Aquatic Ecosystems

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An aquatic ecosystem is a group of interacting organisms dependent on one another and their water environment for nutrients (e.g., nitrogen and phosphorus) and shelter. Familiar examples are ponds, lakes and rivers, but aquatic ecosystems also include areas such as floodplains and wetlands, which are flooded with water for all or only parts of the year. Seemingly inhospitable aquatic ecosystems can sustain life. Thermal springs, for instance, support algae and some insect species at water temperatures near the boiling point; tiny worms live year-round on the Yukon ice fields; and some highly polluted waters can support large populations of bacteria.

Even a drop of water is an aquatic ecosystem, since it contains or can support living organisms. In fact, ecologists often study drops of water -- taken from lakes and rivers -- in the lab to understand how these larger aquatic ecosystems work.

Organisms found in aquatic ecosystems

Aquatic ecosystems usually contain a wide variety of life forms including bacteria, fungi, and protozoans; bottom-dwelling organisms such as insect larvae, snails, and worms; free-floating microscopic plants and animals known as plankton; large plants such as cattails, bulrushes, grasses, and reeds; and also fish, amphibians, reptiles, and birds. Viruses are also a significant part of the microbial ecology in natural waters and have recently been shown to play an important role in the nutrient and energy cycles.

The assemblages of these organisms vary from one ecosystem to another because the habitat conditions unique to each type of ecosystem tend to affect species distributions. For example, many rivers are relatively oxygen-rich and fast-flowing compared to lakes. The species adapted to these particular river conditions are rare or absent in the still waters of lakes and ponds.

Freshwater Aquatic Ecosystems in Canada

Canada contains an abundance of freshwater ecosystems, including lakes, ponds, rivers, streams, prairie potholes, and wetlands.

A lake is a sizable water body surrounded by land and fed by rivers, springs, or local precipitation. The broad geographical distribution of lakes across Canada is primarily the result of extensive glaciation in the past.

Lakes can be classified on the basis of a variety of features, including their formation and their chemical or biological condition. One such classification identifies two types of lakes: oligotrophic and eutrophic. Oligotrophic lakes are characterized by relatively low productivity and are dominated by cold-water bottom fish such as lake trout. Eutrophic lakes, which are shallower, are more productive and are dominated by warm-water fish such as bass. Great Slave Lake (Northwest Territories) and most prairie lakes represent the two types, respectively.

Ponds are smaller bodies of still water located in natural hollows, such as limestone sinks, or that result from the building of dams, either by humans or beavers. Ponds are found in most regions and may exist either seasonally or persist from year to year.

Rivers and streams are bodies of fresh, flowing water. The water runs permanently or seasonally within a natural channel into another body of water such as a lake, sea, or ocean. Rivers and streams are generally more oxygenated than lakes or ponds, and they tend to contain organisms that are adapted to the swiftly moving waters (e.g., black fly larva and darter fish). Some of the larger rivers in Canada include the St. Lawrence, Mackenzie, Fraser, Athabasca, North and South Saskatchewan, and Saint John rivers.

Some rivers flow into oceans, the great saltwater bodies that cover 70% of the earth's surface. The tidal areas where saltwater and fresh water meet are called estuaries. These productive ecosystems, found on Canada's coasts, contain unique assemblages of organisms, including starfish and sea anemones.

Aquatic Ecosystem Health

Healthy aquatic ecosystems are those where human disturbances have not impaired the natural functioning (e.g., nutrient cycling) nor appreciably altered the structure (e.g., species composition) of the system. An unhealthy aquatic ecosystem is one where the natural state is out of balance.

These disturbances can be physical (e.g., injection of abnormally hot water into a stream), chemical (e.g., introduction of toxic wastes at concentrations harmful to the organisms), or biological (e.g., introduction and propagation of non-native animal or plant species). Symptoms of poor ecosystem health include the following:

  • the loss of species;
  • the accelerated proliferation of organisms. One example is algae blooms caused by an excess of phosphorous and nitrogen compounds in the water. This condition is called "eutrophication";
  • increased incidences of tumours or deformities in animals;
  • a change in chemical properties. Perhaps one of the most significant has been a reduction of pH in water caused by acid rain;
  • the presence of certain organisms that indicate unsanitary conditions. Coliform bacteria, for example, are a sign that the system may contain organisms that cause a variety of human diseases such as diarrhea, typhoid, and cholera; and
  • the loss of traditional Aboriginal culture associated with the ecosystem.

Many symptoms of poor ecosystem health occur simultaneously. For instance, increased lake acidity may kill certain species, thereby allowing the temporary proliferation of species more tolerant of acidity.

The importance of healthy aquatic ecosystems

Why is aquatic ecosystem health important to humans? Because everything is connected, where an ecosystem is out of balance eventually humans will begin to suffer as well. Our health and many of our activities are dependent on the health of aquatic ecosystems. Most of the water that we drink is taken from lakes or rivers. If the lake or river system is unhealthy, the water may be unsafe to drink or unsuitable for industry, agriculture, or recreation--even after treatment. Uses of aquatic ecosystems are impaired when these systems are unhealthy. Following are some examples.

  • Inland and coastal commercial fisheries have been shut down due to fish or shellfish contamination or the loss of an important species from the system.
  • The frequency of urban beach closures has escalated as a result of contamination by animal feces and medical waste.
  • Navigation problems for pleasure craft, caused by the rapid expansion of bottom-rooted aquatic plants, have increased.
  • The proliferation of non-native species has created problems. One recent example is the rapidly expanding zebra mussel population, introduced from the ballast waters of a European freighter into the Great Lakes. Zebra mussels have few natural predators, and because the female can produce 30 000 eggs yearly, they are expected to spread throughout most of the freshwater systems of North America. This mussel species is already clogging industrial and municipal water treatment intake pipes, coating boats and piers, and causing beach closures.

Restoring the health of an aquatic ecosystem

Can we restore the health of an aquatic ecosystem? Perhaps, but it takes time and is dependent on the nature of the disturbance. The effects of dredging, for example, may last from one to several years, but many of the displaced organisms such as fish can re-establish themselves. In other cases, more severe disturbances (e.g., dam construction) may cause local extinction of already endangered species. These ecosystems are unlikely to recover naturally.

In many cases, mechanisms exist that allow us to help restore ecosystem health or minimize detrimental impacts caused by human use. Following are some of these mechanisms.

  • Environmental legislation: Legislation such as the Canadian Environmental Protection Act (CEPA) is designed to ensure that Canadians and the aquatic environment are protected from exposure to toxic substances and from the risks associated with the use of chemicals.
  • Integrated resource planning: This approach ensures that relationships among land use, development, water flows, water quality, and aquatic ecosystems are considered prior to an area's land use designation.
  • Technology: Measures to improve the quality of waste discharges and to lower both water demands and effluent loading are being implemented in response to environmental and water use concerns.
  • Environmental monitoring: Monitoring of chemicals in water, sediment, and organisms helps to identify potential ecosystem problems and to track existing problems.
  • Compensatory measures: For example, a fish hatchery operation can produce young fish that a disturbed habitat can no longer supply.

Water and Ecosystem Initiatives

Ecosystems are comprised of four basic components: water, land (rock and soils), air and living things (plants and animals including humans). Everything in an ecosystem is related to everything else. Consequently, anything that occurs in one of these basic components will have an effect on the other three. Thinking in terms of ecosystems is key to achieving sustainability because an ecosystem approach places equal emphasis on concerns related to the environment, the economy and the community.

An ecosystem approach is one of five guiding principles underlying Environment Canada's Ecosystem Initiatives. For more information, visit the Ecosystem Initiatives section

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