Use of a quanta meter to measure attenuation and underwater reflectance of photosynthetically active radiation in some inland and coastal south-eastern Australian waters

Abstract

The attenuation of total photosynthetically active radiation (PAR) in natural waters and its characterization by means of a vertical attenuation coefficient are briefly discussed. The factors determining underwater reflectance (ratio of upward to downward irradiance at a given depth) are considered, and a simple mathematical treatment is presented which leads to the conclusion that within that part of the water body where the asymptotic radiance distribution exists, if reflection from the bottom is negligible then the reflectance is equal to the asymptotic backscattering coefficient (defined in the text) divided by 2K, where K is the (natural logarithm) vertical attenuation coefficient. Data collected using a commercially available quantum irradiance meter over a 2-year period for various inland and coastal waters in south-eastern Australia are presented together with measure- ments of levels of yellow substance and phytoplankton. In the turbid inland waters attenuation of PAR closely follows an exponential law. In the much clearer coastal waters, by contrast, attenuation of PAR is approximately biphasic, the vertical attenuation coefficient in the upper few metres being noticeably higher than that at greater depths. Within any one water body the vertical attenuation coefficient was observed to vary up to four-fold during the 2-year period: nevertheless there were indications that the average attenuation of PAR tended to differ characteristically from one water body to another. In one of the inland waters, measurements at different times of day showed that the vertical attenuation coefficient was not strongly dependent on solar altitude. Underwater reflectance values in the inland waters were surprisingly high (0.04-0.21) compared to values in the literature: this is probably a consequence of the high turbidity of these waters. Calculated values of the asymptotic backscattering coefficient for the inland lakes are presented. It is suggested that measurements of yellow substance and phytoplankton, together with some estimate of light scattering, in parallel with measurements of attenuation of PAR would facilitate an understanding of the factors responsible for that attenuation.