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] = K_H r_gas
                        K_H  (Henry's constant) is a solubility factor, varying from gas to gas
                       O₂ in atmosphere ~20.3% = 0.203 atm
                       K_H @ 20oC = 1.39  (for pure water)
                                           amount of O₂ that will dissolve in water at 20^oC
                     

    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 ( )
                 (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

     B. Spring stratification

        - 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

    D. Breakdown of stratification

        - epilimnetic oxygen decreases slightly as oxygen poor water is mixed to the surface
        - hypolimnetic oxygen continues to decrease
    E. Day of 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

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

III. Oxygen curves

     A. Clinograde

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

     B. Orthograde

        - 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

 

     C. Positive heterograde

        - 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

        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)

        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

    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?

     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?

     C. Lake example 3

            - Three times, each a week apart in summer

     D. Lake example 4

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

     E. River example

            - What caused this pattern?
 

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