A COMPARISON OF METHODS FOR THE MEASUREMENT OF THE ABSORPTION-COEFFICIENT IN NATURAL-WATERS

In the spring of 1992 an optical closure experiment was conducted at L ake Pend Oreille, Idaho, A primary objective of the experiment was to compare techniques for the measurement of the spectral absorption coef ficient and other inherent optical properties of natural waters. Daily averages of absorption coefficients measured using six methods are co mpared at wavelengths of 456, 488, and 532 nm. Overall agreement was w ithin 40% at 456 nm and improved with increasing wavelength to 25% at 532 nm. These absorption measurements were distributed over the final 9 days of the experiment, when bio-optical conditions in Lake Pend Ore ille (as indexed by the beam attenuation coefficient c(p)(660) and chl orophyll a fluorescence profiles) were representative of those observe d throughout the experiment. However, profiles of stimulated chlorophy ll a fluorescence and beam transmission showed that bio-optical proper ties in the lake varied strongly on all time and space scales. Therefo re environmental variability contributed significantly to deviations b etween daily mean absorption coefficients measured using the different techniques.