Differential absorption by wave length gives water colour
•Red light is absorbed much more than blue in distilled water
The extinction coefficient k increases with:
•the concentration of organic matter (colour) of the water
•Deep clean water appears blue because most back-scatter
from depth is blue; shallower waters will back-scatter a mix of
blue and greens so such lakes appear blue-green
•the amount of suspended matter
eg, phytoplankton, fine suspended particles, eg clay
•Organic matter absorbs blue the most—appears yellow/brown
When a lake is rich in humic matter (tea) the organic matter
absorbs most of the blue, and green end of the spectrum,
•Fine colloids of calcite in water absorb blue mostly—water looks
green
•Suspended clay/silt scatter all wave lengths so water appears
milky (no colour)
•A dense phytoplankton bloom appears green because of
chlorophyll in the algal cells
The electromagnetic spectrum
Pure water absorbs preferentially the longer wave lenghts
—at depth short wavelenths predominate-everything gradually looks blue
Incoming spectrum—white light all colours present
10-20 m depth water blue-green
Increasing depth
5-10 m depth water greenish
50 -100 m water blue
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Clean shallow lakes usually appear bluish-green
Deep lakes appear blue because back scattering from deep water is mainly blue
Longer wave lengths have been absorbed already at shallower depths
Glacier meltwater full of
suspended particles looks
milky white since all
wavelengths are absorbed or
back-scattered.
Water from swamps like these
appears brown because of its high
content of dissolved organic matter which absorbs strongly
at the blue end of the spectrum
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The action spectrum for photosynthesis—blue and red work best
green, yellow and brown are least useful
This pond has a dense phyto-plankton bloom, and the green colonial algae make
the water look green
Based on the absorption spectrum for photosynthetic pigments, would
you expect to find algae or plants growing near the lower boundary
of the photic zone is
Where does the exponential equation come from. Another way of
writing it is as a rate equation. The rate of change of light intensity with
depth decreases as a linear function of the light Intensity
Section 10.6
Iz
(a) A clear lake with little organic or particulate matter in the water
(b) A brown-water humic lake
Consider what you know about the spectral composition at depth in
each of these two types of lakes.
z 50%
z 10%
z 1%
z
Photic
zone
dI
= !kI , where Iz = I 0e ! kz
dz
Show that the equation on the right
satisfies the one on the left
Take the derivative of I 0e ! kz with
respect to z
= I0
•
! ke ! kz
= !kI 0 e ! kz
= !kI
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Practice questions
Explain how flow processes contribute to habitat diversity in
rivers and streams.
Outline some examples of human activities that impact
riverine habitats.
Explain why these activities can put aquatic species at risk.
What is a proglacial lake?
Explain how they form and disappear on the landscape and
why they are important in determining the distribution of
aquatic species?
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