巢湖藻类碎屑有机碳调节水华种类演替的机制研究

Lake eutrophication and cyanobacterial bloom act as one of the most serious environmental problems hindering economic advances and social progresses in China. Characterized by shift of blooming species between Microcystis and Anabaena, eutrophication in Lake Chaohu is becoming more and more serious. In this project, using the methods of ecological investigations in large scales, combined with algal cultures and molecular bio-technologies, the biogeochemical cycling processes of nitrogen and phosphorus in sediments subjected to deposition of massive algal detritus is described, together with their microbial basis, linkage between enrichment of organic carbon with different sources and compositions in sediment and deficiency of nitrogen relative to phosphorus in water column is revealed, and finally responding modes by phytoplankton communities are demonstrated.The objective of this study is to test the hypothesis that enrichment of organic carbon alters cycling pathways of both nitrogen and phosphorus thereby driving shift between dominant species of cyanobacterial bloom. In details, large amount of organic carbon produced in process of cyanobacterial bloom can regulate functions of sediment to act as sink or source of nutrient and alter concentration and ratio of nutrients limiting algal growth in water column, and finally trigger shift between dominant species of blooming algae with different strategies of nutrient utilization. The performance of this project will greatly facilitate to have a deep insight into the mechanisms behind lake eutrophication and cyanobacterial bloom, by which suitable target and time for cyanobacterial bloom controlling can be determined. Finally, the principles of this eco-technology can be further reviewed. Hence, the project is of great theoretical and practical significance.

湖泊富营养化与蓝藻水华已成为制约我国经济发展与社会进步的严重环境问题之一。以微囊藻和鱼腥藻水华演替为典型特征的巢湖富营养化日趋严重，但相关机理仍不甚明了。本项研究采用野外调查、实验湖沼学研究与室内模拟相结合的方法，系统描述接受不同水华藻类有机碎屑大量堆积的湖泊沉积物中氮与磷循环的生物地球化学过程与微生物学基础，阐明沉积物不同来源和性质的有机碳富集与水柱氮磷丰缺程度的关系以及藻类的响应模式，验证有机碳改变氮磷循环途径，进而驱动水华藻类演替的科学假设，即藻类水华过程中形成的大量有机碳可调节沉积物作为营养源或汇的功能，并改变水柱中关键限制性营养元素的浓度和比例，进而促发具有不同营养利用策略的藻类优势种群的更迭。项目的实施将极大地有利于深入揭示湖泊富营养化与蓝藻水华的机理，并据此提出蓝藻水华控制的首要目标与适宜时机，从而进一步阐明高效控制水华的技术原理，因而具有重要的理论与实践意义。

湖泊富营养化已成为目前我国面临的最为严重的水环境问题之一，其格局、过程与机理仍亟待深入研究。本项研究以巢湖为实验对象，辅之以武汉市湖泊，系统分析了不同季节水柱和沉积物的理化参数以及浮游植物的密度与组成，测定了沉积物总有机碳和硫化物含量与硝态氮还原速率，并用分子生物学手段分析了蓝藻附着菌的功能和沉积物硝酸盐异化还原为铵（DNRA）细菌的丰度与群落组成。利用统计方法探讨和归纳了上述变量之间的关系，描述了接受不同水华藻类有机碎屑大量堆积的湖泊沉积物中氮与磷循环的生物地球化学过程，以期阐明沉积物藻类碎屑的沉降与水柱氮和磷丰缺程度的关系，进而揭示其在鱼腥藻与微囊藻演替过程中的作用，并据此提出蓝藻水华控制的首要目标与适宜时机。结果表明，巢湖冬春季鱼腥藻占优势，至夏秋季则以微囊藻为主，磷是限制鱼腥藻生长的关键营养元素，而微囊藻的生长则主要受制于无机氮的供应。与微囊藻相比，鱼腥藻具有明显较多的附着解磷细菌（PSB），且二者几乎不存在共有种，但微囊藻附着PSB却显示更强的解磷能力，故能助其在水华过程中形成优势，凋亡沉积之后，该附着菌亦能分解和释放更多的生物可利用性磷，从而有利于磷需求更高的鱼腥藻的繁盛。解磷细菌的添加实验为上述推断提供了进一步的佐证。此外，鱼腥藻附着细菌承载了不同类型的硫酸盐还原基因，而微囊藻缺乏之。再者，藻类密度和组成与环境因子的相关分析结果显示，沉积物DNRA过程是巢湖氮保持的主要途径，相关群落和活性与可利用有机碳以及硫化物等两种电子供体的功能菌密切相关。因此，鱼腥藻衰亡沉积之后，其附着菌将有效介导硫酸盐的还原，促进硫化物的生成，从而有力推动DNRA作用，产生更多的铵态氮，为微囊藻水华的发生提供其迫切需要的丰富的无机氮营养。总之，巢湖鱼腥藻和微囊藻碎屑的产生与沉积可调控水中氮与磷的丰缺程度和转换节律，为二者的演替创造适宜的营养条件。氮磷双控是湖泊治理和管理的重要策略。