THE ROLE OF PHYTOPLANKTON IN DETERMINING THE UNDERWATER LIGHT CLIMATEIN LAKE-CONSTANCE

At all seasons, the underwater light field of meso-eutrophic large (48 0 km(2)) deep (mean: 100 m) Lake Constance was studied in conjunction with the assessments of vertical distributions of phytoplankton chloro phyll concentrations. Vertical profiles of scalar, downwelling and upw elling fluxes of photosynthetically available radiation, as well as fl uxes of spectral irradiance between 400 and 700 nm wavelength were mea sured. The overall transparency of the water for PAR is highly depende nt on chlorophyll concentration. However, the spectral composition of underwater light is narrowing with water depth regardless of phytoplan kton biomass. Green light is transmitted best, even at extremely low c hlorophyll concentrations. This is explained by the selective absorpti on of blue light by dissolved organic substances and red light by the water molecules. Nevertheless, significant correlations were found bet ween vertical attenuation coefficients of downwelling spectral irradia nce and chlorophyll concentrations at all wavelengths. The slopes of t he regression lines were used as estimates of chlorophyll-specific spe ctral vertical light attenuation coefficients (K-c(lambda)). The propo rtions of total upwelling relative to total downwelling irradiance (re flectance) increased with water depth, even when phytoplankton were ho mogeneously distributed over the water column. Under such conditions, reflectance of monochromatic light remained constant. Lower reflectanc e of PAR in shallow water is explained by smaller bandwidths of upwell ing relative to downwelling light near the water surface. In deeper wa ter, by contrast, the spectra of both upwelling and downwelling irradi ance are narrowed to the most penetrating components in the green spec tral range. Reflectance of PAR was significantly correlated with chlor ophyll concentration and varied from similar to 1% and similar to 1-% at low and high phytoplankton biomass, respectively. Over the spectrum , reflectance exhibited a maximum in the green range. Moreover, in dee per layers, a red maximum was observed which is attributed to natural fluorescence by phytoplankton chlorophyll.