In-situ Monitoring of Water Quality on the Basis of Spectral Reflectance. Ship-borne Experiments for the Development of Remote Sensing Algorithms Especially for the Estimation of Algae Content in Natural Waters

When using in-situ methods such as the use of spectrometers aboard a ship, aircraft or satellites to estimate suspended matter (especially phytoplankton concentrations) in waters it is necessary to know the corresponding correlations between the matter concerned and the spectral reflectance R(Λ) or colour indices R(Λi)/R(Λj). Therefore, ship-borne experiments were carried out on mesotrophic and eutrophic waters. The following parameters were measured: reflectance at the wavelengths 450, 550, 670, 705 nm, Secchidisk transparency, chlorophyll-a concentration, fluorescence, extinction, and scattering. Significant correlations (0.92<r²<0.99) were found between chlorophyll-a concentration (10 to 360 μg/l) and the reflectance ratios R(705)/(670), R(705)/R(550), (R(705) -R(670))/R(550). At these concentrations the estimation of chlorophyll-a by means of R(705)/R(670) appears to be more accurate than by the fluorescence method. When developing methods for estimating seston concentrations the use of R(705) is recommended.