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

The increase of ultraviolet radiation (UVR, 280-400nm) 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 3809m 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 [a(lambda)] at 320nm [a(320)] ranged from 0.52 to 14.05 m(-1) (mean standard deviation, 4.40 3.85 m(-1)) and at 380 nm [a(380)] from 0.05 to 4.51 m(-1) (1.40 1.30m(-1)). The exponential slope coefficient for the relationship of wavelength to a(lambda) ranged from 16.2 to 41.4 mu m(-1) (21.74 +/- 4.93 mu m(-1)) over the 280-400nm interval. Normalized fluorescence emission (NFLU) at 450nm from an excitation wavelength of 355 nm, F-n(355), averaged 7.93 +/- 3.22 NFLU. A significant positive relationship was found between a(355) and F,(355). The estimated diffuse attenuation coefficients of UV-B (320nm) and UV-A (380nm) 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 19.42 m. Twenty-five of 38 lakes had 1% UV-13 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,