Riverine network size determined major driving factors of the composition and diversity of aquatic invertebrate communities in a multi-tributary mountain river basin

Abstract

Revealing the spatial variation of aquatic invertebrates and their response to biotic and abiotic factors, from headwaters to estuaries, is crucial for understanding their successional patterns and protecting watershed ecosystems. This study aimed to explore the biogeographic patterns and identify the primary drivers of invertebrate community structure across river networks of varying sizes using environmental DNA (eDNA) technology. To assess the contribution of biotic and abiotic factors to invertebrate communities, we collected six categories of abiotic factors: geography, climate, hydro-morphology, human footprint index, land use, and water quality. For biotic factors, four microbial groups including archaea, bacteria, fungi, and protists were identified using eDNA techniques. Water samples were collected from a total of 187 sample sites in the upper Hanjiang River basin (China) during two seasons (Spring and Autumn), covering the transition from the headwater tributaries to the lower reaches of the main channel. The results revealed that environmental factors explained approximately 6.5 times more variation in invertebrate eDNA communities than geographic factors. Water quality and biotic factors had strong explanatory power for invertebrate eDNA diversity. Ecological succession of invertebrate eDNA communities along the river continuum showed a shift from Arthropoda-dominated communities in the headwaters to a co-dominance of Arthropoda, Rotifera, and Cnidaria downstream. The cumulative dendritic distance upstream, representing the location of each sampling site within the river network, emerged as the most predictive spatial feature. Significant differences were observed in the dominant environmental factors influencing community diversity across different river network sizes. In small river networks, invertebrate eDNA diversity was primarily influenced by biotic factors, while in medium-sized networks, it was shaped by a combination of biotic factors and water quality. In large river networks, water quality emerged as the primary driver. These findings suggest that invertebrate communities throughout the Hanjiang River basin undergo ecological succession along the river continuum, primarily shaped by environmental factors related to river network size.