Freshwater and Freshwater Ecosystems

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Freshwater and Freshwater Ecosystems

Introduction

Freshwater refers to the water found in lakes, ponds, streams, and any other body of water other than the sea. It supports a range of plant and animal ecosystems whose composition is shaped by the availability of food, oxygen (O), temperature, and sunlight. Freshwater environments are less extensive than the sea, but they are important centers of biodiversity. This is especially so in dry environments, like deserts, where isolated ponds and streams provide a haven for plants and animals.

Plants and animals living in freshwater would not usually be able to live in saltwater, because their bodies are adapted to a low-salt content. Freshwater ecosystems are vulnerable to water pollution that arises from a range of human activities, from deforestation to urban development. They are also used as water supplies for human use, and sometimes their natural course is diverted for this purpose as, for instance, when a dam is built.

Historical Background and Scientific Foundations

Aquatic, or watery, environments are divided into freshwater and marine. Freshwater has less than one gram per liter of dissolved solids, mainly salts, of which sodium chloride (NaCl) is the most important as far as living organisms are concerned. It is the main source of water for most human uses. Freshwater ecosystems are found in ponds, lakes, reservoirs, rivers, and streams. Estuaries, which are places where a river meets the sea, such as San Francisco Bay, are part freshwater and part marine in their makeup. The diversity of a freshwater ecosystem depends upon temperature, availability of light, nutrients, oxygen, and salinity.

A wide range of plants, animals, and microbes are found in freshwater ecosystems. The smallest are the microscopic plants and animals known as phytoplankton and zooplankton, which form the bottom layer of freshwater food chains. There are also many freshwater invertebrates including worms and insects. Among the freshwater vertebrates, amphibians, such as frogs, live on land and water, while fish are purely water-dwelling. Many species of birds, such as kingfishers and ducks, live on or near freshwater.

Rivers and streams are lotic, or flowing, freshwater environments. Their water flows in one direction and they begin at a source—which could be a spring, lake, or snowmelt—and travel to their mouths, which may be the sea or another river. The water at the source is generally cooler, clearer, and has a higher oxygen content than at the mouth. Freshwater fish such as trout are often found near the source. There tends to be more biodiversity in the middle of a river or stream, while the water near the mouth is often murky with sediment that decreases the amount of light and the diversity of the ecosystem. Fish requiring less oxygen, like carp, are found near the mouth of a river. Lotic organisms tend to be small, with flattened bodies, so they are not swept away. Fallen leaves, insects, and other detritus are important food sources.

Rivers and streams carry precipitation to oceans, so there are streams in most localities. There is no sharp boundary between water and land with a stream. There is saturated soil both laterally and vertically beyond the banks of a stream, known as the riparian zone.

Lakes, ponds, and reservoirs are lentic, or layered, systems with generally still water varying in size between a few square feet to thousands of square miles. The surface layer is populated by plankton, protists (single-celled organisms like ameba), and insects. Beneath the surface is the epilimnion, which is relatively warm and sometimes mixed by the wind. The penetration of sunlight through the layer depends upon how much silt is suspended in the water. The layer just above the bottom,

WORDS TO KNOW

DETRITUS: Matter produced by decay or disintegration of living material.

LENTIC: The vertically layered nature of a lake.

LITTORAL: The region of a lake near the shore.

LOTIC: Flowing water, as in rivers and streams

WETLAND: A shallow ecosystem where the land is submerged for at least part of the year.

known as the hypolimnion, is cold and unmixed. The interface between these two layers, which marks a sudden fall in temperature, is called the thermocline. The bottom layer of a lake, the benthos, is occupied by burrowing worms and snails, with the ecosystem varying depending upon whether the bottom is rocky, muddy, or sandy. Levels of both oxygen and light decline with-depth. Anaerobic microorganisms, which can live without oxygen, often live in the bottom layers of a lake. A blue lake is lower in productivity than a green lake, but when productivity is too high, algal blooms may result with lower oxygen levels. The littoral layer, near the edge of a lake, generally has a large ecosystem with organisms that can use both land and water, such as dragonflies, frogs, ducks, and turtles. Plants such as rushes, reeds, and cattails grow rooted in the bottom sediments of the littoral zone.

Wetlands are important ecosystems that are part aquatic, part terrestrial. They are submerged, either partially or wholly, for at least part of the year. There are various types of wetland, described by their vegetation. Swamps are wetlands with trees, whereas a marsh is a wetland that does not have any trees. A bog contains areas of ground that are saturated with water and its ground is made of a material called peat, which is composed of accumulated and undecayed vegetation. Fens are like bogs, but their water is groundwater, whereas a bog is wet mainly by precipitation. Swamps and marshes are more nutrient-rich and productive than fens and bogs. Wetlands are often rich in biodiversity and are important for breeding and migrating birds and wild flowers. They play an important role by soaking up storm water, preventing flooding by slowing down the rate at which the water reaches river systems. They also act as a filter for agricultural waste, because wetland plants and microbes can detoxify otherwise harmful residues, thereby purifying this water.

Impacts and Issues

Many human activities threaten the health of freshwater ecosystems. For instance, acid rain created from sulfur (S) and nitrogen oxide (NO) emissions turns many lakes and streams acidic, leaving them unable to support various fish species. The building of dams to create hydro-

electric power plants blocks the routes of migratory fish such as salmon. Deforestation adds silt to a stream or river and slows it down, which may increase flooding.

Wetlands are among the most vulnerable of ecosystems. Five percent of the land area of the United States is occupied by wetlands that are home to one third of its endangered species. They are often situated near urban areas, where they are an attractive target for draining and development. They often fill with sediment, which is sometimes added to by road building and agricultural runoff, and this can convert them into a terrestrial environment. The United States lost nearly 500,000 acres (200,000 hectares) of wetland each year from the 1950s to the mid-1970s before their ecological importance was realized.

Restoring the water supply is sometimes all that is needed to restore a wetland. One example is the delta where the Tigris and Euphrates rivers empty into the Persian Gulf. This area was home to a group of people called the Marsh Arabs, who resided on floating platforms and lived off the marsh. During the Iran-Iraq war of 1980–1988, Saddam Hussein forced most of these people from their homes and drained the marshes, burn-ing them afterward. After the fall of Hussein, the United Nations and the remaining Marsh Arabs restored the water to the area. Some of the original flora and fauna have started to return, although it will be many years until this wetland is restored to its former state.