Variation of chromophoric dissolved organic matter and possible attenuation depth of ultraviolet radiation in Yunnan Plateau lakes

The increase of ultraviolet radiation (UVR, 280–400 nm) caused by stratospheric ozone depletion has profound effects on aquatic ecosystems. High-altitude lakes in the Yunnan Plateau are exposed to high intensities of UVR and contain low concentrations of chromophoric dissolved organic matter (CDOM). Thirty-eight lakes in the Yunnan Plateau with elevations from 1291 to 3809 m above sea level were investigated to study CDOM concentrations and possible effects of UVR on the lake ecosystem. The attenuation of UVR in the Yunnan Plateau lakes was calculated from the absorption coefficient of CDOM based on an empirical relationship from lakes in the Alps and Pyrenees mountains. Absorption coefficients [ α ( λ )] at 320 nm [ α (320)] ranged from 0.52 to 14.05 m −1 (mean ± standard deviation, 4.40 ± 3.85 m −1 ) and at 380 nm [ α (380)] from 0.05 to 4.51 m −1 (1.40 ± 1.30 m −1 ). The exponential slope coefficient for the relationship of wavelength to α ( λ ) ranged from 16.2 to 41.4 μm −1 (21.74 ± 4.93 μm −1 ) over the 280–400 nm interval. Normalized fluorescence emission (NFLU) at 450 nm from an excitation wavelength of 355 nm, F n (355), averaged 7.93 ± 3.22 NFLU. A significant positive relationship was found between α (355) and F n (355). The estimated diffuse attenuation coefficients of UV-B (320 nm) and UV-A (380 nm) ranged from 0.55 to 15.77 m −1 and from 0.24 to 6.73 m −1 ; the corresponding 1% attenuation depths ranged from 0.29 to 8.44 m and from 0.68 to 19.12 m. Twenty-five of 38 lakes had 1% UV-B attenuation depths of 1.5 m or more. The median 1% attenuation depth was 28.8% of the sampling depth for UV-B radiation and 60% for UV-A. In addition to CDOM, chlorophyll α (Chla) and total suspended matter (TSM) also may contribute to attenuation of UVR.