【作者】 李洪斌；

【导师】 叶恒奎； 侯国祥；

【作者基本信息】 华中科技大学 ， 船舶与海洋结构物设计制造， 2007， 博士

【摘要】 本文的研究工作主要着眼于利用神经网络模型对藻类水华进行建模和预测。一方面期望通过结合敏感分析方法来促进我们对于环境因素对藻类水华影响的认识,另一方面期望能够获得具有良好性能的藻类水华预测模型。论文第一章首先介绍水体富营养化和藻类水华问题,以及藻类水华建模研究的主要目的。然后对藻类水华建模(藻类生物量建模)的研究文献进行综述,并介绍了神经网络方法在这一领域的应用。最后,在文献综述的基础上提出了本文的主要研究方法和研究内容。第二章是BP神经网络在滇池藻类水华建模的实证研究。滇池是一个水体浑浊的浅水湖泊,位于中国的西南地区。自1985年以来,滇池已经遭受过多次严重的水华,引起水华发生的藻类中最主要的是微囊藻(Microcystis spp.)。在水华极端严重的情形下,滇池中藻类生物量曾超过每升30亿个。为了预测滇池中微囊藻(Microcystis spp.)的生物量变化以及探索微囊藻(Microcystis spp.)生物量的动力学特性,作者开发了一个基于BP神经网络的模型。将模型预测的藻类生物量和实际观测值进行相关分析,相关系数R2(correlation coefficient)达到0.911。基于训练后得到的神经网络,我们进行了敏感分析。敏感分析的主要目的是研究不同环境因素的变化对微囊藻(Microcystis spp.)生物量变化的影响。敏感分析的结果表明一个较小的pH增量,可以引起微囊藻(Microcystis spp.)生物量的显著减少。并且,针对原始数据的进一步研究表明:微囊藻(Microcystis spp.)生物量对pH值升高的反应依赖于藻类生物量本身以及pH值的高低。当滇池中微囊藻(Microcystis spp.)生物量和pH值处于中等或较低水平时,pH值的增加更倾向于引起微囊藻(Microcystis spp.)生物量的增加;反之,当滇池中微囊藻(Microcystis spp.)生物量和pH值处于较高水平时,pH值的增加则倾向于引起微囊藻(Microcystis spp.)生物量的减少。总之,滇池中微囊藻生物量对pH变化的独特反应特性可由极高的藻类生物量和较高的pH值水平的来进行解释。本章还对其它环境变量的变化对藻类生物量的影响进行了阐述。所有环境变量中,水温一个标准差的增量对微囊藻(Microcystis spp.)生物量的正面影响最强。化学需氧量(chemical oxygen demand,COD)和总磷(total phosphorus,TP)也和微囊藻(Microcystis spp.)生物量具有较强的正相关性。而总氮(total nitrogen,TN)、五日生物需氧量(biological oxygen demand in five days,BOD5)和溶解氧(dissolved oxygen,DO)与微囊藻(Microcystis spp.)生物量之间只有较弱的相关性。此外,透明度(transparency,Tr)和微囊藻(Microcystis spp.)生物量之间具有中等程度的正相关。第三章将径向基函数(Radial basis function,RBF)神经网络应用于澳大利亚Darling河的水华建模研究。结果表明,训练后RBF神经网络能够准确预测Darling河中引起水华的两种主要藻种,Nostocales spp(.后文简写为Nostocales)和Anabaena spp.(后文简写为Anabaena),的生物量。同时,我们还进行了敏感分析研究,以阐明Nostocales和Anabaena的生物量动力学特征。敏感分析的结果表明,总动氮(total kinetic nitrogen)对这两类藻的生长都有非常强的正面影响。其次,水电导率与这两类藻的藻生物量之间具有非常强的负相关关系。第三,河水流量也确认为水华形成的一个非常突出的原因。在本章中,我们还通过散点图技术较为形象地展现了较高的河水流量可以显著减少两种藻的藻生物量这一事实。最后,研究结果表明其它变量,如透明度、水颜色和pH值,对Darling河水华的形成没有前面所述因素那么重要。第四章通过分析澳大利亚Darling河Nostocales藻(Nostocales spp.)的历史数据,首先阐明Nostocales藻(Nostocales spp.)生物量时间序列的非平稳特性。进而说明淡水生态系统倾向于呈现出非平稳特性,而不是平稳特性。而非平稳特性则意味着只有系统“近期的过去”才能够预测系统“近期的未来”。然而,在所有过去的藻生态预测和建模的研究工作中,研究者都没有认真对待和关注系统的非平稳特性。在本章的研究中,作者构造了一种结合时间窗口技术的径向基函数神经网络方法来进行非平稳藻生态时间序列的建模与预测研究,并具体用来预测Darling河Nostocales藻(Nostocales spp.)生物量时间序列。结果表明,结合时间窗口技术的径向基函数神经网络模型能够有效地预测Nostocales藻(Nostocales spp.)水华的发生及其强度。此外,基于多个结合不同尺寸时间窗口的径向基函数神经网络模型,我们特别构造了一个智能组合器将多个径向基函数模型的预测结果进行智能选择,并得到了比单个径向基函数模型更好的预测结果。第五章对全文进行总结,并对未来的研究进行了展望。更多还原

【Abstract】 This paper focuses on the modeling and forecasting algal blooms using neural networks. While we want to further our knowledge with neural network models incorporated with sensitivity analysis, we also want to realize some forecasting models with good performance.Chapter 1 introduced first the eutrophication, algal blooms problem, and the purpose of modeling algal blooms. Then the previous researches were summarized, and the applications of neural networks in this field were introduced. In the end of the chapter, the main research methods and research efforts in this paper were outlined.The empirical research of modeling algal blooms in Lake Dianchi by BP neural network was outlined in Chapter 2. Lake Dianchi is a shallow and turbid lake, located in Southwest China. Since 1985, Lake Dianchi has experienced severe cyanabacterial blooms (dominated by Microcystis spp.). In extreme cases, the algal cell densities have exceeded three billion cells per liter. To predict and elucidate the population dynamics of Microcystis spp. in Lake Dianchi, a neural network based model was developed. The correlation coefficient (R2) between the predicted algal concentrations by the model and the observed values was 0.911. Sensitivity analysis was performed to clarify the algal dynamics to the changes of environmental factors. The results of sensitivity analysis based on the trained neural network model suggested that small increases in pH could cause significant reduction of algal abundance. And further researches revealed that the response of Microcystis spp. biomass to pH increase depended on the algal biomass itself and the pH level. With moderate or low level of algal biomass and pH in Lake Dianchi, pH increase was likely to lead to an increase of Microcystis spp. biomass. Otherwise, when pH and algal biomass were high, pH increase was likely to reduce the Microcystis spp. biomass. It is concluded that the extremely high concentration of algal population and high pH could explain the distinctive response of Microcystis spp. population to +1 SD (standard deviation) pH increase in Lake Dianchi. Further, another hypothesis presented in the paper was that the higher abundance of algal population and the higher pH, an increase of pH would be more likely to have a negative (or strong negative) influence on algal growth in an extremely high eutrophicated water such as Lake Dianchi. And the paper also elucidated the algal dynamics to changes of other environmental factors. One SD (standard deviation) increase of water temperature (WT) had strongest positive relationship with Microcystis spp. biomass. Chemical oxygen demand (COD) and total phosphorus (TP) had strong positive effect on Microcystis spp. abundance while total nitrogen (TN), biological oxygen demand in five days (BOD5), and dissolved oxygen had only weak relationship with Microcystis spp. concentration. And transparency (Tr) had moderate positive relationship with Microcystis spp. concentration.In Chapter 3, radial basis function (RBF) neural network were applied for modelling the abundance of cyanobacteria. The trained RBF neural network could predict with high accuracy the population of the two bloom forming algal taxa, Nostocales spp. and Anabaena spp., in River Darling, Australia. To elucidate the dynamics of algal population for both Nostocales spp. and Anabaena spp. Sensitivity analysis was performed. Some hypothesis could be obtained from the results of sensitivity analysis. First, total kinetic nitrogen had a very strong influence on the abundance of the two algal taxa. Second, electrical conductivity had a very strong negative relation with the population of the two algal species. In other words, more dissolved salts or ions in the water, the algal population are more likely to decrease. Third, flow was identified as one predominant factor on algal blooms after scatter plot clarified the fact that high flow could reduce significantly the algal biomass for both Nostocales spp. and Anabaena spp. in River Darling. Other variables such as turbidity, colour, and pH were less important in determining the abundance and succession of the algal blooms.Chapter 4 demonstrated first the nonstationarity of algal population behaviors by analyzing the historical populations of Nostocales spp. in the River Darling, Australia. Freshwater ecosystems were more likely to be nonstationary, instead of stationary. And nonstionarity implies that only the near past behaviors could forecast the near future for the system. However, nonstionarity was not considered seriously in previous research efforts for modeling and predicting algal population behaviors. Therefore the moving window technique was incorporated with radial basis function neural network (RBFNN) approach to deal with nonstationarity when modeling and forecasting the population behaviors of Nostocales spp. in the River Darling. The results showed that the RBFNN model could predict the timing and magnitude of algal blooms of Nostocales spp. with high accuracy. Moreover, a combined model based on individual RBFNN models was implemented, which showed superiority over the individual RBFNN models. Hence, the combined model was recommended for the modeling and forecasting the phytoplankton populations, especially for the forecasting.The last chapter concluded the paper and made some comments on further researches.更多还原