| Summary: | In polar ecosystems, primary producers have to cope with a very harsh climate that limits the time available for growth and biomass production. In this study, diurnal measurement of photosynthetic processes in biological soil crust and a lichen were carried out in Petuniabukta, Spitsbergen. For field measurements, a method of induced fluorescence of chlorophyll was used. Measurements of photosynthetic activity were taken as repetitive measurements of effective quantum yield of photosystem II (ΦPSII). The short-term field measurements were carried out for 10 days in summer 2014. ΦPSII was recorded each 5 minutes as well as microclimatic data (air temperature, air humidi-ty, photosynthetically active radiation - PAR). The microclimatic parameters were recorded by a datalogger. In general, physiological activity of both biological soil crust and a lichen showed daily courses. Tested lichen was Cladonia rangiferina and the most dominant species in biological soil crust was Nostoc sp. Typically, most of ΦPSII values ranged 0.6 – 0.7 in both model organisms. The results have shown that photosynthetic activity was strongly correlated with all observed abiotic factors in both study objects. Particularly important was the relation found between PAR and ΦPSII in biological soil crust. When the biological soil crust was exposed to high PAR doses of irradiation (about 2300 µmol m-2 s-1) photoinhibition of primary processes of photosynthesis was observed as ΦPSII decrease, while photosynthetic activity of lichen remained at same level. Furthermore, it has been demonstrated increasing that in situ photosynthetic activity increased in both biological soil crust and lichen with a decrease in temperature.
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