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 radi ation on photosynthesis rate. in situ and in laboratory incubations, on siz e fractionated natural algal assemblages (picoplankton: 0.2-2 mum, nanoplan kton: >2 mum) and whole water (total organic carbon TOC). Near surface samp les from a mesotrophic high mountain lake (LCD: L. Cadagno. Swiss Alps, Swi tzerland, 1923 m a.s.l.) and from the oligotrophic pre-alpine L. Lucerne (L LU: Swiss Alps, Switzerland, 434 m a.s.l.) were both incubated at a depth o f 30 cm (50% of surface UV at 323 nm) in L. Cadagno. At the same time, biol ogical weighting functions for UV inhibition of photosynthesis (BWFs) were determined for the autotrophic picoplankton and whole fraction in a spectra l incubator. Photosynthetic assimilation of the pico- and nanoplanktonic al gal communities as well as the assimilated total organic carbon (TOC) was e stimated separately by C-14 uptake under three irradiance conditions: PAR ( photosynthetically active radiation), PAR + UVA and PAR + UVA + UVB. UV rad iation 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 d ue to UVA (Mann Whitney U test, two tailed: P=0.0022). The natural assembla ges from LLU transplanted to LCD were more inhibited by UV than the autocht honous assemblages of LCD. Photosynthetic rates of picoplankton from LLU an d LCD under full UV exposure was reduced by 73% and 55% respectively relati ve 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 signifi cant for the in situ incubations.