The extinction coefficient k increases with:
•the concentration of organic matter (colour) of the water
•the amount of suspended matter
eg, phytoplankton, fine suspended particles, eg clay
Differential absorption by wave length gives water colour
•Red light is absorbed much more than blue in distilled 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
•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
50 -100 m water blue
Increasing depth
5-10 m depth water greenish
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
Water containing dissolved organic matter—organics absorb short wave lengths
and the water absorbs the long wave lengths-spectrum trimmed from both ends
Incoming spectrum—white light all colours present
5-10 m depth water brown
At depth > 10 m virtually no light
Increasing depth
2-5 m depth water yellow-brown
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
Glacier meltwater full of
suspended particles looks milky
white since all wavelengths are
absorbed or back-scattered.
This pond has a dense phyto-plankton bloom, and the green colonial algae make
the water look green
The action spectrum for photosynthesis—blue and red work best
green, yellow and brown are least useful
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
(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.
“Wind streaks” and Langmuir spirals
Larger scale gravitational responses to wind action—The seiche
The oscillation of the
thermocline during a
seiche
The oscillation of the thermocline produced by internal waves during a large seiche