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    dissolved gases of water dissolved gases of water Presentation Transcript

    • RABIYA NASIR
    • DISSOLVED GASES IN SEAWATER
    • H2O: Universal Polar Solvent
    • What is seawater? * Seawater is a solution of about 96.5% water and * 3.5% dissolved salts. • The most abundant salt in seawater is sodium chloride (NaCl). • Most elements on Earth are present in seawater. • Because these substances are dissolved, they are in the form of ions (positive or negative atoms).
    • gases Most gases in the air dissolve readily in seawater at the ocean’s surface  Major gases found in seawater (in order of relative abundance)   Nitrogen  Oxygen  Carbon dioxide
    • Important Gases         6 important gases are dissolved in lakes, streams, seas Nitrogen Oxygen Carbon dioxide Methane Hydrogen sulfide Ammonia All have important functions, but differ in behavior, origin
    • Air Provides Some Gases  Atmosphere has enough nitrogen (78%), oxygen (21%), and carbon dioxide (0.03%) to serve as primary source  Others present only in trace amounts in atmosphere
    • Other Gas Sources Methane - anaerobic breakdown of plants/animals  Hydrogen sulfide chemical/bacterial transformations  Ammonia - breakdown of nitrogenous materials by bacteria, some animals 
    • How much gas is dissolved in water at any given time?  Dependent on several factors:  Solubility factor  Pressure  Temperature  Salinity
    • Solubility Factor  Not all gases dissolve in water to same extent  Some gases dissolve very easily in water, some dissolve very little
    • Pressure (atmosphere)  Amount of gas absorbed by water is proportional to its partial pressure in the atmosphere (conc. = solubility factor X partial pressure)  Altitude decreases saturation level by ~1.4% per 100 m
    • Temperature Solubility of gas in water decreases as temperature rises  Generalization - cold water can hold more gas in solution than warm water  Nearly linear relationship within normal range of natural water temperatures 
    • Salinity  Presence of various minerals in solution lowers the solubility of gases  Generally disregarded in limnology because freshwaters have salinity near zero
    • Relative Saturation   Relation between existing solubility (amount of gas present) and the equilibrium content expected at same temperature and partial pressure Can be less, or more (supersaturation)
    • Oxygen Abundant and dissolves readily in water  Needed for respiration by organisms and for complete breakdown of organic matter  Relatively easy to measure 
    • Oxygen 1/4 as abundant as nitrogen in atmosphere, but twice as soluble  Solubility of oxygen increases as temp. decreases, salinity decreases, and pressure increases 
    • Oxygen  Two sources for oxygen in lakes  Atmosphere  Photosynthesis
    • Atmosphere     Diffusion across airwater interface and down into water column Years to reach depth of 5m Wind-driven waves and currents distribute oxygen to lower levels Too much agitation can prevent water from becoming supersaturated
    • Photosynthesis Most oxygen in standing waters is by-product of photosynthesis  Phytoplankton contribute most  Rooted macrophytes, attached algae, benthic algae mats are chief producers in shallow lakes, lake margins 
    • Loss of Oxygen  Physical - change in temperature, pressure  Biological - most important respiration by plants, animals, bacteria (decay processes)  Other - methane bubbles rising from sediments through water column
    • Oxygen Distribution  Distribution changes as lake goes through seasonal temperature cycle  Orthograde distribution during spring, fall turnovers in dimictic lake  Clinograde distribution during thermal stratification
    • Daily, seasonal variation in oxygen concentrations  The more plant material in a lake or pond, the more prone that system is to both daily and seasonal variations in dissolved oxygen content
    • Seasonal variation in oxygen concentrations O2 high during summer growing season, low in late-summer when plants die  May produce anoxia and die-offs of animals (summerkill) 
    • Seasonal variation in oxygen concentrations O2 also may be low during winter in icecovered lakes  Reduced light transmission, respiration only - Winterkill of animals 
    • Carbon Dioxide CO2 increasing in concentration in atmosphere  High solubility - 200 X > O2  Follows solubility laws (pressure, temp.)  Many sources other than atmosphere: rainwater, runoff, groundwater, respiration, decomposition in sediments 
    • Carbon Dioxide CO2 behaves much differently than other gases once it dissolves in water  Exists in equilibrium with many additional forms of carbon 
    • CO2 + H2O = H2CO3 Carbonic acid H2CO3 = HCO3- + H+ bicarbonate HCO3- = CO32- + H+ carbonate
    • Putting it all together  CO2 + H2O = H2CO3= HCO3- + H+ = CO32- + 2H+ Sensitive to changes in pH Low pH - left side dominates High pH - right side dominates
    • Putting it all together  Addition of CO2 via respiration pushes  equilibrium to right and lowers pH  Removal of CO2 via photosynthesis pulls  equilibrium to left and raises pH
    • Buffer System  CO2 + H2O = H2CO3 = HCO3- + H+ = CO32- + 2H+ CaCO3 + H2CO3 = Ca (HCO3)2 Ca (HCO3)2 = CaCO3 + H2O + CO2 Little change in pH despite additions of lots of acids or base, as long as supply of carbonates & bicarbonates holds out
    • Nitrogen  Exists in many different forms in natural freshwater systems  A major nutrient that affects the productivity of aquatic systems
    • Nitrogen Dissolved gas - N2  Ammonia - NH3 NH4+  Nitrite - NO2 Nitrate - NO3 Dissolved organics   Amino acids  Polypeptides  Proteins  Sources: atmosphere, rain, runoff, groundwater **  Losses: water outflow, adsorption to sediments, dinitrification by bacteria
    • Nitrogen Cycle
    • Ammonia Readily assimilated by plants  Nitrification by bacteria  Present in low concentrations in oxygenated waters 
    • Ammonia Accumulates in hypolimnion  No photosynthesis or nitrification  Release from sediments during anoxia 
    • Nitrate      Nitrates high in presence of oxygen Nitrification Nitrates not assimilated easily by plants Molybdenum needed to reduce nitrate Poor abundance in igneous basins
    • Nitrate Denitrification to N2 only by anaerobic bacteria in hypolimnion  Nitrate:ammonia   Calcareous runoff 25:1  Igneous runoff 1:1  Sewage or fertilizer 1:10
    • Phosphorus Total concentrations in unpolluted waters 0.01-0.05 mg/L  Sources:   Rainfall (unpolluted <0.03 mg/L) (polluted >0.1 mg/L)  Groundwater ~0.02 mg/L  Surface runoff - variable - often major contributor to lakes (especially with pollutants)
    • Phosphorus >90% of P in water is in form of organic phosphates or related materials in living things or their secretions  Great scarcity - limiting factor  Rapid turnover of organic P between living organisms   Bacteria, phytoplankton, zooplankton, others  5-100 minutes, more rapid under deficiency
    • Phosphorus  In presence of O2, various forms of phosphates form complexes, chelates, and insoluble salts with several metal ions  E.g., calcium and iron  Induce precipitation of P in oxygenated waters
    • Confusing, interrelated terms  Alkalinity  Hardness  Salinity
    • Alkalinity  Measure of buffering capacity of water  Carbonates and bicarbonates of alkali metals
    • Hardness Calcium and magnesium salt content  Temporary hardness - carbonates and bicarbonates, can be removed by boiling   Precipitation of CaCO3  Ca(HCO3)2 = CaCO3 + H2O + CO2  Permanent hardness sulfates, chlorides, other anions
    • SALINITY is the total quantity of dissolved inorganic solids in water. You can see here that the most abundant dissolved solids are chloride, sodium, and sulfate.
    • Salinity  Concentrations of Ca2+ Mg2+ Na+ K+ and HCO3- CO32- SO42- Cl-  Plus other ionized components of other elements