Nitrogen reduction causes shifts in winter and spring phytoplankton composition and resource use efficiency in a large subtropical lake in China

Aquatic ecosystems occasionally show ecological thresholds, defined as the point at which there is a sudden shift in production, trait or biomass or where changes in an environmental driver create nonlinear responses at the ecosystem level. Previous studies of lakes have mainly focused on how reduction in particularly phosphorus (P) concentrations helps to create a shift in lakes from a turbid to a clear state (re-oligotrophication), whereas the effect of nitrogen (N) reduction is less well studied. Here, by analysing a 28-year monthly monitoring dataset (from December 1991 to November 2019) from the subtropical, large eutrophic Lake Taihu, China, we identified a sudden shift in phytoplankton biomass and composition that coincided with a pronounced change in ecosystem functions, for example, resource use efficiency (RUE), during a period with reduction of the external nutrient loading. The changes were particularly strong in winter–spring where a sudden decrease in N concentrations was accompanied by a sudden increase in diatom biomass and phytoplankton RUE and a shift from green algae and flagellate co-dominance to dominance of diatoms. Structural equation modelling further indicated that ammonium reduction led directly to increases in winter–spring phytoplankton RUE and diatom biomass. Repeated fish stocking likely also contributed to the changes in biomass and RUE. Our study provides new insight into the ecological responses to N loading reduction and contributes to the understanding of lake responses to early re-oligotrophication, which was pronounced mainly in the colder seasons in subtropical Lake Taihu, similar to findings in the early phase of re-oligotrophication in numerous temperate lakes.

水生生态系统偶尔会出现生态阈值（ecological threshold），即生态系统的生产力、性状或生物量发生突变的临界点，或是环境驱动因子变化引发生态系统层面非线性响应的节点。以往针对湖泊的研究多聚焦于降低磷（P）浓度如何推动湖泊从浊水态向清水态转变（即复贫营养化（re-oligotrophication）），而氮（N）降低的相关效应则较少被深入探讨。本研究通过分析中国亚热带大型富营养化湖泊太湖1991年12月至2019年11月的28年逐月监测数据集，发现在外源营养盐负荷降低的阶段，浮游植物生物量与群落组成发生突变，且这一突变与资源利用效率（Resource Use Efficiency, RUE）等生态系统功能的显著变化相契合。该变化在冬春季节尤为显著：氮浓度骤降的同时，硅藻生物量与浮游植物资源利用效率均大幅提升，浮游植物群落也从绿藻与鞭毛藻共优势态转变为硅藻主导态。结构方程模型（Structural Equation Modeling, SEM）进一步表明，铵态氮浓度降低直接推动了冬春季浮游植物资源利用效率与硅藻生物量的提升。周期性的鱼类放养也可能对生物量与资源利用效率的变化起到了推动作用。本研究为氮负荷降低的生态响应机制提供了新视角，也助力我们理解湖泊早期复贫营养化过程——该过程在亚热带太湖的主要表现为低温季节的显著变化，这与众多温带湖泊复贫营养化初期的研究结果相一致。