RESEARCH PAPER Limnology (2007) 8:311–319 © The Japanese Society of Limnology 2007 DOI 10.1007/s10201-007-0219-z Yun Lin Zhang · En Lou Zhang · Ming Liang Liu Xin Wang · Bo Qiang Qin Variation of chromophoric dissolved organic matter and possible attenuation depth of ultraviolet radiation in Yunnan Plateau lakes Abstract The increase of ultraviolet radiation (UVR, 280– 400 nm) caused by stratospheric ozone depletion has pro- found 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 pos- sible effects of UVR on the lake ecosystem. The attenua- tion 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(λ)] at 320 nm [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 ± 1.30 m -1 ). The exponential slope coefficient for the relationship of wavelength to a(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 a(355) and F n (355). The estimated diffuse attenu- ation 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 a (Chla) and total suspended matter (TSM) also may contribute to attenuation of UVR. Key words Chromophoric dissolved organic matter (CDOM) · Diffuse attenuation coefficient · Fluorescence · UV-B radiation · Yunnan Plateau Introduction Stratospheric ozone depletion resulting from industrial chemicals containing chlorine or bromine [e.g., chlorofluo- rocarbons (CFCs) and halons] released into the atmosphere causes increased potentially damaging ultraviolet radiation (UVR, 280–400 nm), and especially UV-B radiation (280– 320 nm) (Madronich 1993). In the 1990s, global stratospheric ozone thickness decreased an average of 2–3%. A 1% decrease in ozone thickness corresponds to a 2% increase in UV-B radiation. Consequently, many studies carried out during the past two decades focused on determining ozone concentrations (Madronich 1993; Kirchhoff et al. 1997), UVR fluxes reaching the Earth’s surface (Madronich 1993; Bothwell et al. 1994; Frederick et al. 1994), and effects on various organisms and ecosystems (Häder et al. 1998; Perin and Lean 2004). In fact, aquatic organisms are very sensitive to ambient levels of UVR. The deleterious effects produced by the short-wavelength components of the solar spectrum include, among others, damage to genetic material (DNA), inhibi- tion of photosynthetic rates, increased mortality rates, inhi- bition of growth rates, and changes in motility (Cullen and Neale 1994; Häder et al. 1995; Sommaruga et al. 1996; Kirchhoff et al. 1997). The attenuation of UVR in water is controlled by pure water and its contents, including dissolved matter, nonalgal particulate matter, and phytoplankton (Kirk 1994). In dif- ferent aquatic ecosystems, UV-B attenuation coefficients range from 0.02 to 60 m -1 (Williamson et al. 1996; Vincent et al. 1998; Arts et al. 2000; De Lange 2000; Rae et al. 2001; Sommaruga 2001; Huovinen et al. 2003; Frost et al. 2005). Y.L. Zhang (*) · E.L. Zhang · B.Q. Qin State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China Tel. +86-25-8688-2198, Fax +86-25-5771-4759 e-mail: ylzhang@niglas.ac.cn M.L. Liu Nanjing Agriculture University, Nanjing, China X. Wang Metrology Institute of Hebei Province, Shijiazhuang, China Received: February 22, 2007 / Accepted: June 19, 2007 / Published online: October 5, 2007