profundal zone or the hypolimnion. (The prefix hypo means "under.") No plant life exists in this zone because of the absence of light. Most of the biological activity is that of bacteria decomposing dead animals and plants.

Seasonal Changes in Lakes Lakes and ponds are greatly influenced by the temperature changes throughout the seasons. The description below is typical for a lake in a temperate (moderate) climate, which experience seasonal temperature changes. Tropical lakes (those in hot and humid areas) will have less dramatic fluctuations in temperatures.

In the summer, the Sun warms the epilimnion. Warmer water is less dense than colder water, so it floats on top of the cooler water in the hypolimnion. The region between the warm surface waters and the cold deeper waters is a transition zone where the water changes temperature very quickly with depth. This region is called the thermocline. The thermocline acts as a kind of barrier between the surface and the deep waters. In the early summer, the epilimnion is full of life. Phytoplankton can grow quickly because they have plenty of light and nutrients and the water temperature is warm. In turn, zooplankton (animals like crustaceans and small fish that float in the waters) feed on the phytoplankton. These zooplankton are food for larger fish and birds.

As summer progresses, the phytoplankton use up the nutrients in the epilimnion. They begin to die and sink to the bottom of the lake. There, decomposers, like fungi and bacteria, break up the dead phytoplankton and animals and convert them into the nutrients that phytoplankton need to grow. Because the thermocline acts as a barrier between the bottom and the top of the lake, these nutrients are unavailable to the phytoplankton in the epilimnion. Phytoplankton cannot grow in the hypolimnion, where there are nutrients, because there is no light.

In the fall, the air temperature cools, which cools the surface of the lake. Eventually the temperature in the epilimnion becomes the same temperature as that of the hypolimnion. The thermocline disappears and the nutrient-rich waters from the hypolimnion mix with the waters in the surface of the lake. This is called the fall turnover. At this time, the nutrients from the bottom of the lake are mixed throughout the lake. However, because the amount of sunlight decreases in the fall and into the winter, the phytoplankton in the surface cannot grow very quickly.

During the winter, the surface of the lake continues to cool. Freshwater is densest at 39°F (4°C). Ice, with a temperature of 32°F (0°C), is less dense than the deeper waters and so it forms on the surface of the lake. This provides fish and other invertebrates room to live under ice-covered lakes. The ice also acts an blanket-like insulation that helps keep the water underneath from freezing.

In the springtime, the temperatures warm so the ice melts. Eventually the whole lake becomes 39°F (4°C) and so the waters from the bottom mix with the waters from the surface. This is called the spring turnover. As summer begins, the surface waters warm and the thermocline again separates the epilimnion from the hypolimnion. Because of the fall and spring turnovers, the nutrients from the bottom of the lake are available to the phytoplankton in the surface waters. This sets the lake up for the summer's rapid growth of phytoplankton and all the animals that depend on them.

Plant Life in Lakes and Ponds Some of the most plants in lakes and ponds are the smallest. These phytoplankton are usually single-celled plants grouped with the algae. Sometimes they connect themselves together into long strings called colonies. Common phytoplankton in lakes and ponds are diatoms, which have beautiful shells made of silica (the same material that comprises sand); dinoflagellates, which move by snapping their flagella (long whip-like cell extensions that can propel an organism; and cyanobacteria, which are bacteria that perform photosynthesis.

The larger plants in ponds and lakes include large algae and mosses, cattails, reeds, water lilies, bladderworts, willows, and button bush. These plants often grow in mud where the gases that they need to grow—such as oxygen and carbon dioxide—are scarce. Many larger plants have stems that are spongy and they pull gases from the air down into their roots.

Plants on land use their roots to gather water and nutrients, however aquatic plants are surrounded by water, and nutrients are dissolved in the water. Some aquatic plants have given up their roots. For example, duckweed (or water lentil) and watermeal are small pea-sized plants that float on the surface of lakes and ponds in the spring and summer. They absorb nutrients from the water and produce a lot of starches. By the fall, they are so heavy with nutrients that they sink to the bottom of the lake. They live out the winter in the mud at the bottom of the lake, existing on their stores of starch. By spring, they have used up so much of the starch, they are light enough to float again. They pop to the surface just in time to use the strong light of spring and summer for photosynthesis and they begin to use their starch stores once again. Other large plants, like milfoil, water soldier, and water hyacinths also float on the surface of lakes and ponds.

The edges of lakes are often divided into four zones based on the physical environment and the types of plants found there. Beginning farthest from the water, the swamp plant zone contains plants that have roots in the shallow water. At times the water can recede from this zone, leaving the plants roots exposed to the air. Typical plants in the swamp plant zone are rushes and sedges (a type of plant that looks like a stiff grass). The next zone is called the floating-leaf and emergent zone. Here the water never dries up, but the lake is shallow enough that the tops of plants emerge out of the water. A typical plant that lives in this zone is the water lily, which has special gas filled chambers in its leaves that allow it to stay floating on the surface of the water. In the submerged plant zone, plants live entirely underwater. Canadian waterweed and many types of mosses live in this zone. The free-floating plant zone takes up the center of the lake. Here plants without roots, like duckweed and water soldier, float freely on the surface.

Animal life in lakes and ponds Zooplankton float in the epilimnion of lakes and eat phytoplankton and other zooplankton. Usually, these animals are nearly transparent, in order to avoid being seen by their predators. Typical zooplankton in lakes include the water flea, Daphnia, which can reproduce without mating. Under normal conditions all of its offspring are female. However, when the animals are stressed, by lack of food for example, they will produce males. This mixes up the gene pool of the population and creates individuals that are likely to withstand environmental changes. Another typical freshwater zooplankton is the rotifer, which has bristles on top of its head that it whirls like propellers in order to move through the water and capture prey.

Many insects have juvenile stages that are aquatic. Mayflies, caddis flies, mosquitoes, and dragonflies all live for some period underwater in lakes and ponds. They swim among the rocks and plants in the lake bottom for a season or several years. Then they metamorphose (change in appearance) into their adult form and fly away from the water. The bottom of the lake is also home to many different worms, mussels, and crustaceans. These animals feed on the remains of plants and animals that drop to the bottom of the lake from above.

Stream Shredders

In some rivers, nearly all the plant material comes from leaves and other plant parts that fall into the river from land. As soon as the leaf hits the water, an army of invertebrates is involved in tearing the leaves to pieces. These animals, which include insect larvae and crayfish, are called shredders and they play a key role in stream ecology (relationship between organisms and their environment). The parts of the leaves that the shredders do not eat are consumed by worms and snails, which in turn become food for fish, amphibians, and birds. Breaking the leaves into small pieces is the first step in decomposing the plant material. Bacteria and fungi colonize the plant bits and break them into the nutrients that plants growing in the river need to grow.

Larger animals live in lakes and ponds. In particular, fish, birds, and amphibians prey on the invertebrates that live in the lakes. Fish such as bluegills eat juvenile insects that swim in the bottom of the lake, while crappies eat zooplankton near the surface. Birds like flycatchers and warblers fly near the surface of lakes, preying on insects that are hatching from their juvenile stage. Frogs also hunt for insects that live near the pond. Still other birds and fish prey on smaller fish. Bass, salmon, osprey, loons, and heron hunt for fish by using their keen eyesight. Beavers and muskrats are mammals that depend on water for their homes. They build dams and lodges, which provide them with protection from predators.

Juli Berwald, Ph.D.

For More Information

Books

Cunningham, William P., and Barbara Woodworth Saigo. Environmental Science: A Global Concern. Boston: WCB/McGraw-Hill, 1999.

Llamas, Teresa. The Vegetation of Rivers, Lakes, and Swamps. New York: Chelsea House, 1996.

Raven, Peter H., Linda R. Berg, and George B. Johnson. Environment. 2nd ed. Orlando, FL: Saunders College Publishing, 1998.

Rowland-Entwistle, Theodore. Rivers and Lakes. Morristown, NJ: Silver Burdett Press, 1987.

Sayre, April Pulley. Lake and Pond. New York: Twenty-First Century Books, 1996.

Sayre, April Pulley. River and Stream. New York: Twenty-First Century Books, 1996.

Websites

"Anadromous Fish." National Marine Fisheries Service.http://www.nmfs.noaa.gov/habitat/habitatprotection/anadromousfish.htm (accessed on August 16, 2004).

"Freshwater Ecosystems." Missouri Botanical Gardens.http://mbgnet.mobot.org/fresh (accessed on August 16, 2004).

"Freshwater Habitats." Singapore Zoological Gardens Docents.http://www.szgdocent.org/ff/f-wfrsh1.htm (accessed on August 16, 2004).

"FreshwaterLife." United Nations Food and Agricultural Organization.http://www.freshwaterlife.info/fwl/aboutfwl.html (accessed on August 16, 2004).