PHYSICAL LIMNOLOGY OF LAKES

(Disclaimer: these are lecture outlines with some figures; these are not lecture notes)
 

MORPHOMETRY OF LAKES

 What is required for a lake to form on a landscape?

Some lake types:

GLACIAL

-moraine dams - e.g. Finger lakes

-kettle lakes
 

TECTONIC -rift - e.g. Lake Baikal Lake Tanganyika (part of the Dead Sea Rift Valley)

-volcanism - collapsed craters, filled blowholes, lava dams

-earthquakes and landslide - Lake Reelfoot, TN


SOLUTION - sinkhole (solution) lakes, collaspsed saltdomes

WIND - playas, carolina bays?

RIVER - oxbows

ANIMAL - beaver, humans ...


Are lakes permanent features of the landscape?

Natural Lakes versus Artificial Reservoirs

How might
RESERVOIRS (impoundments) and LAKES differ based on:

  • Drainage basin area to lake surface area
  • Flushing rate
  • Sedimentation, turbidity
  • Water level


  •  
     

     
     

      What are some uses of lakes, what are the impacts of those uses, and what should one measure to assess these impacts?


    MATTER IN WATER

    Includes: * dissolved gases (such as ....?)

    * inorganic ions (such as ....?)

    * free organic molecules (such as ....?)

    * inorganic particles (such as ....?)

    * detritus

    * living cells and multicellular organisms
     
     

    Can everything in water be measured easily as separate entities?   Do substances tend to have single source and single impacts? 
    What can be estimated more easily?

     
     
     

    Measurements used to characterize matter in water:

    Total solids


    How can either of the two parameters above be estimated more directly?


    Estimating organic matter:
    The amount of organic matter in water can be estimated as biochemical oxygen demand (BOD) or chemical oxygen demand (COD).   What will happen to the level of oxygen in water as more organic carbon is added?

     
     
     
     
    Summary of matter in water:
    LIGHT Why might light be important to measures in aquatic systems?
     
     

    Fate of light in aquatic systems:


     
     

    * Reflection - prevented from entering water by air-water surface interface

    * Scattering - suspended particles reflect light at a massive array of angles
     
     
     

    * Absorption - diminution of light by transformation.  What is light energy typically transformed into upon being absorbed (or is the energy simply destroyed)?
     
     


     


     

    Light entering pure water:

    * longer visable light (reds) absorbed shallower

    * shorter wavelengths (blues) tend to scatter

     

    Light entering lake water with other matter:

    * organic compounds - often absorb blues and greens

    * silts and clays - reds and oranges more likely to be scattered - How do color and clarity differ from water with high concentrations of organic compounds?

    * phytoplankton chlorophyll - What color is not absorbed by chlorophyll a?


     

     Measuring light properties in water (usefulness depends on question asked):
        

    Turbidity (nephlometric) - an expression of the optical properties that cause light to be scattered and absorbed rather than transmitted in a straight line. Turbidity is primarily caused by total suspended solids but a direct relationship varies from system to system. Why?

    Turbidometers  (nephlometer) measures intensity of light scattered at 90°.  Units are Nepholometic Turbidity Units (NTU's).


     
     

    Vertical illumination (light penetration) - illumination at some depth as measured by underwater photometer. Light is absorbed exponentially with depth (a constant percentage of light available is extinguished at each meter)
      Iz = Io e-kz
    where: Io = intensity of light at surface
    Iz  = intensity of light at depth z in meters
    k = vertical absorption coefficient

    When plotted as depth vs log % incidence of light, line is straight in a homogeneous solution.

       


       How are vertical illumination and turbidity related? 


    What would a deflection in the line indicate?   

    Why might it be important to measure vertical illumination?

    Compensation depth - Respiration exceeds photosynthesis within a cell at about 1% of incident surface light (the photic zone is the region from the surface to where 99% of light has disappeared).

     

    Visibility - measure of the depth at which one can see into the water.  Measured using a secchi disc.  
    What factors affect this measurement?

    Visibility can be used to estimate photic depth and trophic state
    What factors affect these estimate?
     

    http://dipin.kent.edu/secchi.htm           http://www.hao.ucar.edu/public/education/sp/images/secchi.html
     In the above three methodologies, where is the light source and light sensor in each?
    Why not use just one?
    Which provides more information, secchi depth or  a vertical profile from photometer measurements? Which is less expensive?

    TWO SUMMARY POINTS:


     
     
     
    TEMPERATURE
    What happens to the energy when light is absorbed?
    Properties of water:

    * density-temperature relationship

    * high specific heat

    * high specific gravity


    Why is there a temperature difference between lake inputs and output?
     
     

    Sources of heat:

    * direct absorption of solar radiation - DOMINANT

    * transfer of heat from air

    * inflows

     
     
     
    Sinks of heat: * conduction of heat to air

    * evaporation

    * outflow


     



    How should heat energy be distributed vertically within a lake (what should a temperature-depth profile look like)?


    Consider:
  • exponential decline in light
  • change in density with temperature
  • water movement
  • ...
  • Major sources of water movement in lakes:

    * wind-generated waves (animation)  What determines wave height?
    * currents

     * langmuir spirals spirals
     
     

    * seiches
     
     

    * internal waves


     
     
     
    Thermal stratification is a function of water movement and heat distribution

     

    http://wow.nrri.umn.edu/wow/under/primer/page5.html
    Three layers identified in thermal stratification:

    Why is the warmest water shallowest?


    What factors affect intensity (depth) of thermal structure?


    Which would take more wind energy to mix, a lake with a shallow thermocline or a deep one?

    Is reverse thermal stratification possible?


    What will happen to
    thermal stratification as air temperatures cool in the fall?

     
     

    Lake types based on mixing vs. stratification:

         Holomictic
              Monomictic
              Dimictic
              Polymictic
         Meromictic
         Amictic


    How will thermal stratification change as a lake becomes more eutrophic?


    Quantifying the intensity of thermal stratification - Relative Thermal  Resistance to Mixing (RTRM)

          RTRM    =      Density of Upper Layer - Density of Lower Layer
                                         Density at 5°C - Density at 4°C

         RTRM is a relative, non-dimensional value used to:


    So depth of light penetration is important in determining depth of thermal stratification.  How can we test this idea in Lake Allatoona?


    What are the ecological consequences of thermal stratifcation?