Investigating the influence of biodiversity on ecosystem functioning over environmental gradients is needed to anticipate ecosystem responses to global change. However, our understanding of the functional role of freshwater biodiversity, especially for microbes, is mainly based on manipulative experiments, where biodiversity and environmental variability are minimized. Here, we combined observational and manipulative experiments to analyse how fungal biodiversity responds to and mediates the impacts of drying on two key ecosystem processes: organic matter decomposition and fungal biomass accrual. Our observational data set consists of fungal biodiversity and ecosystem processes from 15 streams spanning a natural gradient of flow intermittence. Our manipulative design evaluates the responses of ecosystem processes to two fungal richness levels crossed with three levels of drying. For the observational experiment, we found that increasing the duration of drying reduced fungal species richness and caused compositional changes. Changes in species composition were driven by species turnover, suggesting resistance mechanisms to cope with drying. We also found that fungal richness had a positive effect on organic matter decomposition and fungal biomass accrual. Positive effects of fungal biodiversity were consistent when controlling for the effects of drying duration on richness by means of structural equation modelling. In addition, our results for the manipulative experiment showed that the positive effects of higher richness on both ecosystem processes were evident even when exposed to short or long simulated drying. Overall, our study suggests that maintaining high levels of biodiversity is crucial for maintaining functional freshwater ecosystems in response to ongoing and future environmental changes.

需要调查生物多样性对环境梯度生态系统功能的影响，以预测生态系统对全球变化的反应。然而，我们对淡水生物多样性的功能作用的理解，特别是对微生物的作用，主要基于操纵实验，其中生物多样性和环境可变性被最小化。在这里，我们结合观察和操作实验来分析真菌生物多样性如何响应和调节干燥对两个关键生态系统过程的影响：有机物分解和真菌生物量累积。我们的观测数据集包括来自 15 条河流的真菌生物多样性和生态系统过程，这些河流跨越自然的流动间歇梯度。我们的操纵设计评估了生态系统过程对两个真菌丰富度水平与三个干燥水平交叉的反应。对于观察实验，我们发现增加干燥时间会降低真菌物种的丰富度并导致成分变化。物种组成的变化是由物种周转驱动的，这表明应对干燥的抵抗机制。我们还发现真菌丰富度对有机物分解和真菌生物量产生有积极影响。通过结构方程模型控制干燥时间对丰富度的影响时，真菌生物多样性的积极影响是一致的。此外，我们的操纵实验结果表明，即使暴露于短期或长期模拟干燥，更高的丰富度对两个生态系统过程的积极影响也很明显。总体而言，我们的研究表明，维持高水平的生物多样性对于维持功能性淡水生态系统以应对持续和未来的环境变化至关重要。