Measurement of photosynthetically available radiation in freshwater: comparative tests of some current instruments used in studies of primary production

Instruments now in common use among biologists for measuring underwater photosynthetically available radiation (PAR) were compared as part of the Group on Aquatic Primary Production workshop. The effect of differences in spectral response (narrow versus broad wavebands) and collecting properties (cosine versus scalar) on measurements of vertical (diffuse) attenuation coefficient (ε) were tested. As instruments measuring spectral quality can be expensive a low-cost manual interference-band-filter photometer is described and compared with a scanning spectroradiometer. In Bodensee there was a progressive narrowing of the spectral waveband with depth, so that by 15 m nearly 75% of the total quanta were between 525 and 575 nm. Therefore values of ε for PAR, measured with broad waveband sensors, decreased with depth. With narrow waveband sensors values of ε were constant with depth. Attenuation was similar with both scalar and cosine instruments. However, comparisons of absolute values were difficult because of incorrect calibrations. It is recommended that the calibration of scalar instruments should be more precisely defined, and that the intrinsic depth-variability of ε for PAR should not be neglected when interpreting measurements.