基于生态化学计量学的三峡水库典型支流食物网养分循环及驱动机制研究

Eutrophication of the Three Gorges Reservoir tributaries is the prominent expression of ecological and environmental issues for the Three Gorges Reservoir. Due to the imperfection of the tributary aquatic ecosystem structure, feeding pressure to the phytoplankton would be weak, and the nutrients (carbon, nitrogen, phosphorus, etc.,) circulation would be blocking, which may be the main cause of algal bloom occurrences. The nutrient cycle of trophic levels in the food web based on ecological stoichiometry provides an insight into the study of eutrophication mechanisms for water bodies. This project focuses on understanding the temporal and spatial dynamics of the Three Gorges Reservoir tributary food webs through ecological stoichiometry and field investigations, and to study the ecological stoichiometry relationship between different trophic levels within the food web. We intend to analyze the impact of environmental factors on the ecological stoichiometry characteristics of each trophic level in food web from the bottom-up effect, and to evaluate the influence of the consumer-driven nutrient cycle on the producers’ nutrient limitation from the top-down effect. Additionally, we also plan to study the inherent relationship between the ecological stoichiometry characteristics of each trophic level within the food web, and to investigate the blocking effects of nutrient cycles during phytoplankton bloom events, and to clarify the relationship between ecological stoichiometry cycle and algal bloom occurrences. The results from this study will underpin the ecological restoration of the eutrophic waters in the Three Gorges Reservoir tributaries.

三峡水库支流富营养化问题是三峡水库生态环境问题的突出表现，支流水生态系统结构不完善，对浮游植物的牧食压力较弱，碳、氮、磷等营养物质循环不畅，可能是导致水华发生的主要原因。基于生态化学计量的食物网营养级养分循环研究为水体富营养化机制研究的提供了一个新思路。本项目拟通过调查试验掌握三峡水库典型支流水体食物网生态化学计量的时空变化格局，研究食物网各营养级之间的生态化学计量关系。从上行效应角度分析环境因子对食物网各营养级生物的生态化学计量特征的影响，从下行效应角度分析消费者驱动养分循环对生产者营养限制的影响；研究水华过程中食物网各营养级的生态化学计量特征的内在关系，探索水华期间食物网营养物质循环的阻隔效应，阐明食物网生态化学计量循环与水华发生的响应关系，为三峡库区支流富营养化水体生态恢复提供技术支撑。

本项目通过调查试验掌握三峡水库典型支流水体食物网生态化学计量的时空变化格局，研究了食物网各营养级之间的生态化学计量关系。小江鱼类、浮游植物、浮游动物碳（C）、氮（N）、磷（P）含量变化较大，其中P含量的变异系数最大。小江不同食性的鱼类C、N、P含量差异不显著，表明消费者的生态化学计量特征不像初级生产者那样易受环境变化的影响，进一步印证了化学计量稳态机制。小江水体食物网中，被捕食者会影响捕食者的生态化学计量，但跨级向上层传递效果并不明显，浮游植物的C、N、P含量可以显著影响浮游动物的C、N、P含量，但对鱼类体内C、N、P含量尚未有显著影响，而浮游动物的C、N、P含量对除草食性鱼类以外的鱼类C、N、P含量均有显著影响。小江食物网不同种群N:P呈现鱼类 > 浮游动物 > 浮游植物，说明食物网中捕食者能储存更多的P在体内，在营养物质再循环过程中释放较高的N:P，一定程度加强了浮游植物的P限制，使水体和浮游植物的N:P升高，减少水体富营养化的可能性。此外，研究结果还发现水华期鱼类N:P低于非水华期，这可能是水华期鱼体内P的滞留和N的释放造成的，表明鱼类对P的循环利用能力显著低于N，水体P含量是水生态系统中养分循环的主要限制因素。水华期采取放养鱼类措施，通过下行效应，降低P在食物网中的养分循环，一定程度上减缓水体富营养化。消费者驱动的养分循环研究表明，滤食性鱼类氮排泄率最高，滤食性鱼类和肉食性鱼类磷排泄率相对较高；通过初步估算，小江鱼类氮、磷排泄率分别为17.01ng/L/d、2.11ng/L/d；氮磷排泄对藻类养分循环贡献较小，分别占初级生产力所需氮、磷含量的0.147‰和0.220‰，不会对藻类生物量的增加产生显著影响。