Chemistry in Lakes - Water Chemistry in Lakes Lecture 6...

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Lecture 6 EVPP/BIOL 350 Freshwater Ecosystems Dr. Kim de Mutsert Fall 2011 Water Chemistry in Lakes
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Chemical Cycles in Lakes Carbon Oxygen Nitrogen Phosphorus Others
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Carbon
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The Carbon Cylce Global carbon cycle includes: Photosynthesis Respiration Fossil Fuel combustion Ocean interactions Rock interactions (over long term)
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Forms of Carbon • Carbon Dioxide (CO 2 ) - 0.035% of the atmosphere - Readily dissolves in water Dissolved inorganic carbon (DIC) - Includes CO 2 , H 2 CO 3 , HCO 3 - , and some CO 3 -2 (last precipitates readily) - Form of DIC depends on pH Dissolved organic carbon (DOC) - Methane (CH 4 ) and organic acids Particulate organic carbon (POC) - Detritus and any living or dead organism
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Forms of Carbon
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CO 2 Earth’s atmosphere contains relatively small amounts of CO 2 as compared to O 2 But the amount has increased greatly over the past several decades As a greenhouse gas, CO 2 is a major factor in the warming of Earth surface temperatures CO 2 is also intimately involved in the carbonate- bicarbonate buffering system that controls pH in most freshwaters Ice core data Direct Measurements
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Forms of DIC Carbon dioxide dissolves in water to produce carbonic acid Carbonic acid dissociates to produce bicarbonate and hydrogen ion (1 st dissociation of carbonic acid) Bicarbonate dissociates to produce carbonate and another hydrogen ion (2 nd dissociation of carbonic acid) CO 2 + H 2 0 ↔ H 2 CO 3 H 2 CO 3 ↔ HCO 3 - + H + HCO 3 - ↔ CO 3 -2 + H +
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Revisit pH pH = -log [H+] pH is the negative log of the hydrogen ion concentration pH = 4 means [H+] = 10 -4 pH = 7 means [H+] = 10 -7 pH = 10 means [H+] = 10 -10
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Forms of DIC and pH The relative amounts of carbonate, bicarbonate, and carbon dioxide-carbonic acid change with pH in a predictable manner based on dissociation equations At high pH, carbonate dominates At intermediate pH, bicarbonate dominates At low pH, carbon dioxide-carbonic acid dominates
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Forms of DIC and pH CO 2 + H 2 O H 2 CO 3 H + + HCO 3 -1 2H + + CO 3 -2 Low pH High pH
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DIC, alkalinity, and pH Alkalinity is the ability of water to resist acidification If the carbonate-bicarbonate system is the major buffer, then pH change can be resisted as long as bicarbonate and carbonate are present since they can absorb hydrogen ions • Alkalinity is based, not only on the input of CO 2 from the atmosphere, but even more so on sources of carbonate and bicarbonate from the watershed
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Effect of photosynthesis Effect of photosynthesis on pH and carbonate system Effect of respiration on pH and carbonate system - Psyn consumes CO 2 , equilibrium shifts to right resulting in consumption of H + and increase in pH - Resp releases CO 2 , equilibrium shift to left resulting in release of H + and decrease in pH CO 2 + H 2 0 ↔ H 2 CO 3 ↔ HCO 3 - + H + ↔ CO 3 -2 + H +
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Temporal Changes in pH at Belmont Continuous Monitor
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This note was uploaded on 01/23/2012 for the course BIOL/EVPP 350 taught by Professor Kimdemutsert during the Fall '11 term at George Mason.

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Chemistry in Lakes - Water Chemistry in Lakes Lecture 6...

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