Unterwasser-Photometer zur spektralen Absorptionsmessung in Binnengewässern

The paper presents an underwater photometer for spectral extinction measurement in inland waters. The monitoring of the water quality of lakes needs quantitative registration of their trophic state, i.e. the nutrient content. The spectral extinction measurement of natural light in the water should result in an effective in-situ method for the classification of lakes. Therefore, this photometer has been constructed and tested. The following spectral ranges were chosen: 360 nm for the detection of DOC (dissolved organic carbon), 430 nm to detect yellow substances (humic and fulvic acids), 662 nm for measurements of the chlorophyll-a concentration and a reference at 550 nm. The halfbandwidth is 36 nm for the 360 nm range, but about 11 nm for the others. The spectral selectivity is reached by appropriate small interference filters. The incoming light is detected by photodiodes. The signal is electronically amplified in the measuring head. The angle of light incidence is limited to a halfwi dth of about 20 deg by appropriate diaphragms for each wavelength. Furthermore, four additional detectors with the same properties as the upper ones are pointing downwards to detect light scattered back by the water and the bottom. First results of field measurements are presented. Here, a comparison with traditional analytical laboratory methods and a first calibration of the 662 nm detector with respect to the chlorophyll-a concentration are given. The influence of different properties of the natural illumination (i.e. direct sunlight or diffuse light from the sky) are discussed.