太湖梅梁湾沿岸带水体生物学与光学特性

Biological and optical properties of Meiliang Bay in Lake Taihu

基于 1998～ 1999年周年 4季原位水下光场观测资料及中国科学院太湖湖泊生态系统研究站 1992～ 2 0 0 1年悬浮物、叶绿素 a、透明度长期历史观测资料分析了太湖梅梁湾沿岸带第 2号站点水体的生物学与光学特性 ,探讨了水下光合有效辐射(PAR)总量的日变化、垂直分布 ;光衰减系数的季节变化及光谱分布 ;影响光衰减系数的主要水色因子。结果表明 ,无论是 PAR还是光谱衰减系数其值都很高 ,其中 PAR衰减系数在 1.4 0～ 5 .30 / m间变化 ,均值为 2 .4 3± 0 .5 5 / m,秋季最大、夏季最小 ,真光层深度在 0 .87～ 3.2 9m间变化 ,均值为 1.98± 0 .4 1m;水下光谱在蓝光波段衰减最强烈 ,其次是红光、绿光 ,随着深度增加光谱成分出现绿移和红移现象 ,绿红光占得比例越来越大 ;光谱衰减系数随着波长的增加大致呈下降趋势 ,但在 6 70 nm附近有个峰值 ;基于线性相关分析发现在混浊的沿岸带水体中影响光衰减主要因子为水体中的悬浮物和有色可溶性有机物 ,叶绿素 a对 PAR衰减系数的贡献率只占到 1.5 9%～ 14 .2 1%。

The underwater light field, both light intensity and quality, has a significant influence on phytoplankton abundance and photosynthetic activity in shallow lakes. Light conditions in the lake depended on the inherent optical properties of the water and the incident irradiance. In this study, underwater light climate, biological and optical properties in the highly turbid and eutrophic littoral region of Meiliang Bay in Lake Taihu were studied by applying in situ measurement data from 1998 to 1999 and total suspended solid, chlorophyll a and Secchi disc transparency of No.2 sampling station from 1992 to 2001. The seasonal dynamics and vertical profile of light attenuation, the spectral attenuation coefficients, and the relative roles of total suspended solid, chlorophyll a as light attenuators were assessed for the period 1998 to 1999. Incident photosynthetically available irradiance (PAR, 400～700 nm) at the water surface and at different depths was measured using a 192SA light sensor. Underwater spectral irradiance was measured with a high-resolution spectroradiometer (SR-9910; Macam Photometrics, Livingston, Scotland) equipped via a 4 m light guide with a spherical light collector of 0.7 cm diameter. Phytoplankton chlorophyll a concentration was calculated according to Lorenzen from spectrophotometric measurements after extraction in 90% hot ethanol. Other parameters including Secchi disc transparency, suspended solid (SS), chemical oxygen demand (COD (Mn)), dissolved oxygen (DO), total phosphorus (TP), total dissolved phosphorus (TDP), total nitrogen (TN) and total dissolved nitrogen (TDN) 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. The attenuation of light in the waters of Meiliang Bay was rather high, with a downward attenuation coefficient of photosynthetically available radiation (PAR) ranging fro 1.40 to 5.30 /m (mean value being 2.43±0.55 /m). The corresponding euphotic depths were 0.87～3.29 m with a mean value of 1.98±0.41 m, which indicated that phytoplankton and hydrophyte basically could not acquire enough solar radiation for photosynthesis below 2 m depth especially during winter. From the subsurface irradiance, on average only a mean of 3.00% reached a depth of 1.5 m. The highest attenuation coefficient and the lowest euphotic depth of PAR were found in autumn during the period of 1998～1999. The blue waveband was the most strongly attenuated because of the higher levels of colored dissolved organic matter. Red light basically penetrated as deep as green although green was the most penetrating waveband. An obvious absorption peak near 670 nm was recorded in spring due to high chlorophyll a concentration. The contribution of chlorophyll a to PAR attenuation coefficient was 1.59%～14.21% from 1998 to 1999 at No.2 sampling station using the value of 0.016 m~~2/mgChla as average specific attenuation coefficient of chlorophyll a. The higher correlation coefficient was obtained between attenuation coefficient and total suspended solid than chlorophyll a based on linear regressions between attenuation coefficient and total suspended solid, chlorophyll a concentration. The main light attenuators were total suspended solid and colored dissolved organic matter in the littoral region of Meiliang Bay in Lake Taihu.