Photosynthetic response of pico- and nanoplanktonic algae to UVB, UVA and PAR in a high mountain lake

The objective of the present study was to evaluate the influence of UV radiation on photosynthesis rate. in situ and in laboratory incubations, on size fractionated natural algal assemblages (picoplankton: 0.2-2 mum, nanoplankton: >2 mum) and whole water (total organic carbon TOC). Near surface 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. Photosynthetic assimilation of the pico- and nanoplanktonic algal communities as well as the assimilated total organic carbon (TOC) was estimated separately by C-14 uptake under three irradiance conditions: 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). The natural assemblages from LLU transplanted to LCD were more inhibited by UV than the autochthonous assemblages of LCD. Photosynthetic rates of picoplankton from LLU and LCD under full UV exposure was reduced by 73% and 55% respectively relative to PAR only. A higher sensitivity of autotrophic picoplankton to the UV radiation, with respect to the nanoplankton, was observed in the biological weighting functions. However this difference was not statistically significant for the in situ incubations.