洪水脉冲前后鄱阳湖湖盆内"碟形湖"浮游植物生产力及群落结构变化研究

The principal driving force responsible for the existence, productivity, and interactions of the major biota in river-floodplain systems is the flood pulse. It is reported that the species composition and population densities in floodplain lakes can be affected directly or indirectly by periodic flooding. Compared with flooding process, drying process within the floodplain is also of crucial importance for ecosystem. However, the ecological meaning of low water period has long been neglected. Thus, there are few systematic studies on the variations of aquatic biota, especially for phytoplankton, in floodplain lakes after floods. According to the definition of river-floodplain systems, “saucer-like lakes” are floodplain lakes in the Poyang Lake. They play very important roles in maintaining the biodiversity and ecosystem integrity of Poyang Lake. During the low water periods, there is no hydraulic connection between the “saucer-like lakes” and main Poyang Lake. Therefore, the “saucer-like lakes” with different flooding time (exposure time) in different areas of the Poyang Lake are selected in this study. Based on the sensitivity of phytoplankton to environmental changes, variations in the productivity and community structure of phytoplankton before and after flood pulse in “saucer-like lakes”, are investigated. The combined methods of field sampling, semi-in situ nutrient enrichment bioassays, and multiple statistical analyses are conducted to determine nutrient thresholds for phytoplankton growth in “saucer-like lakes”. We believe that these research results would be helpful in enriching the application of flood pulse and reducing the risk of regional ecological disaster.

洪水脉冲是河流-洪泛区系统生物生存、生产与交互作用的主要驱动力，洪水周期性涨落直接或间接影响洪泛水体生物群落组成、种群密度等方面。与洪水过程相比，枯水过程对洪泛水体的生物生命进程具有同等重要的作用，但是以往研究却忽视了洪泛水体枯水期的生态过程，而以浮游植物为主要对象来体现洪泛水体枯水期生态过程的研究更是匮乏。“碟形湖”是鄱阳湖湖盆内枯水季节显露于洲滩之中的洪泛湖群，在维护鄱阳湖湿地生物多样性和生态系统完整性上具有十分重要的作用。本项目针对浮游植物的环境敏感性，拟选取不同出露时间、不同湖区的典型“碟形湖”，通过原位监测结合多元统计分析手段，阐明洪水脉冲前后春秋季“碟形湖”浮游植物群落结构特征和生产力变化规律及其影响机制，基于半原位营养盐添加实验，明确“碟形湖”藻类水华发生的关键因子。研究结果有助于完善洪水脉冲理论的应用，为降低洪泛湖泊的生态灾害风险提供科学依据。

洪水脉冲是河流-洪泛区系统生物生存、生产与交互作用的主要驱动力，洪水周期性涨落直接或间接影响洪泛水体中生物群落组成、种群密度等方面。“碟形湖”是鄱阳湖湖盆内枯水季节显露于洲滩之中的洪泛湖群，在维护鄱阳湖湿地生物多样性和生态系统完整性上具有十分重要的作用。本项目选取了不同出露时间、不同湖区的典型“碟形湖”，分析了洪水脉冲前后不同淹没时间的“碟形湖”浮游植物群落结构，剖析了“碟形湖”DOC和TP的变化及其对浮游植物的影响，比较分析了“碟形湖”与主湖区营养盐及chl a浓度季节变化趋势。结果显示（1）“碟形湖”营养盐TP、PO4-P、TN、HN4-N、NO3-N浓度在枯水期(12~翌年2月)明显高于其他月份，这可能是洲滩植被的腐烂分解引起的。而DOC浓度在秋季(9~11月)明显高于其它季节。硅藻(0.53~1.27 mg/L)是“碟形湖”全年优势门类，隐藻(0.04~1.15 mg/L)在冬-春季生物量显著增加，绿藻(0.11~1.03 mg/L)在夏-秋季生物量显著增加，蓝藻(4.54 mg/L)在仅6月占优。颗粒直链硅藻和脆杆藻是硅藻的优势种，隐藻的优势种是卵形隐藻，小球藻和纤维藻是绿藻的优势种，蓝藻的优势种是浮游鱼腥藻、微囊藻和浮游蓝丝藻；（2）“碟形湖”冬季DOC: TP比(14~85)最低，秋季DOC: TP比(50~298)最高。冬-春季，浮游植物生物量随着DOC: TP比增加而增加，夏-秋季，当DOC: TP < 100时，生物量随着其比值增加而显著减少的，当DOC: TP > 100后，生物量变化不显著；（3）主湖区DTN和DTP浓度显著高于“碟形湖”DTN和DTP浓度，但主湖区丰水期(6~8月)DTN和DTP浓度高于枯水期，而“碟形湖”DTN和DTP浓度枯水期高于其它季节。8月份，季节性水位连通使得两湖区DTN和DTP浓度差异逐渐减小。另外，主湖区DOC和chl a浓度均低于“碟形湖”，前者DOC浓度随季节变化不明显，而后者DOC浓度在8~10月显著增加。chl a浓度在主湖区8月份出现峰值(13.4 mg/m3)， “碟形湖”chl a浓度在7月(23.9 mg/m3)和12月(17.2 mg/m3)出现峰值。以上研究结论初步表明，洲滩植被的生长和季节性水位变化的耦合作用影响了“碟形湖”和主湖区的浮游植物初级生产力及其群落构成。