Regime shifts in a shallow lake: Consequences for taxonomic and functional diversity, and ecosystem multifunctionality

Under increasing nutrient loading, shallow lakes may shift from a state of clear water dominated by submerged macrophytes to a turbid state dominated by phytoplankton or a shaded state dominated by floating macrophytes. How such regime shifts mediate the relationship between taxonomic and functional diversity and lake multifunctionality is poorly understood. We employed a detailed database describing a shallow lake over a 12-year period during which the lake has displayed all the three states (clear, turbid, and shaded) to investigate how species richness, functional diversity of fish and zooplankton, ecosystem multifunctionality, and five individual ecosystem functions (nitrogen and phosphorus concentrations, standing fish biomass, algae production, and light availability) differ among states. We also evaluated how the relationship between biodiversity (species richness and functional diversity) and multifunctionality is affected by regime shifts. We showed that species richness and the functional diversity of fish and zooplankton were highest during the clear state. The clear state also maintained the highest values of multifunctionality as well as standing fish biomass production, algae production, and light availability, whereas the turbid and shaded states had higher nutrient concentrations. Functional diversity was the best predictor of multifunctionality. The relationship between functional diversity and multifunctionality was strongly positive during the clear state, but such relationship became flatter after the shift to the turbid or shaded state. Our findings illustrate that focusing on functional traits may provide a more mechanistic understanding of how regime shifts affect biodiversity and the consequences for ecosystem functioning. Regime shifts towards a turbid or shaded state negatively affect the taxonomic and functional diversity of fish and zooplankton, which in turn impairs the multifunctionality of shallow lakes.随着营养负荷（nutrient loading）持续升高，浅水湖泊可能会从沉水大型水生植物（submerged macrophytes）主导的清水稳态，转变为浮游植物（phytoplankton）主导的浊水稳态，或是浮水大型水生植物（floating macrophytes）主导的遮光稳态。目前，关于此类稳态转换（regime shifts）如何介导分类多样性（taxonomic diversity）、功能多样性（functional diversity）与湖泊生态系统多功能性（lake multifunctionality）之间的关系，学界尚缺乏充分认知。本研究依托一套覆盖12年监测周期的详细数据库，该数据库对应的浅水湖泊曾先后呈现上述三种稳态状态（清水态、浊水态与遮光态），旨在探究不同稳态下物种丰富度（species richness）、鱼类与浮游动物（zooplankton）的功能多样性、生态系统多功能性，以及五项单一生态系统功能——氮、磷浓度、鱼类现存量生物量（standing fish biomass）、藻类生产力（algae production）与光照可获得性（light availability）——的差异。本研究同时评估了稳态转换对生物多样性（物种丰富度与功能多样性）与多功能性之间关系的影响。研究结果显示，清水态下鱼类与浮游动物的物种丰富度及功能多样性均达到最高水平。清水态同时维持了最高的生态系统多功能性，以及鱼类现存量生物量、藻类生产力与光照可获得性，而浊水态与遮光态的营养浓度更高。功能多样性是生态系统多功能性的最佳预测因子。在清水态下，功能多样性与多功能性之间呈现显著正相关关系，但当湖泊转换至浊水态或遮光态后，二者的相关关系趋于平缓。本研究结果表明，聚焦功能性状（functional traits）可获得对稳态转换影响生物多样性及其生态系统功能效应的更具机制性的理解。向浊水态或遮光态的稳态转换，会对鱼类与浮游动物的分类多样性和功能多样性产生负面影响，进而削弱浅水湖泊的生态系统多功能性。