Photosynthetic activity of epilithic algal communities in sections of the Torna stream (Hungary) with natural and modified riparian shading

The photosynthetic activity of epilithon grown in the Torna-stream (Hungary) was studied. Samples were collected from natural sections with closed canopy and natural stream bed and from modified sections with trapezoid river bed and shores lacking riparian vegetation. We assumed that human modifications would have a significant impact on epilithic photosynthesis, and therefore expected to observe corresponding changes in photosynthesis–irradiance (P–I) parametric. The laboratory P–I measurements were carried out monthly between April 2008 and September 2009 with epilithon after 3 weeks of in situ colonization. The maximum rate of the photosynthesis (P _max) correlated positively with the chlorophyll a content of the samples. Natural sites had lower biomass and P _max values than modified sites, and the biomass differed substantially in spring and summer, the P _max differed significantly only in spring. Natural sections had higher biomass-specific photosynthesis values than modified sections in all seasons, but the difference was significant only in summer–autumn: in summer the samples from the natural sections had significantly higher photosynthetic efficiency. In spring and summer, the photoadaptation parameters (I _k ) of communities grown in modified sections were substantially higher than in natural sections. Light availability appeared to be the major factor regulating the seasonal photosynthetic activity of epilithic communities.