太湖表层混合层与真光层深度的配置对藻类生产力的影响研究

With the combination effect of the wind-induced mixture and the heat exchange between air and water, the thermal stratification may be appear, which means that the surface mixed layer might establish in the upper layer. The water temperature and density in mixed layer is vertically homogenized. The ratio of the surface mixed-layer depth to the euphotic depth is a key physical factor which influences algal photosynthesis efficiency and the succession of the phytoplankton. Under the effect of the surrounding terrain and the suitable meteorological fields, there exits diurnal stratification in Taihu Lake where the surface mixed layer depth varies frequently and its spatial distribution is remarkably different. Additionally, due to the spatial difference of the euphotic depth and the vertical transportation of nutrients, which are caused by sediment resuspension, the ratio of the surface mixed layer depth to the euphotic depth influences greatly on the algae growth in seas and deep lakes.But the study on that influence is very little so far in such large and shallow lakes as Taihu Lake. Based on the driving fileld originated from the high spatial-temporal resolution of the meteorological data, the baroclinic dynamic model with unstructured grid and finite element method would be used to simulate the three-dimensional water temperature, and the spatial-temporal variation law of the surface mixed layer depth would furthur analyzed. At the same time, the euphotic layer depth and the profile of the algal primitive productivity were synchronously observed to explore the law of the influence of the ratio of the surface mixed-layer depth to the euphotic depth on the algal primary production. The study on Taihu Lake would be helpful for the improvement of the aquatic ecological environment in large shallow lakes.

在风生混合及水-气之间的热量交换作用下，水体可能出现分层现象，即在其上部形成一种温度、密度均匀的表层混合层，该混合层深度与真光层深度的配置是影响藻类光合作用效率及其种群演替的关键物理过程。太湖在周边地形和适合的气象条件作用下，存在日分层现象且其表层混合层深度变化频繁、空间分异明显；加之底泥再悬浮作用造成真光层深度的空间变化和营养盐的垂直迁移，使得太湖表层混合层深度与真光层深度的配置对藻类生长的影响较海洋或深水湖泊更为突出。然而在太湖中，针对该配置对藻类生长影响的研究相对甚少。本研究拟通过模拟高时空分辨率的气象要素场作为驱动场，利用非结构有限元的斜压动力模式，对水体三维水温进行模拟，并由此分析表层混合层深度的时空变化规律，同时通过同步的野外观测确定真光层深度和藻类初级生产力廓线，以研究太湖表层混合层深度与真光层深度的配置对藻类生产力的影响规律，该研究为大型浅水湖泊的水环境治理和修复奠定基础。

本项目旨在探究真光层深度与混合层深度的配置对浮游植物生长的影响机制，探究蓝藻水华暴发机制。. 依据野外不同湖区的水体浊度及风速风向等气象场的观测，探讨了风速风向对太湖水体浊度的影响，风向对风速效应的修整作用可用三角函数表征，给出了耦合风向及风场时间累积效应的风速对水体浊度影响模型。同时利用FVCOM模式，较好地模拟了水体中悬浮物浓度的时空分布。. 依据水温廓线和气象场数据，诊断了水体势能异常（热力分层）的特征，揭示了太湖水体势能异常的时空变化的机制。. 依据水下光场廓线分布，利用MontleCarlo和多次散射模拟，分析了水下光能流动、漫射衰减系数模拟的不确定性及真光层深度谱的时空特征。揭示了太湖真光层深度谱的空间分异特征。. 依据春夏秋三季的水下藻类在不同的深处光能获取量、水温及营养盐水平，探究了物理、化学等过程对藻类初级生产力的定量影响。得到了太湖浮游植物光合作用的关键参数，及对浮游植物生长影响贡献的季节转换。 三季中，相对水温与营养盐而言，光能是影响浮游植物生长率最为重要的关键因子，20-30cm以下，水体中光能是浮游植物生长的关键性限制因子；水温及营养盐对浮游植物生长率的影响存在明显的季节转换：春季，温度是影响浮游植物生长次要因素。温度的升高促进藻类对光的获取和利用，提高光抑制的光能阈值，造成光限制程度的加强，藻类生长呈现光限制的深度变浅，该季中，浮游植物生长基本仅受物理过程的影响。夏、秋两季中，水温基本不影响藻类生长，营养盐是影响浮游植物生长率次要因子，随夏季向秋季的过度，磷的影响减弱而氮的影响增强。. 该研究揭示了影响太湖浮游植物生长的关键因素随季节的变化。有利于针对不同季节制定水生生态修复策略，促进健康淡水生态系统的建立。