Chapter 3 Life in Water 1 Chapter 3 Life in Water 3 1 The Hydrological Cycle The hydrological cycle exchanges water among reservoirs Over 71 of the earth s surface is covered by water The oceans contain over 97 of the water in the biosphere and the polar ice caps and glaciers contain an addition 2 Less than 1 is fresh water in rivers lakes and actively exchanged groundwater Reservoirs are places where water is stored for some period of time such as lakes rivers and oceans plus the atmosphere and ice Hydrologic cycle is the dynamic movement of water among these reservoirs in a global exchange With a renewal time of about 3 100 years the total volume of the oceans over 1 3 billion km 3 of water has turned over nearly 50 times in the last 150 000 years or so 3 2 The Natural History of Aquatic Environments The biology of aquatic environments corresponds broadly to variations in physical factors such as light temperature and water movements and to chemical factors such as salinity and oxygen The Oceans Geography Structure The largest ocean basin is the Pacific nearly 180 million km2 The Pacific ocean has the deepest depth of over 4 000 m The second largest ocean basin is the Atlantic 106 million km2 The third largest ocean basin is the Indian under 75 million km2 The oceans can be divided into several horizontal zones The shallow shoreline under the influence of rise and fall of the tides is called the littoral or intertidal zone The neritic zone extends from the coast to the margin of the continental shelf about 200 m deep Beyond the continental shelf lies the oceanic zone The ocean is also generally divided vertically into several depth zones Habitats on the bottom of the ocean are referred to as benthic Habitats off the bottom regardless of depth are called pelagic The epipelagic zone is the surface layer of the oceans that extends about 200 m The mesopelagic zone extends from 200 1 000 m The bathypelagic zone extends from 1 000 4 000 m The abyssal zone extends from 4 000 6 000 m The deepest parts of the oceans belong to the hadal zone Physical Conditions Light Temperature Approximately 80 of solar energy is absorbed in the first 10 m Oceans appear blue because the visible ranges red orange yellow and green light are absorbed more rapidly than blue light Kinetic energy increases as temperature increases The solar energy absorbed by water increases the kinetic energy or velocity of motion of water molecules The layers of warm and cold water are separated by a thermocline a layer of water through which temperature changes rapidly with depth This layering of the water column by temperature thermal stratification is a permanent feature of tropical oceans At the ocean surface average annual temperature and variation in temperature change with latitude The lowest average temperature is around the Antarctic about 1 5 C The highest average surface temperatures occur near the equator a bit over 27 C The greatest stability in oceanic temperatures is below the surface At just 100 m depth annual variation in temperature is often less than 1 degree Celsius Water Movements Chapter 3 Life in Water 2 Oceanic currents moderate climates fertilize the surface waters off the continents stimulate photosynthesis and promote gene flow among populations of marine organisms Wind driven surface currents sweep across vast expanses of open ocean to create great circulation systems called gyres that under the influence of the Coriolis effect move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere Deep water may also be moved to the surface in a process called upwelling Upwelling occurs along the west coasts of continents and around Antarctica where winds blow surface water offshore allowing colder water to rise to the surface Chemical Conditions Salinity Oxygen Biology The amount of salt dissolved in water called salinity varies with latitude and among the seas that fringe the oceans In the open ocean it varies from about 34 g of salt per kilogram of water The lowest salinity occurs near the equator and above 40 N and S latitudes where precipitation exceeds evaporation The highest salinity occurs in the subtropics at about 20 30 N and S latitude where precipitation is low and evaporation is high A liter of air contains about 20 mL of oxygen at seal level while a liter of seawater contains a maximum of about 9 mL of oxygen Oxygen concentration is highest near the ocean surface and decreases with depth Because of the limited penetration of sunlight into seawater photosynthetic organisms are limited to the brightly lighted upper epipelagic zone of the ocean The most significant photosynthetic inhabitants of this zone also called the photic zone are microscopic organisms called phytoplankton There are entire biological communities on the sea floor that are nourished not by photosynthesis at the surface but by chemosynthesis on the ocean floor The average rate of photosynthesis per square meter of ocean contributes approximately one half of the total photosynthesis in the biosphere The first Census of Marine Life which took place between 2000 and 2010 resulted in an estimate of at least 1 million marine species Human Influences Geography Structure Physical Conditions Light Temperature In temperate to sub polar regions wherever there is a solid bottom and no overgrazing there are profuse growths of kelp As you get closer to the equator these kelp forests are gradually replaced by coral reefs Coral reefs are confined to low latitude between 30 N and S Fringing reefs hug the shore of a continent or an island Barrier reefs stand between the open sea and a lagoon Coral atolls consist of coral islets that have built up from a submerged oceanic island ring a lagoon Reef crests are where corals grow in the surge zone created by waves coming from the open sea extending to a depth of about 15 m Below the reef crest is a buttress zone where coral formations alternate with sand bottomed canyons Behind the reef crest lays the lagoon which contains numerous small coral reefs called path reefs and sea grass beds Both seaweeds and reef building corals grow only in surface waters where there is sufficient light to support photosynthesis The depth of light penetration to support kelp and coral varies from a few meters to nearly 100 meters Life in Shallow Marine Waters Kelp Forests and Coral Gardens Overfishing pollution and hunting are all human influences that decreased the marine population Most