Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 41

toward the collector. The appropriate correction is a cosine curve, so co-
sine collectors are the sensors of choice to measure radiation from above.
In contrast, some biological processes such as photosynthesis are depen-
dent on total light received from all directions. In this case, a scalar or
spherical (360°) response collector is the sensor of choice (Kirk. 1994).
To measure the total energy entering a water body per unit area per
unit time, a pyrheliometer is used which compares the temperatures of
a reflective metal surface and one that absorbs all incoming radiation. These
measurements are useful for determining the heat budgets of water bodies.
A meter that is used to estimate light available for photosynthesis
should measure the number of photons that are available to excite chloro-
phyll. These sensors measure photons between 400 and 700 nm and the
results generally are reported as photosynthetically available radiation or
photosynthetic photon flux density. Units are in mol quanta m 2 s 1 , or

Einsteins m 2 s 1 sometimes [the Einstein is not an internationally rec-

10 23 ) of photons]. Spherical sensors are
often used for these measurements.
The relative absorption of specific wavelengths may also be of interest.
In this case, selective filters can be fitted over sensors, or a spectral ra-
diometer can be used. Spectral radiometers have diffraction gratings that
allow photodetectors to sense the intensity of specific wavelengths of light.
Problems arise if light is measured in algal mats or sediments because
very small sensors are needed (Jøregensen and Des Marais, 1988). In such
cases, spherical tips on fiber optic collectors have been used (Dodds, 1992).
Photodetectors or spectral radiometers can be used to analyze light col-
lected by such sensors.

ognized unit for a mole (6.02

EXAMPLE 3.2.
Use of Light Attenuation Equations

Calculate the attenuation coefficient and percentage transmission per me-
ter at the surface of two lakes, one that is productive and one that is less
productive. In both lakes, the light is 1500 mol quanta m 2 s 1

at the

mol quanta m 2 s 1 at 1 m,

surface. In the productive lake, the light is 1

mol quanta m 2 s 1

and in the unproductive lake it is 1200 at 1 m.

Calculation Unproductive Highly productive

I 0 (ln I ), intensity at first depth 1500 (7.31) 1500 (7.31)

I 1 (ln I ), intensity at second depth 1200 (7.09) 1 1 (0)

% transmission m 1

1 80 1 0.07

11 0.22 1 7.31

Note that transmission is higher and attenuation coefficient is lower
for the unproductive lake.