Sept. 20, 2000 Light under Water in Lakes

BIO321

Sept. 20, 2000

Light under Water in Lakes
I: Review

A

. Light reaching the surface of a lake is determined by:

• 
  latitude
  
• 
  season
  
• 
  time of day
  
• 
  altitude
  
• 
  meteorological conditions
  

B. Reflective loss is about 50% of the light leaving the lakes surface

• 
  (90˚=3%, 15˚=12%, 10˚=16%)
  
• 
  S_A = The sun's altitude or angular height in degrees
  
• 
  R_s ≈ 6.5% in summer
  
• 
  R_w ≈ 10% in winter
  

C. Scattering or internal reflection makes up the other 50% of reflective loss (about 5 - 10 percent of total light loss)

1. Causes

• 
  Water molecules
  
• 
  Solutes
  
• 
  Particles
  
• 
  Sediment (bottom)
  

2. Impacts on scattering

• 
  increases with concentration of particulates
  
• 
  Scattering coefficient varies for different wavelengths()
  

• 
  A
  
  s in the sky, clear water scatters blue wavelengths (clear lakes blue)
  
• 
  As the size of the molecule/particle increases, the wavelength of light scattered increases
  
• 
  particulates make water blue-green
  
• 
  organics make lakes green-yellow
  

I
I. Light in its various radiant forms plays important roles as radiant energy in lakes

During the Day: Q_B = Q_S + Q_H + Q_A - Q_R - Q_U - Q_W

Q_B = Net Radiation Surplus: the net amount of solar radiation affecting a lake

Q_S = direct solar radiation (the sun)

Q_H = indirect solar radiation: scattered and reflected radiation from clouds & sky

Q_A = long wavelength thermal radiation gain from atmosphere and surrounds

Q_R = reflected light - radiation reflected from the lake's surface

Q_U = upscatter: light scattered from water column and benthos

Q_W = emission from the lake of long wavelength thermal radiation

During the Night: Q_B = Q_A - Q_W

Light regulates the biotic components of lakes via photosynthesis.

Light also regulates the thermal properties of lakes, which in turn regulates the lake "metabolism" and respiration of biotic components.
III. F.A. Forel was one of the first limnologists to examine light as a variable in lakes. In 1865, using photographic plates placed at different depths, he discovered light in some lakes penetrating to 200 meters, but in others penetrated only a few meters.

• 
  Light decreases with depth exponentially (see figure 7.3 above)
  
• 
  Attenuation
  
• 
  upscatter
  
• 
  absorbtion
  
• 
  light energy is transformed to heat energy by collision with stuff, with 53% of all light being converted to energy within the first meter of water
  
• 
  stuff =water, dissolved salts, organic molecules, seston = bioseston (plankton) and abioseston(non-living POM (Particulate Organic Matter))
  

• 
  Coefficient of extinction of light in water
  

__w + _p + _d (Epsilon = coefficient of extinction, w = water, p = particulates(POM or PIM), d = dissolved color (DOM or DIM)

• 
  Light is quantified via absorption or transmission as in spectrophotometry
  

increased A means a decrease in T resulting from a HIGHER _

• 
  I_z = I₀e^-z (See Horne and Goldman page 37)
  
• 
  light is not monochromatic and different wavelengths are absorbed differently (each wavelength has a different _ or  is wavelength dependent)
  

- Distilled water ROY is strongly absorbed in the first meter while BG penetrates the most

• 
  Various factors modify light penetration in lakes
  

water (_w)

particulates (_p)absorb ROY and BIV allowing green to penetrate, but have little impact on transmission at low concentrations

color caused by organics absorbs R(less selectively) and IR leading to heating

dissolved color (_d) absorbs UV and BG within the first meter

• 
  Transmission of light in lakes varies with lake type and season and is diagrammed via isopleths as in the diagram above (Figure 5-9).
  

In Oligotrophic lakes (Lake Lawrence)  averages 0.39/meter and is more or less constant.

In Eutrophic lakes (Wintergreen Lake)  averages 1.0 /meter and varies a lot due to changes in algal populations

• 
  C
  ompensation Depth = 1% T level separating the upper productive trophogenic zone from the lower decompositional tropholytic zone
  

• 
  Algae differ by light useage depending on the pigments that they contain (Chl. A, B, C, D, E and more)
  

1. green algae with Chlorophyll A absorbs red (640nm) and blue (405nm), also possess Chlorphyll B (620nm and 440nm) and thus live in surface waters where these wavelengths are abundant

2. Diatoms, bluegreens (and also greens) possess mainly Accessory pigments like carotenoids and xanthophylls which absorb in the yellow-green (450-550nm) and/or phycocanin (Bluegreens and Cryptomonads (540-583nm). These light wavelengths penetrate deeper and so these organisms can live at depths or in winter.

• 
  Thus lake color is an important parameter that is quantified in one of several ways.
  

1. Platinum units range from 0 (clear) to 300 (stained like weak tea) and are a set of standards compared to a mixture of Potassium hexachloroplatinate and Cobalt Chloride and HCl in water.

2. Forel-Ule Scale (Based upon CuSO₄ + K₂CrO₄ + CoSO₄

3. True color must be distinguised from apparent color (bottom/particulate effects)

• 
  Transmission through ice and snow
  

-Clear ice = 85%T

-Bubbled ice = 20%T

-Snow covered ice T may be much lower

-Decreased light leads to decreased photosynthesis leading to decreased O₂ which of course is needed by the life of the lake from plants to consumers to decomposers. The result may lead to either hypoxia or anoxia leading to winter kill typical in small, shallow lake systems in North Temperate Regions

-Green light (480-550nm) passes best through ice restricting life to organisms that can use green light (flagellated cryptomonads for example)

• 
  Transparency is an approximation of %T &  and A.
  

- is usually measured instrumentally with photometers or estimated with the Secchi Disk.

- Secchi Disk is a 20 cm white or white and black disk lowered on the shaded side of a boat (avoid dawn and dusk) until it disappears (note distance) and then pulled up until it reappears (note distance) and then averaged (down + up)/2.

-Secchi Disk Transparency is a function of the light reflected from the disk and is influenced by dissolved, particulates, and water.

-Secchi Disk Depth(SDD) = 5% Transparency

-compensation depth = 1.3(Z_SD) = 1%T = the as a general rule, but use with caution since sediments and organics (color) can change the relationship.

- You can estimate the extinction coefficient for a lake:  ≈1.7/ Z_SD