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Water – An ecosystem perspective
What is an aquatic ecosystem?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 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. What is the range of 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. What types of freshwater aquatic ecosystems do we have 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.
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. How does an ecosystem work?Energy from the sun is the driving force of an ecosystem. This light energy is captured by primary producers (mainly green plants and algae) and converted by a process called photosynthesis into chemical energy such as carbohydrates. The chemical energy is then used by the plants to perform a variety of functions including the production of plant parts such as leaves, stems, and flowers. The raw materials used for this purpose are nutrients (e.g., nitrogen, phosphorus, oxygen, and calcium): substances necessary for the growth of all plants and animals. Animals are incapable of photosynthesis. They therefore eat either plants, other animals, or dead tissue to obtain their energy and required nutrients. In ecosystems, the transfer of energy and nutrients from plants to animals occurs along pathways called food chains. The first link in a food chain consists of primary producers: green plants and other organisms capable of photosynthesis. Plant-eating organisms, known as primary consumers, are the next link in the food chain. They, in turn, are eaten by secondary consumers: carnivores (flesh eaters) or omnivores (plant and animal eaters). Decomposers such as bacteria and fungi make up the final link in the food chain. They break down dead tissues and cells, providing nutrients for a new generation of producers. Most organisms in an ecosystem have more than one food source (e.g., fish feed on both insects and plants) and therefore belong to more than one food chain. The consequent overlapping food chains make up food webs: complex phenomena with links that are constantly changing. What is the significance of a toxic substance to the food web?A toxic substance is one that can cause harm to the environment or human life. Most are synthetic and include PCBs, pesticides, dioxins, and furans. Some toxic substances can enter a food web and be transferred through it. The uptake of any environmental substance by an organism is called bioconcentration. Although nutrients taken up through this process are usually converted into proteins or excreted as waste, many toxic compounds accumulate in the fat or certain organs (e.g., liver) of animals. As contaminated organisms are eaten by others, the toxic substances are transferred up the levels in the food web and become more concentrated, sometimes to harmful levels. This process is called biomagnification. The species at the top level of the food web, including humans, are often subjected to higher concentrations of toxic substances than those at the bottom. Toxic substances reaching harmful levels is one sign that the aquatic ecosystem is unhealthy. What do we mean by an "unhealthy aquatic ecosystem"?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:
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. 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.
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.
What are wetlands?Wetlands are defined as lands saturated by surface or near surface waters for periods long enough to promote the development of hydrophytic vegetation (e.g., weeds, bulrushes, and sedges) and gleyed (poorly drained) or peaty soils. There are five basic classes of wetlands: bogs, fens, saltwater and freshwater marshes, swamps, and shallow water. Where are wetlands found in Canada?Wetlands cover about 14% of the land area of Canada. They were once abundantly distributed throughout the country. Recently, however, wetlands have become an increasingly scarce resource in settled areas of the country. Throughout Canada, wetlands have been adversely affected by land use practices that have resulted in vegetation destruction, nutrient and toxic loading, sedimentation, and altered flow regimes. For example, in southern Ontario, 68% of the original wetlands have been converted from their natural state to support alternative uses such as agriculture and housing. Similarly, only about 25% of the original wetlands of the "pothole" region of southwestern Manitoba remain in existence. In the North, however, most of the wetlands are intact. What kinds of animals use wetlands?Wetlands are important to species from many familiar classes of animals, as well as to less commonly known creatures. Every drop of water contains microscopic zooplankton, which are a vital component of the food chain. The water's surface and the wetland bottom are covered with insect eggs, larvae, and nymphs. Members of the fish, amphibian, and reptile groups are all dependent on the habitat provided by wetlands. Numerous bird and mammal species make extensive use of the water and its adjacent shores. These species can be important to humans economically or as indicators of environmental health. How do wildlife species use wetlands?Food and shelter are the primary requirements of life. Wetlands provide these functions for many species of animals that either live permanently within the wetland or visit periodically. Almost every part of a wetland, from the bottom up, is important to wildlife in some way. Frogs bury themselves in the muddy substrate to survive the winter, and some insects use bottom debris to form a protective covering. Fish swim and feed in wetlands, often eating the eggs of insects that have been deposited in the water. Wetland vegetation provides nesting materials and support structures to several bird species and is a major source of food to mammals, even those as large as moose. Small mammals use the lush vegetation at the edge of wetlands for cover and as a source of food, and they themselves are a food source for birds of prey. Each species has adapted to using the wetland and its surrounding area in a particular way. Why are wetlands important?Wetlands provide a critical habitat for a wide range of plants and animals and support one third of the wildlife species identified as endangered, threatened, or vulnerable by the Committee on the Status of Endangered Wildlife in Canada. Wetlands provide "outdoor laboratories" for education and scientific research and contribute to landscape variety and open space. Wetlands are among Canada's most valuable and productive ecosystems. They generate between 5 billion and 10 billion dollars annually in economic returns to Canadians. Wetlands support commercial fishing, sportfishing, waterfowl hunting, trapping, recreation, mining, peatland agriculture and forestry, water purification, groundwater discharge, and flood peak modification. Up to 80% of all North American waterfowl are born on prairie pothole wetlands. In Alberta, for example, 204 species of birds, 16 species of mammals, and 61 species of amphibians and reptiles depend on wetlands. How can we protect our remaining wetlands?Through conservation programs. Wetland conservation encompasses the protection, enhancement, and use of wetland resources according to principles that will assure their highest long-term social, economic, and ecological benefits. It is recognized that some wetlands should be protected and managed in their natural state; some actively managed to allow sustained, appropriate use of wetland renewable resources; and some developed for their non-renewable resource values. A significant program that aims at protecting our remaining wetlands is the North American Waterfowl Management Plan (NAWMP). In 1986, the governments of Canada and the United States signed the plan in reaction to the sharp decline in waterfowl populations associated with the destruction of their habitat. They were joined by Mexico in 1993. The plan itself outlines the scope of the work to be done on a continental basis and provides broad guidelines for habitat protection and management actions. Many partners – from federal and provincial or state governments to nongovernmental organizations and landowners – representing various interests, work in partnership to achieve the NAWMP's goal to restore, protect, and enhance wetland habitat for the benefit of waterfowl, biodiversity, and humans. To receive additional information on the NAWMP, contact:
North American Waterfowl Management Plan |
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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.
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.
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