Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 42

3.0

A

2.5

2.0

1.5

1.0

0.5

0.0

100

B

80

60

40

20

0

200 300 400 500 600 700 800

Wavelength (nm)

FIGURE 3.8 The absorption (A) and transmission (B) of light by pure water as a function
of wavelength of light (data from Kirk, 1994).

Blackwater swamps, other wetlands, rivers, and some small lakes can have
high concentrations of humic compounds.
Not only does total light intensity change with depth but also the rel-
ative amounts of different wavelengths vary. Pure water has maximum ab-
sorption in the visible wavelengths of red light and maximum transmission
of blue (Fig. 3.8). This is why clear (oligotrophic) lakes appear blue; they
absorb green to red wavelengths and transmit blue. The blue wavelengths
are more likely than longer visible wavelengths to be reflected back out be-
fore they are absorbed. The pigments of suspended algae (phytoplankton)
absorb light in specific regions. The most important of these is chlorophyll
a , which absorbs blue and red light. As a lake becomes more eutrophic,
more blue is absorbed and relatively more green is transmitted and
reflected (Fig. 3.9). Thus, the spectral quality of light at depth is a function
of the absorptive properties of the lake. Spectral transmission influences
perceived colors of lakes and the objects within them (Sidebar 3.2).
Cyanobacteria (blue-green algae) have evolved a pigment system that
uses green light. This fact, combined with an understanding of optical prop-
erties including wavelength-specific attenuation, can be useful in describing
some of the ecology of cyanobacteria. The relatively high transmission of
green light in eutrophic lakes results in areas below the surface that are poor