Attenuation of ultraviolet radiation in mountain lakes: Factors controlling the among‐ and within‐lake variability

Abstract

High-altitude lakes are exposed to high fluence rates of solar ultraviolet radiation (UVR; 290-400 nm) and contain low concentrations of dissolved organic carbon (DOC). While in most lowland lakes, DOC can be used to predict UV transparency with sufficient accuracy, current models fail to estimate UVR in clear alpine lakes. In these lakes, phytoplankton may contribute significantly to the UV attenuation either as particles or as a source of chromophoric dissolved organic matter (CDOM) with distinctive properties. We investigated a series of 26 lakes in the Alps and Pyrenees, situated at elevations ranging from 422 to 2,799 m above sea level and having DOC concentrations ranging from 0.2 to 3.5 mg L Ϫ1 . CDOM, as measured by the absorptivity of filtered lake water, explained most of the variability in the attenuation of underwater UVR among lakes (r 2 ϭ 0.94, P Ͻ 0.001). However, within-lake variation in the UV attenuation revealed a significant contribution from phytoplankton in deeper waters (UV attenuation increasing with chlorophyll a concentration; r 2 ϭ 0.97, P ϭ 0.002), only apparent when DOC concentrations were low (ϳ0.3 mg L Ϫ1 ). The DOC-specific absorptivity (a g *) was also important for characterizing the optical conditions in this series of lakes. Epilimnetic values of a g * were significantly lower in lakes located at high elevations (with low allochthonous CDOM inputs from the catchment), compared to lakes surrounded by trees and meadows. Moreover, a g * was generally lower in surface waters than in deeper water layers, suggesting the influence of photobleaching on UV transparency. The slope S of the exponential regression between CDOM absorptivity and wavelength did not show clear patterns, such as found in marine systems, and often presented lower values in the epilimnetic waters (in association with lower a g *). Collectively, our results suggest that in transparent alpine lakes, the dynamics of the CDOM pool and phytoplankton production will have a strong effect on temporal changes in UV underwater attenuation. Solar ultraviolet-B radiation (UVB; 290-320 nm) has increased during the last 15 yr over many Earth's locations as a consequence of the degradation of the stratospheric ozone layer. Beside Antarctica, where the increment is notorious,

Photochemical production of organic matter triplet states in water samples from mountain lakes, located below or above the tree line

“…On the other hand, negligible R T* was observed in the case of Lago Rouen, which is mostly surrounded by exposed rocks. Such an environment is expected to decrease the importance of soil runoff as organic matter source (Laurion et al, 2000). Soil-derived organic matter usually includes a considerable fraction of humic substances (Vinebrooke and Leavitt, 1998) with significant photochemical activity (Halladja et al, 2007).…”

“…A combination of altitude and low concentration of UV-absorbing substances favours the penetration of harmful UV radiation in the water column (Laurion et al, 2000). Despite the development of several adaptations by aquatic organisms (Sommaruga, 2010), alpine lakes are a potentially vulnerable environment to climate change because of the possible increase of UV doses connected to variations in organic matter inputs and, most notably, to decreases in ice cover (Neale et al, 2001; Leavitt et al, 2003;Rogora et al, 2003).…”

Study of influencing factors to chromophoric dissolved organic matter absorption properties from fluorescence features in Taihu lake in autumn

“…A shoulder value was found at approximately 275 nm [a CDOM (275 nm)] in Taihu lake (Bidigare, 1989;Laurion et al, 2000;Bontempi et al, 2004;Repetad et al, 2004;Kieber et al, 2007), which also appeared in the data of Zhao and Qin (2008) and Warnock and Gieskes (1999). This shoulder value destroys the consistency of CDOM exponential decrease with wavelength.…”

“…However, there is slight fluctuation at approximately 334 nm in this study. A flat peak, which might be associated with the absorption by proteins and nucleic acids (Bidigare, 1989;Laurion et al, 2000), was found at approximately 275 nm. There may be two reasons for this: first, there were small numbers of phytoplankton in studied water; second, damage of phytoplankton cell was avoided by using low pressure during filtration.…”

Long-term trends of chemical and modelled photochemical parameters in four Alpine lakes

“…Changes in DOC concentration are important because they affect the penetration of sunlight (and most notably of UV radiation) into the water column (Laurion et al, 2000), as well as several other processes including photochemical reactions (Schindler, 1997).…”