Contrasting Abiotic and Biotic Predictors of Diatom Resource Use Efficiency in a Megadam‐Regulated River

ABSTRACT

Megadams can profoundly alter river environments and biotic communities. However, their effects on ecosystem functioning such as resource use efficiency (RUE) remain poorly understood. The biodiversity‐ecosystem functioning (BEF) framework posits that ecosystem functioning is associated with abiotic conditions and biotic community properties, yet whether the relative importance of these predictors varies in a megadam‐regulated river remains unclear. Here, we studied the influence of the Xiaolangdi Dam in the Yellow River on river environmental conditions and diatom community properties including taxonomic and phylogenetic diversity, community composition, complexity, and stability. Planktonic diatom samples were analyzed using environmental DNA metabarcoding, and RUE was estimated as chlorophyll‐a per unit resource (e.g., nitrogen and phosphorus). The reservoir communities had significantly higher chlorophyll‐based RUE and community complexity than the downstream communities, but had lower Faith's phylogenetic diversity and no consistent increase in community stability. Random forest models showed that downstream chlorophyll‐based RUE was mainly associated with abiotic predictors such as turbidity, temperature, and TN:TP. In contrast, the relative importance of biotic community properties, including Faith's phylogenetic diversity, community complexity, and stability, increased in the reservoir. These results suggest that abiotic conditions and biotic community properties jointly predict chlorophyll‐based RUE, and that their relative importance can differ between reservoir and downstream reaches. Our study provides field evidence that BEF relationships can depend on the context in a megadam‐regulated river.