Comparative reflectance properties of algal cultures with manipulated densities

A comparative study was conducted with representatives of four ecologically important freshwater algal phyla (Chlorophyta, Cyanophyta, Bacillariophyta and Pyrrophyta) in order to (1) analyze the relationship between reflectan ce spectral features and algal density, and (2) to elucidate and characteri ze possible diagnostic spectral reflectance features for identification of dominant groups in algal bloom states. Algae (two chlorophytes, a cyanophyt e and a bacillariophyte) were cultured in mesocosm tanks and then their den sities were reduced through dilution with clear water in four experiments. In a fifth experiment, use was made of the pyrrophyte Peridinium gatunense, which bloomed in Lake Kinneret, Israel, practically forming a unialgal 'cu lture'. The basic quantitative procedures for spectroradiometeric measureme nts and support data collection (pigment concentrations) were consistent am ong the experiments. Several spectral reflectance characteristics were found to be common to all the species examined in the study: a depression between 440 and 500 nm, a salient trough around 670 nm, and prominent peaks centered around 570 nm an d 700 nm. The increase in magnitude of a near infra-red peak and shift of i ts position towards longer wavelengths with increased chlorophyll-a concent ration was found to be a common trait for all the species studied. Other sp ecific spectral reflectance features were also found and are discussed here . Quantitatively different relationships of magnitude and position of spect ral reflectance features with chlorophyll-a concentration reflect different optical properties of algae (their scattering and absorption). This may be useful for taxonomic characterization by remote sensing.