Dissolved Organic Carbon Concentration and Phytoplankton Biomass in High-mountain Lakes of the Austrian Alps: Potential Effect of Climatic Warming on UV Underwater Attenuation

Abstract

Concentrations of dissolved organic carbon (DOC) and phytoplankton biomass (chlorophyll-a and biovolume) were measured during a survey of 57 high-mountain lakes of the Austrian Alps ranging from 1970 to 2890 m a.s.l. in elevation. Glacier-fed lakes had lower DOC (mean: 0.39 mg L⁻¹) than lakes not fed by glaciers (mean: 0.70 mg L⁻¹). Concentrations of DOC decreased with lake elevation and was 64% lower in lakes located between 2600 and 2800 (mean = 0.51, median = 0.47 mg L⁻¹) than between 1970 and 2200 m a.s.l. (mean = 1.40, median = 1.10 mg L⁻¹). Chlorophyll-a concentrations ranged from 0.2 to 10.6 μg L⁻¹ with a typical deep water maximum found in transparent lakes and a maximum close to the surface in glacier-fed lakes. Chlorophyll-a and direct estimations of biomass were highly significant correlated among lakes (r = 0.938, P < 0.001). DOC concentrations and chlorophyll-a were not significantly correlated (P > 0.05). The potential impact of climatic warming on the underwater ultraviolet radiation (UVR) was assessed on the basis of expected changes in concentration of the two most important factors controlling the attenuation of UVR in the water column of alpine lakes not fed by glaciers, i.e. the chromophoric DOC and phytoplankton. Considering a scenario of further climatic warming in the Alps, we hypothesize that the attenuation of underwater UVR will increase mainly as a consequence of higher inputs of allochthonous DOC to surface waters of alpine lakes. Application of an empirical model based on the attenuation of underwater UVR by DOC, indicated that between 25 and 50% of the lakes studied can be considered as sensitive to UVR, i.e., the value of the 1% UV attenuation depth was higher than the lake maximum depth.