Light climate and plankton in the deep chlorophyll maxima in North Patagonian Andean lakes

The light climate at the deep chlorophyll maxima (DCM) was analysed in a set of lakes of the North Patagonian Andean region. Apparent and inherent optical properties in relation to the chlorophyll a vertical distribution were investigated in seven lakes including deep (Z(max) > 90 m) and shallow (Z(max) < 12 m) ones. Sampling was carried out during the thermal stratification period (summer) of the deep lakes since in the shallower lakes no stable thermal stratification was detected. The large deep lakes presented very low diffuse attenuation coefficients of photosynthetically active radiation (K-d PAR), and a DCM situated at 0.98-2% of surface PAR irradiance, coinciding with the maximum abundance of the mixotrophic ciliate Ophrydium naumanni and autotrophic picoplankton, Both fractions seemed to be favoured by dim light conditions of particular wavelengths, since at these DCM layers mainly green and blue wavelengths prevailed (<600 mn). In contrast, shallow lakes showed higher K-d PAR values, with a higher concentration of dissolved yellow substances, which caused substantial differences in the spectral quality that may have contributed to explain the absence of this ciliate population in these lakes.