Photoreactivity of Dissolved Organic Matter from High-Mountain Lakes of Sierra Nevada, Spain

Abstract

The effect of many environmental stressors can be mediated by dissolved organic matter (DOM) properties. In this study, DOM from four high mountain lakes (two surrounded by meadows and two located on rocky terrain) was optically characterized and its photoreactivity was experimentally evaluated. To evaluate DOM photoreactivity, photobleaching rates of absorptivity, and fluorescence and their effects on DOM spectral properties were analyzed. Dissolved organic carbon (DOC) concentration ranged from 37 to 69 μM, absorptivity at 320 nm (a₃₂₀) from 0.60 to 3.09 m⁻¹, and fluorescence from 1.30 to 5.70 QSU. Photobleaching of absorptivity was significant only at 320 nm (a₃₂₀) resulting in half lives that varied from 2.6 to 6.7 d. Photobleaching of a₃₂₀ was significantly higher for DOM from lakes located on rocky terrain than for DOM from lakes surrounded by meadows. Photobleaching of fluorescence emission at 450 nm (F₄₅₀) was significant only for three lakes and their half lives varied from 4.1 to 6.3 d. No significant differences were observed among the lakes studied. The changes over sunlight exposure of spectral slopes (S_UV), ratios of absorptivity at 250 nm to 365 nm (a₂₅₀:a₃₆₅) and ratios of fluorescence emission at 450 nm to 500 nm (F₄₅₀:F₅₀₀) did not show consistent trends with alternate increases and decreases. Lakes surrounded by meadows showed higher DOC concentrations, higher absorptivities, and lower a₃₂₀ photobleaching coefficients, suggesting that these lakes could be less vulnerable to UVR than lakes located on rocky terrain.