Impacts of cyanobacterial blooms outbreak and decline on chromophoric dissolved organic matter in Lake Taihu

Abstract

利用平行因子分析（PARAFAC）技术对藻华暴发季节太湖梅梁湾和开敞区水样中有色可溶性有机物（CDOM）的三维荧光光谱进行分析，获得代表类酪氨酸、类色氨酸和类腐殖酸等3种荧光组分，这3个组分的荧光得分值均与叶绿素a浓度呈极显著正相关，其中类腐殖酸物质荧光得分值占总分值的比例也与叶绿素a浓度极显著正相关，由此可以推测，蓝藻水华可能是太湖CDOM的一个重要来源，并极大地影响了湖泊CDOM的组成结构.为进一步探索蓝藻水华对湖泊CDOM的影响，于太湖梅梁湾湖岸进行藻华暴发和衰亡模拟试验.PARAFAC结果显示，模拟试验水样CDOM的三维荧光图谱同样含有3个组分，其中类酪氨酸和类色氨酸荧光峰位置与湖泊水样基本相同，而类腐殖酸物质荧光峰则有一定程度的蓝移，表明培养试验中有新的内源类腐殖酸物质产生.利用PARAFAC模型中的得分值计算后发现，初始叶绿素a浓度越高，类腐殖酸物质所占比例越大，而实验结束时添加了高浓度和中等浓度藻的2个处理中类腐殖酸物质对荧光的贡献率也有显著提高.因此，藻华的长期暴发可以显著改变湖泊CDOM的组成，导致类腐殖酸物质比例提高.;Parallel Factor Analysis (PARAFAC) was applied to assess the composition of chromophoric dissolved organic matter (CDOM) from the excitation-emission matrix (EEM) fluorescence spectra of the water samples collected from Meiliang Bay and the open area in Lake Taihu during a cyanobacterial bloom event, and three fluorescent components were identified:a tyrosine-like component, a tryptophan-like component and a humic-like component. Correlation analysis showed that the concentrations of chlorophyll-a(Chl.a) were significantly correlated with the fluorescence intensities of the three components, respectively, as well as the proportion of humic-like to total fluorescence intensities, suggesting that cyanobacteria might be an important source of CDOM and can greatly change the composition of CDOM in Lake Taihu. For further investigation, a simulation experiment was performed on the shore of Meiliang Bay. PARAFAC model showed that the number of components and the spectral characteristics of CDOM EEM data from the simulation experiment are similar in terms of the number of fluorescence peaks and their position to previously identified components in the water samples from Lake Taihu, except that the fluorescence peaks of humic acids showed a certain degree of blue shift, which suggested that endogenous humic substances were produced during the simulation experiment. It was found that the higher initial concentrations of Chl.a, the greater contribution of humic acids to total CDOM fluorescence intensity derived by the PARAFAC model. Moreover, at the end of the experiment, the ratios of humic substances to total fluorescent substances increased significantly in the two treatments with relatively high concentrations of cyanobacteria. Thus, the long-term outbreak of cyanobacterial blooms can significantly alter the composition of the CDOM in lakes, resulting in an increase of the proportion of humic-like substances.