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       Marine ecosystems cover approximately 71 % of the Earth's surface and contain

approximately 97 % of the planet's water. They generate 32 % of the world's net

primary production. They are distinguished from freshwater ecosystems bу the

presence of dissolved compounds, especially salts, in the water. Marine ecosystems саn bе divided into the following zones: oceanic (the relatively shallow part of the осеаn that lies over the continental shelf); profundal (bottom or deep water); benthic (bottom substrates); intertidal (the area between high and low tides); estuaries; salt marshes; coral reefs and hydrothermal vents (where chemosynthetic sulphur bacteria form the food base). Classes of organisms found in marine ecosystems include brown algae, dinoflagellates, corals, cephalopods, echinoderms and sharks. Fish caught in marine ecosystems are the biggest source of commercial foods obtained from wild populations.

       The seas and oceans are rich in animal life, from whales and fish to starfish and sponges. The nature of marine communities depends upon depth, with the surface and the ocean floor being particularly rich in biodiversity. Similar communities are often found at similar depth, even though they may be widely separated geographically. There are also complex marine habitats, such as mangrove swamps, estuaries, and coral reefs, occurring near the shore.

       Marine ecosystems depend largely upon phytoplankton, which are photosynthetic algae living near the surface of the water where the sun penetrates. Tiny herbivores feed on the phytoplankton and these, in turn, are eaten by increasingly larger animals, ending with larger fish and sharks at the top of the marine food web. The seas and oceans are important for humans as a resource for fish and other marine products, but its ecosystems are threatened by exploitation, such as overfishing and pollution.

       Until the middle of the nineteenth century, it was assumed that few, if any, animals and plants lived in the seas and oceans because the waters were dark and cold. The first clues to the existence of rich and complex marine ecosystems came from broken underwater telegraph wires. When these were retrieved, various unusual and previously unknown creatures were found clinging onto them. The HMS Challenger, a British naval vessel, carried out the first-ever oceanographic survey between 1872 and 1876, exploring as deep as 18,700 ft (5,700 m) in the Pacific Ocean. The expedition returned with thousands of specimens, many of which were previously unknown to science.

       The marine environment is where life evolved in the first place. Seawater contains sodium chloride  (NaCl) and other mineral salts, and has remained at roughly the same salt concentration for millions of years. Salinity, as the salt concentration is called, is around one ounce per liter, of which 90% is composed of sodium chloride. This happens to be the same sodium chloride concentration as living cells, making it a natural environment for organisms to deal with.

       Water covers around 70% of Earth’s surface. Its average depth is just over 2 mi (3.2 km)—ranging from a few inches close to the shore to around 7 mi (11.2 km) at its greatest depth. The oceans alone provide more than 170 times more living space than land, air, and freshwater put together. The weight of water does exert pressure upon those organisms living there. This pressure will increase by one atmosphere for every 33 ft (10 m) of depth. But, like organisms living with atmospheric pressure on land, deep-sea animals such as fish and snails have the same pressure inside and outside their bodies. Most sea creatures are composed mainly of water and, since liquids are incompressible, they do not experience adverse effects on moving from one depth to another.

       The seas are constantly in motion due to surface currents and deeper circulation currents, which means that cold salty water, which is heavier, sinks and is replaced by warmer, less salty water. This mixes the water, making its chemistry uniform as well as carrying oxygen (O) from the surface to deeper layers, making life down there possible. The temperature of surface water varies from 104°F (40°C) in tropical waters to about 35°F (1.9°C) for seawater in the Artic  and Antarctic. The depths of the oceans are always very cold, even in tropical regions, at 32 to 37°F (0 to 3°C). Compared to the land, the marine environment is not so rich in different species. Only one tenth of the nearly 2 million animal species known are found in the sea, and only around 4,000 plants, compared to a quarter of a million on land. But marine ecosystems tend to be more diverse, with 28 different phyla existing in the oceans, compared to only 11 on land.

Exercise 20. Read and translate the following worda and word combinations from Russian into English.

Растворенный кислород, континентальный шельф, коралловые рифы, низина, солончак, донный субстрат, гидротермальные отверстия, атмосферное давление, морские животные, морские сообщества, мангровые болота, хемосинтезирующие бактерии серы, фотосинтезирующие водоросли, побережье, морская звезда, кит, губка, иглокожий, головоногий моллюск, динофлагелляты, бентический, акула, устье, профундаль, бентический.