Lecture  - Water Chemistry -- Dissolved Gases -- Oxygen

I. Properties of dissolved gases
    A. Solubility -- How much gas will dissolve in water?
        1. Pressure -- as pressure increases, solubility increases
        2. Temperature -- as temperature increases, solubility decreases

solubility of oxygen with temperature

        3. Salts -- as TDS increase, solubility decreases exponentially
        4. Concentration -- Henry's law

                At constant temp. the amount of gas absorbed by a given volume of liquid is proportional to the pressure
                        in atmospheres that the gas exerts

              [gas] = KH rgas
                        KH  (Henry's constant) is a solubility factor, varying from gas to gas
                       O2 in atmosphere ~20.3% = 0.203 atm
                       KH @ 20oC = 1.39 mmolO2/kgH2O*atm (for pure water)
                                           amount of O2 that will dissolve in water at 20oC
                      example of solubility of oxygen in water

    B. Oxygen
            1. Used in respiration
            2. Important in chemical reactions
            3. Oxygen dynamics:
                 (a) respiration uses oxygen
                 (b) bacterial decomposition uses oxygen
                 (c) photosynthesis makes oxygen ( general photosynthesis equation)
                 (d) atmosphere contains a large reserve of oxygen
     C. Carbon dioxide -- about 200 X more soluble than oxygen.

II. Seasonal Cycles of Oxygen
         Dimictic pattern for a mesotrophic or eutrophic lake
    A. Spring turnover
        - Ice cover over winter isolates from atmospheric exchange.
        - Some depletion occurs over winter (decomposition and respiration)
        - Oxygen increases until 100% saturation is reached
        - Summer oxygen debt -- this occurs if stratification sets up before the lake is saturated with oxygen

increase in oxygen content during spring mixing

     B. Spring stratification
oxygen profiles at beginning of spring stratification

oxygen saturation later in spring

        - Follows the 100% saturation curve.
        - Organism effects:
            i. Respiration
            ii. Photosynthesis

    C. Summer stratification
        - epilimnetic oxygen fluctuates due to balance of photosynthesis, respiration, and wind mixing
        - hypolimnetic oxygen drops -- respiration is important
summer oxygen patterns (during stratification)

    D. Breakdown of stratification
summer stratified pattern

beginning of stratification breakdown

continuing stratification breakdown
        - epilimnetic oxygen decreases slightly as oxygen poor water is mixed to the surface
        - hypolimnetic oxygen continues to decrease
    E. Day of turnover
increasing oxygen saturation during turnover
        - temperature >4 degrees C because of leftover summer heat
        - oxygen <100% because of respiration in hypolimnion
        - oxygen goes toward 100% by wind pumping
        - but, if ice sets in early, whole lake may not come to 100%, and this is a 'winter oxygen debt'

    F. Winter
oxygen profile at beginning of ice-on period

oxygen profiles as winter progresses
 

continuing depletion of oxygen under the ice

        - Inverse stratification
        - most intense respiration is in and near the sediments
        - 'winter kills' possible

III. Oxygen curves

     A. Clinograde

clinograde oxygen curve

        - Hypolimnetic oxygen depletion
        - Respiration and decomposition increase as lake productivity increases

     B. Orthograde
orthograde oxygen curves
        - Oligotrophic lakes -- low production, little respiration
        - Also higher oxygen solubility due to lower temperature
        - If two lakes have the same productivity on an areal basis, one may be clinograde and one orthograde based on basin shape
basins of same surface area, with differing volumes
 

     C. Positive heterograde

positive heterograde oxygen curve
        - due to photosynthesis at the thermocline
        - light penetration to metalimnion where there is slow mixing so that any oxygen produced stays around and builds up;
                some input of nutrients from hypolimnion increases growth
            from hypolimnion increases growth
        - 'deep chlorophyll layer' - depends on water transparency
        - could also be due to input of oxygen-rich river water that is denser than surface water

     D. Negative heterograde
negative heterograde oxygen curve

        1. can be due to respiration of algae at night, or respiration of dense layer of zooplankton
        2. density gradient slows the rain of detritus ('marine/lake snow') around thermocline, more respiration of organic matter
            (could also be effluent output there)
lake with a large sediment interface at the metalimnion; this will decrease oxygen at that level due to decomposition in the sediments

        3. morphology -- bench; more sediment area per water volume at a given depth so lots of respiration at that depth

    E. Diel cycle of epilimnetic oxygen content

example of diurnal (diel) cycle of oxygen concentrations in water

    F. Oxygen deficit -- how much oxygen is used up from decomposition of material falling from the productive trophogenic zone
        to the tropholytic zone
        amount of oxygen consumed by decomposition in the hypolimnion gives an estimate of the productivity of the lake

IV. Integration of temperature and oxygen profiles
     A. Lake example 1
        - Summer stratification period
        - Same temperature profile
        - Sample two lakes of similar morphology during the day and find the following two profiles.  Which lake is more productive?

oxygen curves of two lakes differing in productivity

     B. Lake example 2
        - Lakes 1 and 2 have the same:
            i. productivity
            ii. depth
            iii. transparency/light penetration
        - Why are there two different curves?

temperature and oxygen profiles of two lakes differing in thermocline depth
 

     C. Lake example 3

            - Three times, each a week apart in summer
oxygen and temperature profiles of a temperate lake three weeks apart

     D. Lake example 4
temperature and oxygen profiles of a lake on three consecutive days

            - Three times, each a day apart
            - Get change in oxygen, not temperature
            - What caused this?
 

     E. River example
oxygen and temperature profiles in a river

            - What caused this pattern?
 

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