Photosynthetic response of pico- and nanoplank1732 Medina-Sánchez et al. tonic algae to UVB

The objective of the present study was to evaluate the influence of UV radiation on photosynthe-sis rate, in situ and in laboratory incubations, on size fractionated natural algal assemblages (pico-plankton: 0.2–2µm, nanoplankton:>2µm) and whole water (total organic carbon TOC). Near sur-face samples from a mesotrophic high mountain lake (LCD: L. Cadagno, Swiss Alps, Switzerland, 1923 m a.s.l.) and from the oligotrophic pre-alpine L. Lucerne (LLU: Swiss Alps, Switzerland, 434 m a.s.l.) were both incubated at a depth of 30 cm (50 % of surface UV at 323 nm) in L. Cadagno. At the same time, biological weighting functions for UV inhibition of photosynthesis (BWFs) were determined for the autotrophic picoplankton and whole fraction in a spectral incubator. Photo-synthetic assimilation of the pico- and nanoplanktonic algal communities as well as the assimilated total organic carbon (TOC) was estimated separately by 14C uptake under three irradiance con-ditions: PAR (photosynthetically active radiation), PAR + UVA and PAR + UVA + UVB. UV radiation reduced significantly photosynthesis rate in samples from both lakes (LLU: P = 0.0012; LCD: P = 0.0001). It appears that UVA plus UVB significantly affect the algal assemblage in both lakes; however most of the effect is due to UVA (Mann Whitney U test, two tailed: P = 0.0022)