太湖典型草、藻型湖区紫外辐射的衰减及影响因素分析

Attenuation of solar ultraviolet radiation and analysis of attenuators in typical macrophytic, algal lake zones of Lake Taihu

2004年4月通过野外原位观测和实验室测定相结合的方法对东太湖和梅梁湾典型草、藻型湖区紫外辐射光谱衰减及影响因素进行了研究.结果表明,320 nm(UV-B)、380 nm(UV-A)的衰减系数在6.33～19.59 m-1、3.41～13.64 m-1间变化,对应的1%表面光强穿透深度分别为0.24～0.73 m、0.35～1.35 m,到达湖面的99%UV-B辐射在0.5 m左右表层水就衰减完毕,东太湖和梅梁湾紫外辐射衰减系数存在明显的湖区差异;溶解性有机碳(DOC)的浓度在6.60～17.17 mg/L间变化,其均值为(9.99±2.48)mg/L;375 nm波长处CDOM吸收系数为1.78～6.25 m-1,均值为(3.70±1.10)m-1;在短波部分CDOM吸收与DOC浓度存在显著性相关,相关性大致随波长降低而增加,320 nm处的线性关系式:ad320=0.885DOC+2.182;紫外辐射衰减主要受制于水体中的CDOM浓度,衰减系数与DOC浓度、CDOM吸收系数存在显著性相关,340 nm处的关系式分别为:Kd340=0.82+1.05DOC、Kd340=1.98+1.49ad340.在太湖紫外辐射衰减还要受悬浮物和叶绿素a浓度的影响,衰减系数与DOC、叶绿素a和悬浮物浓度多元回归的结果明显要高于单独与DOC浓度或CDOM吸收系数的回归结果.

The attenuation of solar ultraviolet radiation （UV） in typical macrophytic, algal lake zones of Lake Taihu was evaluated based on two field investigations and lab analysis in April 2004. Spectral underwater UV and photosynthetically available irradiance （PAR, 400-700nm） was measured at several depths at 19 sampling stations. PAR was measured usinga 192SA cosine sensor, and UV was measured with a scanning spectroradiometer （SR-9910, Macam Photometrics, Livingston, Scotland ） equipped via a 4 m light guide with a side-view cosine sensor positioned horizontally. Chlorophyll a concentration was calculated from spectrophotometric measurements after extraction in 90% hot ethanol. Absorption spectra were obtained between 240 and 800 nm at 1-nm intervals using a Shimadzu UV-2401PC UV-VIS recording spectrophotometer equipped with matching 4-cm quartz cells. Other parameters including Secchi disc transparency, total suspended solid （SS） were measured according to the Chinese CERN standard methods of ＂Observation and analysis of lake ecological survey＂. SPSS 11.0 software was applied to all data analysis. Diffuse attenuation coefficients at 320 nm and 380 nm were ranging from 6.33 to 19.59 m^-1, and from 3.41 to 13.64 m^-1 , respectively. The euphoric depths of 1% subsurface irradiance at 320 nm and 380 nm ranged from 0.24 to 0.73 m and from 0.35 to 1.35 m. About 99% of the UV-B radiation was attenuated in the upper half a meter of the water column in Lake Taihu. The corresponding attenuation in the UV-A was attenuated in the upper one meter. In the humic water in two river mouths of sampling 0# and 6# , UV-B radiation was attenuated to 1% of the subsurface irradiance within the top 25 cm of the water column. Significant differences in diffuse attenuation coefficients and attenuation depths were found in East Lake Taihu and Meiliang Bay. Dissolved organic carbon （DOC） concentration ranged 6.60 to 17.17 mg/L with an average of 9.99±2.48 mg/L and CDOM absorption coefficient ranged from 1.78-6.25 m^-1 with an average of 3. 704±1.10 m^-1 at 375 nm. The results showed a good linear correlation between CDOM absorption and DOC concentration for the 280 to 600 nm wavelength interval. The linear regression equation between DOC concentration （mg/L） and absorption coefficient （a： unit m^-1） at 320 nm was： ad32o = 0. 885DOC＋2. 182. The UV diffuse attenuation coefficients were strongly influenced by DOC concentrations and CDOM absorption coefficients. The linear regression equations between attenuation coefficients at 340 nm, DOC concentration and CDOM absorption coefficients at 340 nm were： Kd340 = 0. 82＋1.05DOC and Kd340 = 1.98 ＋ 1. 49ad340, respectively. The coefficients of determination between UV radiation diffuse attenuation coefficients and CDOM absorption coefficients are higher than those between UV radiation diffuse attenuation coefficients and DOC concentration, which indicated that not only DOC concentration but also DOC chemical composition was important in determining the penetration of UV into the lake. Differences in the carbon specific absorption can result in different UV attenuation at similar DOC concentration. Furthermore, the UV radiation attenuation was also affected by total suspended solids and chlorophyll a in addition to DOC in Lake Taihu. The multiple linear regressions show that the coefficients of determination for the relationship between UV diffuse attenuation coefficients and DOC concentration （or CDOM absorption coefficients）, total suspended solids, chlorophyll a are obvious higher than those of between UV diffuse attenuation coefficients and DOC concentration （or CDOM absorption coefficients）.