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Advanced Biology: Bahe & Deken Biogeochemical Cycles Chapter 5 Text Page - 45 - Advanced Biology: Bahe & Deken 5.1 Nutrients in an Ecosystem vapor in the atmosphere, 84%, comes from the oceans. An ecosystem needs more than energy to function. It also needs matter that is used by organisms in the ecosystems for life processes such as growth. Most ecosystems need greater than 20 elements for life processes. They are so important to living things that they are called nutrients. We already know that energy is lost along a food chain. Little or no energy is left at the end of the food chain to be recycled back to producers. However, this is not the case for nutrients. Nutrients flowing through the food chain are returned to the producers. Producers get their nutrients from the soil, water and air. Herbivores get most of their nutrients when they eat producers. Carnivores also get the same nutrients when they eat the herbivores. Then decomposers break down animal wastes and dead organisms. This releases the nutrients back into the soil, water and air so producers can use them again. In this way, nutrients are recycled through an ecosystem. The path each nutrient follows is called a nutrient cycle, or biogeochemical cycle, and each cycle is subject to disruption by human activity. The important cycles, all driven directly or indirectly by incoming solar energy and gravity, include water, carbon, nitrogen, and phosphorus cycles. The amount of water vapor air can hold depends on its temperature, with warm air holding more water vapor than cold. Relative humidity is the amount of water vapor in a certain mass of air, expressed as a percentage of the maximum amount it could hold at that temperature. For example, a relative humidity of 60% at 27°C (80°F) means that each kilogram of air contains 60% of the maximum amount of water vapor it could hold at that temperature. In the cool upper atmosphere this vapor condenses, forming clouds or fog. In time, enough water collects in the clouds to cause precipitation. In order for precipitation to occur, the air must contain condensation nuclei, tiny particles on which droplets of water vapor can collect such as volcanic ash, soil dust, smoke, sea salts and particulate matter from factories, coal-burning power plants and automobiles. When rain, sleet, and snow occur, some of the water falling on the ground runs along the surface of the ground to a stream, pond or ocean where it can resume the cycle. This water is called surface runoff. Some of the water also soaks into the ground by percolation 5.2 The Water Cycle The global water cycle is driven by heat energy from the sun. Water is moved continuously between land, oceans and the atmosphere by several processes. Water enters the atmosphere through transpiration from vegetation. Transpiration is the loss of water through pores in the leaves of plants. Water also enters the atmosphere by evaporation from oceans, streams, and lakes as well as from soil. The majority of water Text Page - 46 - Advanced Biology: Bahe & Deken through the soil and permeable rock formations to groundwater storage areas called aquifers. Groundwater can also run along bedrock back to lakes and rivers that eventually carry it back to the oceans to be evaporated and cycle again. Some of the water in the soil moves up to the roots of plants by capillarity. The roots absorb the water and distribute the water to the rest of the plant. This is how most plants get the water they need. Animals obtain water by eating plants or animals and by drinking it directly from a body of water. When plants and animals die, they decompose and the water in their tissues is released into the environment. Besides replenishing streams and lakes, surface runoff can also cause soil erosion. Because the water dissolves many nutrient compounds, it is a major medium for transporting nutrients within and between ecosystems. During the water cycle, many natural processes act to purify water. When water evaporates it leaves behind impurities. Water flowing above ground through streams and lakes and below ground in aquifers is naturally filtered and purified by chemical and biological processes. The water cycle is a cycle of natural renewal of water quality. Humans affect the water cycle in a number of ways. One of the main sources of atmospheric water is transpiration from the dense vegetation making up tropical rain forests. The destruction of these forests for agriculture, timber and mining, which is occurring rapidly today, will change the amount of water vapor in the air. This in turn will likely alter local, and possibly, global weather patterns. In addition such deforestation increases runoff, reduces infiltration that recharges groundwater supplies, increases flooding, and accelerates soil erosion. Another change in the water cycle results from pumping large amounts of groundwater to the surface to use for irrigation. This practice can increase the rate of evaporation over land, and unless this loss is balanced by increased rainfall over land, groundwater supplies can be depleted. The Midwestern United States, the Southwestern American desert, parts of California and areas bordering the Gulf of Mexico currently face this problem. Withdrawing large quantities of fresh water from underground sources can also lead to intrusion of ocean salt water into the underground water supplies. Humans are also modifying water quality through the addition of nutrients, especially phosphates and nitrates from fertilizers, and other pollutants. 5.3 The Carbon Cycle Carbon is the basic building block of all living things. Carbohydrates, fats, proteins and nucleic acids all require carbon. Carbon dioxide (CO2) in the atmosphere (only 0.03% carbon dioxide by volume) forms the basis of the carbon cycle. Carbon dioxide is taken in by terrestrial and aquatic plants, algae, and cyanobacteria to make food through photosynthesis. These producers are the source of all carbon for all biotic components of the ecosystem. Herbivores eat some of the plants and carnivores eat some of the herbivores. Now the carbon is in animals. Both plants and animals respire. Their respiration returns carbon dioxide to the atmosphere and water. Detritus feeders and decomposers breakdown dead plants, leaf litter, dead Text Page - 47 - Advanced Biology: Bahe & Deken Name: animals, and animal wastes. This also returns carbon dioxide to the atmosphere where it can be taken in again by producers for photosynthesis. The resulting global warming could disrupt global food production and wildlife habitats and raise sea levels in various parts of the world. Some organic matter does not decompose readily. Instead, it builds up in the earth’s crust. Over millions of years, these buried deposits of dead plants and bacteria are compressed between layers of sediment, where they form carbon-containing fossil fuels such as coal and oil. This stored carbon is not released back into the carbon cycle until the fossil fuels are burned. Over the past few hundred years, people have extracted and burned fossil fuels that took millions of years to form. Carbon dioxide is a key component of nature’s thermostat. If the carbon cycle removes too much CO2 from the atmosphere, the atmosphere will cool. If it generates too much, the atmosphere will get warmer. Slight changes in the carbon cycle can affect climate and ultimately the types of life that can exist on various parts of the planet. Questions (answer on a separate sheet of paper) Since 1800 and especially since 1950 humans have been intervening in the earth’s carbon cycle in two ways that add carbon dioxide to the atmosphere: Clearing trees and other plants that absorb CO2 through photosynthesis Adding large amounts of CO2 by burning fossil fuels and wood Computer models of the earth’s climate systems suggest that increased concentrations of atmospheric CO2 and other gases we are adding to the atmosphere could enhance the planet’s natural greenhouse effect that helps warm the lower atmosphere and the earth’s surface. Trace a carbon atom through the carbon cycle. 1. In what chemical form is carbon in the air? 2. How does a carbon atom enter the food chain? 3. In what chemical form might the carbon atom be obtained by a consumer? 4. What chemical process puts carbon atoms back into the atmosphere? Text Page - 48 - Advanced Biology: Bahe & Deken 5.4 The Nitrogen Cycle Plants and animals need nitrogen to make amino acids, proteins and DNA. The atmosphere contains a huge reservoir of nitrogen. Almost 80% of the atmosphere is N2. However, because of the strong triple bonds holding N2 together neither plants nor animals can use atmospheric nitrogen directly. Plants must absorb nitrogen in the form of nitrates (NO3-) or ammonium ions (NH4+) with their roots. Lightning forms some nitrate ions by causing oxygen and nitrogen gas in the atmosphere to join. These nitrate ions reach the soil in precipitation and dust. Rhizobium bacteria in the soil can also convert molecular nitrogen into nitrates. These bacteria live on the roots of legumes, such as soybeans, alfalfa, peas, clover, and beans. In addition, many free-living soil bacteria and aquatic cyanobacteria can form nitrates. The changing of molecular nitrogen (N2) into nitrates (NO3-) is called nitrogen fixation. Plants use the nitrates that they absorb to make plant DNA and proteins. In turn, animals get the nitrogen they need to make proteins by eating plants or other animals. When plants and animals die and decompose, bacteria change their nitrogen containing organic molecules into ammonia (NH3), which quickly becomes ammonium ions (NH4+). The nitrogen in urine and fecal matter of animals is also changed to ammonia by bacteria, keeping the nitrogen available for use by plants. The pungent odor of outhouses, chicken pens, hog yards, cat litter boxes and wet baby diapers is ample evidence of this fact. Ammonia, in turn, is converted to nitrites and then to nitrates by bacteria. This completes the main part of the cycle. Denitrifying bacteria (mostly anaerobic bacteria in waterlogged soil or in the bottom sediments of lakes, oceans, swamps, and bogs) convert some nitrites and nitrates to atmospheric nitrogen (N2) to complete the total cycle. The nitrogen cycle need not and often does not Text Page - 49 - Advanced Biology: Bahe & Deken involve this last denitrification step. Humans, however, have altered the nitrogen cycle balance in many ecosystems. Sewagetreatment facilities usually empty large amounts of dissolved inorganic nitrogen compounds into rivers and streams. Farmers routinely apply large amounts of inorganic nitrogen fertilizers, mainly ammonium compounds and nitrates to their fields. Lawns and golf courses receive sizable doses of fertilizers, and denitrifying bacteria convert some into atmospheric nitrogen. But chemical fertilizers usually exceed the soil’s natural recycling capacity. The excess nitrogen compounds often enter streams, lakes, soil, and groundwater. In lakes and streams, these nitrogen compounds continue to fertilize, causing heavy growth of algae (nitrogen is normally a limiting nutrient). The subsequent breakdown of dead algae and other aquatic plants can deplete the water of dissolved oxygen and disrupt aquatic ecosystems by killing some types of fish and other oxygenusing organisms. Ground water pollution by nitrogen fertilizers is a serious problem in agricultural areas. Nitrates in drinking water are converted to nitrites, which can be toxic, in the human digestive tract. Humans also add large amounts of nitric oxide (NO) into the atmosphere when we burn fuels (N2 + O2 --> 2NO). In the atmosphere, this nitric oxide combines with oxygen to form nitrogen dioxide gas (NO2), which can react with water vapor to form nitric acid (HNO3). Droplets of HNO3, dissolved in rain or snow are components of acid deposition, commonly called acid rain. Nitric acid, along with other air pollution, can damage and weaken trees, corrode metals and damage marble, stone and other building materials. Some bacteria in the soil will also convert fertilizer and livestock waste to nitrous oxide (N2O) that enters the atmosphere. In the atmosphere, N2O reaches the stratosphere where it can help warm the atmosphere by enhancing the natural greenhouse effect and contributing to the depletion of the earth’s ozone shield, which filters out harmful ultraviolet radiation from the sun. 5.5 The Phosphorus Cycle Plants and animals need phosphorus for the production of nucleic acids (DNA, RNA, and ATP) and phospholipids. Many animals also need phosphorus for teeth, bones, and/or shells. The atmosphere does not contain phosphorus and thus all the phosphorus available for plants and animals comes from rocks and soil. Because phosphorus does not enter the atmosphere it is called a sedimentary cycle. Rocks and sediment at the ocean floor contain the largest reservoir of phosphorus. Text Page - 50 - Advanced Biology: Bahe & Deken Terrestrial rocks upon geological uplift and weathering exposes phosphate ions (PO43and HPO42-) that are absorbed into the soil. Plants take up phosphorus from the soil and then consumers incorporate phosphorus by eating plants and other animals. As the plants and animals die and decompose phosphorus is again put back into the soil. The weathering of rock and soil runoff also puts phosphorus into aquatic ecosystems. As the aquatic biota die and decompose phosphorus gets trapped in ocean floor sediments to return phosphorus back to rock. Like nitrogen humans have altered the phosphorus cycle. Phosphate mining for fertilizers and detergent production put excess phosphorus into the system. Phosphorus, like nitrogen, is a limiting nutrient for plant and algae growth. Increased phosphorus and nitrogen into aquatic ecosystems can results in eutrophication (over-enrichment of water). Eutrophication can lead to algal blooms, apparent by a green scum on top of the water. When the algae die off, decomposers use up all the available oxygen during cellular respiration. Because there is no oxygen available fish and other aquatic life begin to all die off. An abundance of phosphate in the water can also be linked to red tide that can produce deadly toxins. 5.6 Disruption of Biogeochemical Cycles Obtaining an accurate picture of chemical cycling requires long-term studies. One experiment that continues today was begun in 1963 by F. H. Borman from Yale and Gene Likens from Cornell at the HubbardBrook Experimental Forest in the White Mountains of New Hampshire. The site has a number of watersheds; small valleys that are each drained by a stream that is a tributary of Hubbard Brook. Thick rock is close to the soil surface and water does not seep into the rock from the soil. As a result, water drains out of each watershed only via its own stream. The research team began by carrying out a controlled experiment to compare the loss of water and nutrients from an uncut forest ecosystem (the control system) with one that was stripped of its trees (the experimental system). To do this, V-shaped concrete catchment dams were built across the creeks at the bottom of six valleys. The dams were anchored on impenetrable bedrock so all surface water that leaves each watershed had to flow across the dams, where scientists could measure its volume and dissolved nutrient content. When monitoring began, in the control system about 60% of the water that fell as rain and snow exited through the streams, and the remaining 40% was lost by transpiration from plants and evaporation from the soil. Preliminary data also indicated that the flow of nutrients into and out of the watershed was nearly balanced, and was relatively small compared with the quantity of nutrients being recycled within the forest itself. Thus, an undisturbed mature forest ecosystem is very efficient at retaining chemical nutrients. In the next experiment the scientists disturbed the system and observed any changes that occurred. One winter, investigators cut down all trees and shrubs in one valley, left them where they fell and sprayed them with herbicide for the next three years to prevent regrowth. They then compared the inflow and outflow of water and nutrients in this modified experimental Text Page - 51 - Advanced Biology: Bahe & Deken valley with those in the control valley for three years. With no plants to absorb and transpire water from the soil, water runoff in the deforested valley increased 30-40%. As this excess water ran rapidly over the surface of the ground, it eroded soil and carried nutrients out of the ecosystem. Overall, the loss of minerals from the cut forest was six to eight times that in a nearby-undisturbed forest. Chemical analysis of the water flowing through the dams in the experimental valley showed a 60-fold rise in the concentration of nitrate (NO3-) ions. So much nitrogen was lost in the experimental valley that the water flowing out of the valley was unsafe to drink and the over-fertilized stream below this valley became covered with populations of cyanobacteria and algae. After a few years, however, vegetation slowly grew back and nitrate levels began to return to normal. Addition of large amounts of chemical nutrients to an aquatic ecosystem can lead to pollution where too much of a good thing is bad. One can trace the chain of events that occur when a freshwater lake, for instance, receives an overload of nutrients from the surrounding terrestrial ecosystems. A natural lake typically has a moderate growth of algae and plants and often a rich diversity of fish and invertebrates. When a lake receives an excess of mineral nutrients, its entire trophic structure can change very quickly. A lake today may receive runoff of inorganic matter from fertilizers from agriculture, from sewage, from factory and animal wastes, and from pastures and stockyards. The water becomes polluted with these materials - actually, overfertilized - and the lake’s photosynthetic organisms, especially algae, multiply rapidly. The lake will undergo accelerated eutrophication. More nutrients stimulate the growth of algae, cyanobacteria, water hyacinths and duckweed. Heavy plant growth increases oxygen production during the day but greatly reduce oxygen levels at night when they respire. As the plants and animals die and accumulate at the bottom of the lake, the bacteria decomposing them can use up much of the oxygen dissolved in deep waters and near the edges. This oxygen depletion can kill fish and other aerobic aquatic animals. If excess nutrients continue to flow into a lake, anaerobic bacteria take over and produce gaseous decomposition products like smelly, highly toxic hydrogen sulfide and flammable methane. When this happens, the lake may lose most of its species diversity, with only the organisms tolerant of low oxygen conditions surviving. Human-induced eutrophication, for example, wiped out commercially important fish in Lake Erie in the 1950s and 1960s. Since then, tighter regulations on the dumping of wastes into the lake have enabled some fish populations to rebound, but many of the native species of fish and invertebrates have not recovered. Today, accelerated eutrophication is the most common problem affecting lakes throughout the world. Ways to prevent or reduce this culturally induced eutrophication include using advanced waste treatment to remove nitrates and phosphates, banning and limiting phosphates in household detergents and other cleaning agents and using soil conservation land-use control to reduce nutrient runoff. Text Page - 52 - Advanced Biology: Bahe & Deken Name: Questions Each of the statements below describes a situation in a healthy ecosystem. Next to each sentence briefly describe the corresponding situation in an unhealthy system affected by human disturbance. 1. The amount of CO2 released into the atmosphere by respiration equals the CO2 used in photosynthesis. 2. Over time, nutrients from the surrounding land gradually accumulate in a lake, and the lake becomes more productive, a process called eutrophication. 3. Much of the water that falls on tropical forests is returned to the atmosphere by transpiration. 4. Organic fertilizers release nitrogen and phosphorus gradually, so they are absorbed by crop plants and do not run off to pollute rivers and lakes. Multiple Choice 5. Most plants get nitrogen from a. nitrates in the soil. b. N2 gas in the air. c. proteins. d. ammonium in the soil. e. rainfall. 6. Bacteria are especially important in a. the water cycle. b. the nitrogen cycle. c. ecological succession. d. the phosphorus cycle. e. the calcium cycle. 7. The biggest difference between the flow of energy and the flow of chemical nutrients in an ecosystem is that a. the amount of energy is much greater than the amount of nutrients. b. energy is recycled, but nutrients are not. c. organisms always need nutrients, but they don’t always need energy. d. nutrients are recycled, but energy is not. e. organisms always need energy, but they don’t always need nutrients. 8. An ecosystem is unlikely to be limited by the supply of ______ because it is obtained from the air. a. water b. carbon c. phosphorus d. nitrogen e. calcium Text Page - 53 - Advanced Biology: Bahe & Deken Text Page - 54 -