Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 38

Reflected
light

Incident
light

Water
surface

Scattered light

Particle

FIGURE 3.5 Schematic of light entering water, where it can be reflected back, scatter off of
a particle, or be absorbed in the water column.

(freons) and bromide-containing compounds have caused destruction of
ozone in the upper atmosphere and have led to significant increases in the
solar UV that reaches the surface, particularly at higher latitudes. Some of
the consequences of increased UV to aquatic ecosystems will be discussed
in Chapter 11. The absorption of light in the atmosphere is wavelength
specific; the same is true once light enters water.
When light reaches the water surface, it can either be reflected or en-
ter the water (Fig. 3.5). The amount of light reflected is highly variable be-
cause it can be altered by waves on the water surface, the incident angle
of the sun, and the type of waves (e.g., whitecaps and size). If the surface
is snow-covered ice, almost all the light is reflected away. Clear ice cover
does not absorb much light. Once light enters a parcel of water, it can be
absorbed, reflected, or transmitted. Almost all light that is absorbed by wa-
ter, suspended particles, or dissolved materials is converted to heat.
Biological activities related to light and water column heating are con-
strained by the amounts of light at different depths. Light intensity de-
creases logarithmically with depth in a water column (attenuation). The at-
tenuation rate is related to reflection and absorption by water, dissolved
compounds, and suspended particles. In more productive water columns
with a large biomass of suspended photosynthetic organisms (eutrophic),
or those with large amounts of suspended inorganic materials or high con-
centrations of dissolved colored materials, the water contains more mate-
rial to absorb or reflect out the light; thus, it is not transmitted as far. In
less productive (oligotrophic) lakes with low amounts of suspended par-
ticulate or dissolved colored material, light is transmitted to greater depths
(Fig. 3.6).
Light attenuation is logarithmic (Fig. 3.6), a process that may be un-
derstood using the following example. Assume that 1/10 of the full sun-
light entering the water column is transmitted to 10 m below a lake sur-
face. If attenuation is constant, only 1/10 of the light remaining at 10 m is
transmitted to the second 10 m (by 20 m); thus, only 1/100 remains. By
30 m only 1/1000 remains (1/10 of 1/100), and so on. The example is pre-
sented in units convenient to a log base 10 scale, but a plot of the light re-