SEASONAL-VARIATIONS IN PHOTOSYNTHESIS-IRRADIANCE RESPONSES BY BIOFILMS IN MEDITERRANEAN STREAMS

The relationships between primary production and irradiance were analy zed over an annual cycle in natural biofilms of two undisturbed stream s: La Sobana (LS), an open calcareous stream, and Riera Major (RM), a shaded siliceous stream. In LS, low photosynthetic efficiency (alpha(c hl) and alpha(area)) and high values of both the light saturation para meter (I-k) and the carotenoid / chlorophyll ratio indicated adaptatio n to high light regimes. On the other hand, higher photosynthetic effi ciency and lower I-k as well as photoinhibition at high irradiance fou nd in the biofilms of RM indicated shade adaptation. However, the back of correlation between light availability in nature and the photosynt hetic parameters studied in the laboratory suggested that light was no t the most important factor in determining seasonal changes in the pho tosynthetic behavior in this stream. Both in the open and shaded strea ms, algal patches collected simultaneously exhibited different photosy nthesis-irradiance (P-I) curves, showing that community composition in fluenced the P-I parameters. In the open stream (LS) however, signific ant negative correlations between alpha(area) and chlorophyll a and be tween P-max(chl) and chlorophyll a (r = -0.994, P < 0.001, and r = -0. 929, P < 0.05, respectively suggested that photosynthesis was affected by self-shading. Due to the absence of photoinhibition in the biofilm s of LS, high photosynthetic rates were maintained at the ambient high light environment, thus compensating for low photosynthesis at low ir radiance. In the shaded stream (RM), because photosynthesis was satura ted at low irradiances, primary production was relatively high given t he low light conditions.