Michigan Department of Conservation
Research and Development Report No. 131
Institute for Fisheries Research Report No. 1744, 1968
Measurement of Incident Stream Illumination with an
Inexpensive Light Integrating Unit
Thomas A. Wojtalik, Robert C. Ball and Frank F. Hooper
Calculation of the efficiency of autotrophic
production depends upon assessment of the quantity of solar energy received at the
stream surface. Orientation of the sun’s arc with respect to direction of flow
together with its relationship to shading by adjacent vegetation, stream banks and
surrounding terrain, make precise measurement of incident light received by a stream
a difficult task. To make possible comparison of the photosynthetic efficiencies of
several sections of stream, which have different vegetational cover and different
orientation to the sun, we have constructed a simple but efficient light integrating
unit which measures the total light received at any desired point under the water
surface for any desired length of time. In this paper we wish to describe this unit
and compare the quantity of radiation it receives on stream sections with different
cover and sun-orientation.
In a section of stream shaded by trees,
intensity of light changes from point to point depending upon the orientation of the
sun in relation to openings. Since openings are more or less random the only feasible
approach to measurement of the quantity of incident radiation over a given surface
area is a random sampling procedure, using light recording units that can be easily
moved from point to point. Increasing the number of available units increases the
efficiency of sampling during a given period and under a single set of meteorological
conditions. To permit construction of a large number of such units with a modest
budget we have designed a unit that is relatively simple and inexpensive.