Light in Lakes


I. Why study light in lakes?
    A. Drives photosynthesis and lake metabolism
    B. Affects thermal structure
    C. Regulates biota
    D. Can damage biota

II. Electromagnetic spectrum

    A. Background
            SEE FIGURE 5-1, WETZEL

        1. photosynthetically active radiation
        2. Insolation – solar radiation
        3. Radiant flux – quantity of electromagnetic energy flow over time (quanta/sec)
        4. Irradiance – all wavelengths of light; intensity of radiant flux; flux per unit of surface area (quanta x sec-1 m-2)
        5. Wavelengths and energy (note, not all symbols display accurately on all browsers -- double check equations from notes printed off the web)
            E = hn (Planck's Law)
                 h = Planck’s constant = 6.63 X 10-34 J s
                 n = frequency (cycles per sec)
           Wavelength, l = c/n
                 c = speed of light (3 x 108 m s-1)
           Therefore E=hc/l        so photons have less energy at longer wavelengths

    B. Dual nature of light

         1. Energy – heat flux
              e.g., cal/min/cm2 or Joules/min/cm2
         2.Particle – biochemical processes
              Photons (quanta)
              1 mol photons = 1 Einstein
    C. Solar constant
            Amount of radiation reaching the earth’s outer atmosphere
            ~1.94 cal/cm2/min (or 1353 W m-2) reaches earth
            Most common ('maximum') wavelength is ~480 nm
III. Intensity and Quality of Light
     A. Factors affecting light intensity and quality
        1. Latitude
        2. Solar angle (time of day and season)
        3. Altitude
        4. Atmospheric transparency – haze, smoke, particles
        5. Cloud cover

    B. Processes affecting light intensity and quality

        1. Scattering
            a. Atmosphere and water
            b. Dependent on wavelength (for small particles it is proportional to 1/ l4)
            c. Selective scattering of short wavelengths
            d. Decrease of UV bands (200-400 nm)

            e. Rayleigh – scattering due to small molecules (why sky is blue)
            f. Mie – scattering due to dust (forward scattering preferentially)

        2. Refraction – speed of light changes in different substances

           a.  Speed of light changes in medium (generally cited speed is for a vacuum)
           b. This causes the angle of light to change when it enters a new media, based on the refractive index
           c. sin(angle) air      = nwater  =  (1.33)
                sin(angle) water    nair         (1.00028)
            d. Refractive index is affected by temperature, salt content
            e. Net effect is to move the angle closer to vertical in the water

        3. Reflection
           a. angle of light
           b. wave height and foam
           c. ice and snow

        4. Absorption
           a. decrease of light energy by transformation to heat
           b. atmospheric gases, O2, O3, H2O
           c. water itself

        5. Attenuation of light in the water column – due to absorption and scattering

            a. Transmittance  (amount of light left) = Iz/ I0 x 100
                where I = irradiance,
                         I0 = irradiance just below surface
                         Iz = irrad. at depth z

            b. Absorbance [100 x (I0 - Iz)]/I0

            c. Attenuation equation

                i.   Iz = I0 e - kz
                        where e = natural logarithm
                        k = attenuation coefficient (extinction coefficient; use h in Wetzel)
                ii. characteristic for each water body and each wavelength
plot of light extinction with depth
               iii. often converted to a linear plot by taking the log of both sides:

                ln Iz = ln I0 – kz

plot of light extinction with depth, log scale

        d. components of the attenuation/extinction coefficient

              Kl = Kabs + Kscattering
              K = Kwater + Kdissolved organics + K particulates

                1) Kwater
                    - for pure water, absorption at long wavelengths dominates (>550 nm; red and IR)
                    - So, IR disappears in the top 1-2 m of most lakes
                    - Scattering at short wavelengths, <380 nm
                    - Pure water does not absorb UV (only scatters it)
                    - Dissolved salts do not increase attenuation

                2) Kdissolved organics
                    - dissolved organics "Gelbstoff" -- humic and fulvic acids
                    - absorb strongly at short wavelengths -- blues and UV's (<500 nm)

                3) Kparticulates
                    - absorbs light evenly over the entire spectrum
                    - often the particulates are predominantly tripton and phytoplankton
                    -detritus may have higher absorbance at the blue end

                4) Examples from various natural lakes with different amounts of dissolved substances

examples of light extinction in an oligotrophic and a eutrophic lake

differential penetration of wavelengths in lakes
IV. Other interesting facts about light in lakes

     A. Measurement of light
        1. Secchi disc
            - Visual contrast between light reflected off the disc and all other upwelling irradiance, so it is independent of surface light intensity
            - Has been used to predict chlorophyll a (within lake measurements)
            - Good way to communicate light penetration information to non-scientists

    B. Old measures - lumens

old measures of light

    C. Complications

          Lagged attenuation coefficients

Interested in Light in Aquatic Systems?  A good additional reference is:

 Kirk, J.T.O. 1994. Light and photosynthesis in aquatic systems. 2nd edition.  Cambridge University Press


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