Seasonality in UV transparency of an alpine lake is associated to changes in phytoplankton biomass

We studied the temporal variation in ultraviolet (UV) radiation transparency in the water column of a low dissolved organic carbon (DOC) lake (range: 0.12-0.65 mg L-1) located above the treeline. After ice-break, the water column was spatially segregated into an upper zone (0-4 m depth) with relatively constant UV transparency and a lower zone (4-9 m depth) where transparency decreased over time, for example at 380 nm, by up to 70%. This temporal decrease in UV transparency was significantly correlated with the increase in phytoplankton chlorophyll a. For the entire study period, there was no clear relationship between changes in UV transparency and colored dissolved organic matter (CDOM) absorptivity. Furthermore, changes in DOC concentration were not reflected in CDOM absorptivity or fluorescence, indicating a discrepancy between estimates of concentration and optical properties of this material. Use of spectral slope to track signals of CDOM photobleaching resulted in no consistent pattern. An indication of the low proportion of UV-absorbing chromophores in the dissolved organic matter (DOM) pool was obtained by applying size-exclusion chromatography combined with simultaneous measurements of DOC and UV absorption. These results indicated that more than the 50% of the DOM did not have significant UV absorption. Electrospray mass spectrometry analysis of DOM also showed a low diversity of organic compounds which exhibited relatively low molecular weight (< 250 D). These results, in combination with measurements of DOC-specific absorptivity and fluorescence, indicated that DOM in this lake is mainly autochthonous. Overall, our results show that factors which have been established to explain temporal changes in UV transparency of high DOC lakes may have little application in this and similar alpine lakes. Finally, reconstruction of the UV-exposure history in alpine lakes may have to take into account the discrepancy between DOC and CDOM.