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Fresh Water! Ch. 15 Of all the water on Earth, only 2.5% is considered fresh water, or water with few dissolved salts most of which is tied up in glaciers, ice caps, and aquifers. This water is renewed and recycled as it moves through the hydrologic cycle. Guiding Questions • What is water’s importance to people and to ecosystems? • What are the major types of freshwater ecosystems? • How do humans use and alter freshwater systems? Describing Aquatic Ecosystems Salinity: the amount of dissolved salt present in water. Ecosystems are classified as salt water, fresh water, or brackish depending on salinity. Photosynthesis tends to be limited by light availability, which is a function of depth and water clarity. Aquatic ecosystems are either flowing or standing. Aquatic ecosystem zones: photic, aphotic, benthic Freshwater Ecosystems: Ponds, Lakes, Inland Seas Salinity is less than 0.5 ppt (parts per thousand) Ponds and lakes are similar, except in size, but inland seas contain organisms adapted for open water. Ponds and lakes are divided horizontally into zones: littoral and limnetic Freshwater Ecosystems: Wetlands Areas of land flooded with water at least part of the year Include freshwater marshes, swamps, bogs, and fens Wetlands prevent flooding, recharge aquifers, filter pollutants, and provide habitats. Wetlands are valuable • Wetlands are extremely valuable for wildlife - They slow runoff, reduce flooding, recharge aquifers, and filter pollutants • People have drained wetlands, mostly for agriculture - Southern Canada and the U.S. have lost over half of their wetlands • In 2006 the Supreme Court told the Army Corps of Engineers it must create guidelines to determine when wetlands are valuable enough to protect by law © 2011 Pearson Education, Inc. Freshwater Ecosystems: Rivers and Streams Bodies of surface water that flow downhill, eventually reaching an ocean or inland sea Watershed: The area of land drained by a river and its tributaries Characteristics, such as dissolved oxygen, temperature, water speed, organisms, and others, change from source to mouth. Groundwater plays a key role • Groundwater = water beneath the surface held in pores in soil or rock - 20% of the Earth’s freshwater supply • Aquifers = porous, sponge-like formations of rock, sand, or gravel that hold water - Zone of aeration = pore spaces are partly filled with water - Zone of saturation = spaces are filled with water - Water table = boundary between the two zones © 2011 Pearson Education, Inc. The Ogallala Aquifer • The world’s largest known aquifer • Underlies the Great Plains of the U.S. Its water has allowed farmers to create the most bountiful grain-producing region in the world © 2011 Pearson Education, Inc. Water is unequally distributed across Earth • Water is unevenly distributed in space and time - Different areas possess different amounts of water - People erect dams to store water Many densely populated areas are water-poor and face serious water shortages © 2011 Pearson Education, Inc. How we use water • We have achieved impressive engineering accomplishments to harness fresh water - 60% of the world’s largest 227 rivers have been strongly or moderately affected - Dams, dikes, and diversions • Consumption of water in most of the world is unsustainable - We are depleting many sources of surface water and groundwater © 2011 Pearson Education, Inc. Water supplies houses, agriculture, and industry • Proportions of these three types of use vary dramatically among nations - Arid countries use water for agriculture - Developed countries use water for industry • Consumptive use = water is removed from an aquifer or surface water body and is not returned - Irrigation = the provision of water to crops • Nonconsumptive use = does not remove, or only temporarily removes, water - Electricity generation at hydroelectric dams © 2011 Pearson Education, Inc. Water-poor regions take water from others • Politically strong, water-poor areas forcibly take water from weaker communities • Los Angeles commandeered water from rural areas - Turning the environment into desert, creating dustbowls, and destroying the economy • In 1941, L.A. diverted streams that fed Lake Mono - Lake levels fell, salt concentrations doubled • Las Vegas wants to import water from sparsely populated eastern Nevada - An ecologically sensitive area © 2011 Pearson Education, Inc. We have erected thousands of dams • Dam = any obstruction placed in a river or stream to block the flow of water to prevent floods, provide drinking water, allow irrigation, and generate electricity - 45,000 large dams have been erected in more than 140 nations • Only a few major rivers remain undammed - In remote regions of Canada, Alaska, and Russia © 2011 Pearson Education, Inc. A typical dam © 2011 Pearson Education, Inc. We are depleting surface water • In many places, we are withdrawing water at unsustainable rates - Reduced flow drastically changes the river’s ecology, plant community, and destroys fish and invertebrates The Colorado River often does not reach the Gulf of California © 2011 Pearson Education, Inc. Can we quench our thirst for bottled water? • Groundwater is being withdrawn for bottled water - An average American drinks 29 gallons/year • People drink bottled water for portability, convenience - They think it tastes better or is healthier • Bottled water is no better than tap water - It is heavily packaged and travels long distances using fossil fuels - Bottles are not recycled - Corporations move in, deplete water, and move away © 2011 Pearson Education, Inc. Will we see a future of water wars? • Freshwater depletion leads to shortages, which can lead to conflict - 261 major rivers cross national borders • Water is a key element in hostilities among Israel, Palestinians, and neighboring countries • Many nations have cooperated with neighbors to resolve disputes - They sign water-sharing treaties © 2011 Pearson Education, Inc.