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

The relationships between primary production and irradiance were analyzed over an annual cycle in natural biofilms of two undisturbed streams: La Solana (LS), an open calcareous stream, and Riera Major (RM), a shaded siliceous stream. In LS, low photosynthetic efficiency (α^chl and α^area) and high values of both the light saturation parameter (I_k) and the carotenoid / chlorophyll ratio indicated adaptation to high light regimes. On the other hand, higher photosynthetic efficiency and lower I_k as well as photoinhibition at high irradiance found in the biofilms of RM indicated shade adaptation. However, the lack of correlation between light availability in nature and the photosynthetic parameters studied in the laboratory suggested that light was not the most important factor in determining seasonal changes in the photosynthetic behavior in this stream. Both in the open and shaded streams, algal patches collected simultaneously exhibited different photosynthesis-irradiance (P-I) curues, showing that community composition influenced the P-I parameters. In the open stream (LS), however, significant negative correlations between α^area and chlorophyll a and between 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 biofilms of LS, high photosynthetic rates were maintained at the ambient high light environment, thus compensating for low photosynthesis at low irradiance. In the shaded stream (RM), because photosynthesis was saturated at low irradiances, primary production was relatively high given the low light conditions .