Freshwater

Freshwater ecosystems cover 0.8% of the Earth’s surface and contain 0,009% of its total ______. They generate nearly 3% of its net primary production. Freshwater ecosystems contain 41% of the world’s known fish _______. There are three basic types of freshwater ecosystems:

1. Lentic – slow-moving water, including pools, ponds and lakes.

2. Lotic – rapidly moving water, such as streams and rivers.

3. Wetlands – areas where the soil is saturated or inundated for at least part of the time (nearshore shallow waters).

4. Riparian – areas of land bordering a body of water.

Two important subclasses of lakes are ponds, which typically are small lakes that integrade with wetlands and water ______. Many lakes and bays whithin them gradually become enriched by nutrients and fill in with ogranic _______, a process called eutrophication. Eutrophication is accelerated by human ________ within the water catchment area of the lake. The major zones in river ecosystems are determined by the river bed’s gradient or by the velocity of the current. Faster moving turbulent water typically contains greater concentrations of dissolved oxygen, which supports greater biodiversity than the slow moving water of pools. These distinctions form the basis for the dicision of rivers into upland and lowland _______. The food base of streams within forests is mostly derived from the trees, but wider streams get the majority of their food base from algae. Anadromous ____ are also an important source of nutrients. Environmental threats to rivers include loss of water, dams, chemical pollution and introduced species.

eutrophication - заболачивание

river bed – русло реки

current – течение

Functions

water, recreation, environment, alterations, species

Aquatic ecosystems perform many important environmental functions. For example, they recycle nutrients, purify water, attenuate floods, recharge ground ______ and provide habitats for wildlife. Aquatic ecosystems are also used for human ________ and are very important to the tourism industry, especially in tourist regions. The health of an aquatic ecosystem is degraded when the ecosystem’s ability to absord a stress has been exceeded. The stress on an aquatic ecosystem can be a result of physical, chemical or biological alterations of the _________. Physical alterations include changes in water temperature, water flow and light availability. Chemical alterations include changes in water temperature, water flow and light availability. Chemical _________ include changes in the loading rates of biostimulatory nutrients and oxygen consuming materials. Biological alterations include the introduction of exotic _______. Human populations can impose excessive stresses on aquatic ecosystems.

Abiotic characteristics

substance, salinity, absorb, compounds energy

An ecosystem is composed of biotic communities and biotic environmental factors which form a self-regulating and self-sustaining unit. Abiotic environmental factors of aquatic ecosystems include temperature, salinity and flow.

The amount of water of dissolved oxygen in a water body is frequently the key _________ in determining the extent and kinds of organic life in the water body. Fsh need dissolved oxygen to survive. Conversely, oxygen is fatal to many kinds of anaerobic bacteris. The ________ of the water body is also a determining factor in the kinds of species found in the water body. Organisms in marine ecosystems tolerate salinity, while many freshwater organisms are intolerant of salt. Freshwater used for irrigation purposes often _______ levels of salt that are harmful to freshwater organisms. Though, some salt can be good for organisms. Autotrophic organisms are producers that generate organic ________from inorganic material. Algae use solar _______ to generate biomass from carbon duioxide and are the most important autotrophic organisms in aquatic environments. These organisms are able to feed on hydrogen sulfide in water that comes from volcanic vents. Great concentrations of animals that feed on this bacteria are found around volcanic vents.

Make questions to the following sentences:

1. Northern Eurasia accommodates a variety of hydrological conditions and water bodies.

2. Regions with an abundant moisture supply border on areas with poor water resources such as, for example, the Caucasus mountains and semi-deserts of the Caspian plain or mountains and deserts of Central Asia.

3. Rivers, which are among the world's largest (the Yenisey, Ob, Lena, and Volga) drain the continent; the largest lakes, the Caspian and the Aral, and the deepest in the world (Lake Baikal), which accounts for 20 per cent of the world fresh water resources, are located here.

4. Nowhere in the world do wetlands cover such an enormous area as on the West Siberian plain and nowhere in the world does the development of water resources take place in such an enormous permafrost area as in Eastern Siberia.

5. One of the world's most dramatic examples of critical human-induced changes is the modern condition of the Aral Sea.

6. At the same time, due to the remoteness of many regions, especially in the north and in the east, many water bodies remain in their natural state.

7. Regular hydrological observations began in Northern Eurasia in the 18th century and by the beginning of the 20-th century a relatively dense hydrological network had been established.

8. Typical of Northern Eurasia is a water regime with powerful spring floods and low winter and summer water levels which develop because of the northern location of the region and continentality of its climate.