Characteristics and driving factors of the long-term shifts between <i>Microcystis</i> and <i>Dolichospermum</i> in Lake Taihu and Lake Chaohu

Abstract

随着我国湖泊治理力度的加大，太湖和巢湖的营养盐水平，特别是氮水平近年来明显下降，如2007年以后太湖总氮和近年来巢湖的氨氮水平都呈现下降趋势，但是2个湖泊水华蓝藻的优势种却向相反的方向演化，太湖的长孢藻（Dolichospermum）比例在增加，而巢湖的长孢藻比例却在降低，为阐明这种变化过程和驱动因素，本研究利用太湖（1993—2015年）和巢湖（2012—2018年）的历史数据分析了2个湖泊中的水华蓝藻——微囊藻（Microcystis）和长孢藻生物量的历史变化过程，并结合营养盐数据分析了影响2种水华蓝藻变动的驱动因素.结果显示：太湖和巢湖的微囊藻生物量多年来始终保持高位波动，近年来均有升高的趋势，这与2个湖泊磷的高位波动具有明显的相关性，磷是决定微囊藻生物量长尺度变化的主要驱动因素；太湖的长孢藻生物量呈现较大波动变化，2007年以后明显升高，巢湖的长孢藻生物量则明显下降，氮与长孢藻生物量呈现负相关关系，而且这种负相关仅在低磷浓度时具有显著性.微囊藻生物量对磷浓度变化敏感的正反馈响应是其水华形成的重要机制之一，在高温高磷条件下，微囊藻可以快速繁殖，并竞争性排除长孢藻，从而形成优势；而长孢藻可以通过温度生态位和固氮两种方式占据优势，在氮浓度相对较低，且温度低于微囊藻形成水华的温度范围时，长孢藻可以依靠温度生态位的优势形成水华，而在氮限制的条件下，即使在夏季高温时，长孢藻依然可以利用固氮作用形成水华，但是关键的温度阈值和开始固氮的氮浓度阈值仍不清楚.基于2种水华蓝藻对营养盐变化响应的差异，建议在进行蓝藻水华治理、污染削减过程中，应针对不同水华蓝藻的特性进行分时段分类别的营养盐控制策略.;With increasing nutrient management efforts in China, the nutrient levels, especially nitrogen level, have shown significantly decreasing trends recently in Lake Taihu and Lake Chaohu. For example, the total nitrogen in Lake Taihu has decreased significantly since 2007, and NH₃-N in Lake Chaohu also showed decreasing trend. However, the interspecific variation of bloom-forming cyanobacteria in the two lakes showed different patterns. The dominance of Dolichospermum increased in Lake Taihu, and decreased in Lake Chaohu. In order to illuminate the shift process and driving factors, we analysed the long-term dynamics between Microcystis and Dolichospermum biomass in Lake Taihu (1993-2015) and Lake Chaohu (2012-2018). Our results showed that Microcystis biomass in the two lakes maintained high level in the past years. The dynamics of Microcystis biomass was related to the variation of phosphorus level in the two lakes, and phosphorus was the primary driving factor for the variation of Microcystis biomass. Dolichospermum biomass fluctuated during the past decades in Lake Taihu, and increased significantly since 2007. While that in Lake Chaohu decreased obviously. Nitrogen was the main factor for the variation of Dolichospermum biomass. The Microcystis biomass was sensitive to the variation in phosphorus level, and Microcystis would thrive rapidly and competitively exclude Dolichospermum under high temperature and phosphorus level condition. Dolichospermum could dominate by two ways: the advantage in low temperature niche and fixing N₂. If the nitrogen level was relatively low, and the temperature was lower than that in where Microcystis dominate, Dolichospermum would dominate by the advantage in low temperature niche. If the nitrogen was limiting, Dolichospermum would dominate by fixing N₂, even in summer. However, the key temperature and nitrogen values are not clear. Our findings will be helpful for making policy about nutrient management, which will be different in window period and nutrient classes depending on Microcystis and Dolichospermum.