Spectral attenuation of ultraviolet and visible radiation in lakes in the Yunnan Plateau, and the middle and lower reaches of the Yangtze River, China

Solar radiation spectra were measured in China in 2006 and 2007 in situ in 16 lakes with 42 stations in the Yunnan Plateau (YP) and in 4 lakes with 41 stations in the middle and lower reaches of the Yangtze River (YR) to determine the principal factors influencing ultraviolet radiation (UVR) and photosynthetically active radiation (PAR). The diffuse attenuation coefficient for PAR (Kd(PAR)) varied from 0.19 to 2.88 m-1 in the YP, which was significantly lower than the values that were measured in the YR, which ranged from 0.55 to 4.52 m-1 (t-test, p < 0.001). In the UV-B region (320 nm), Kd(320) ranged from 0.59 to 18.56 m−1 in the YP, significantly lower than that in the YR, which ranged from 4.80 to 20.64 m−1 (t-test, p < 0.001). The corresponding 1% attenuation depths ranged from 0.25 to 7.81 m in the YP lakes and 0.22 to 0.96 m in the YR lakes (t-test, p < 0.001). Partial correlation analyses were used to partition the importance of every optically-active substance on UVR and PAR attenuation in the two different regions. In the YP lakes, chromophoric dissolved organic matter (CDOM) absorption explained most of the variability in the UV-B (320 nm) and UV-A (360 nm) attenuations (partial correlation: r = 0.911, p<0.001; r = 0.820, p <0.001, respectively). Tripton and phytoplankton made no significant contributions to UVR attenuation. Tripton, phytoplankton and CDOM all made significant contributions to PAR attenuation with the highest partial correlation corresponding to tripton. In the YR lakes, tripton, phytoplankton, and CDOM all made significant contributions to UVR attenuation. The contribution of CDOM to UVR decreased with increasing wavelength, but the contribution of tripton and phytoplankton to UVR increased with increasing wavelength. Tripton contributed the most to UV-A attenuation. Tripton and phytoplankton made significant contributions to PAR attenuation with a higher partial correlation for tripton but CDOM had no significant contribution to PAR attenuation. The absorption and attenuation data of UVR and PAR in this study showed a shift from absorption-dominated Kd in the YP lakes to scattering-dominated Kd in YR lakes.