Coupling between bacterial phylogenetic diversity and heterotrophic productivity in a coastal ecosystem affected by estuarine plumes.

Understanding the diversity-productivity relationship (DPR) is crucial for elucidating the ecological functions of marine bacterioplankton. However, studies have often focused on species diversity, neglecting phylogenetic diversity, which may offer deeper insights into the complex ecological processes shaping DPR in natural systems. This study addressed this gap by exploring the role of phylogenetic diversity in bacterioplankton productivity in the northern South China Sea, a coastal ecosystem influenced by estuarine plumes. We aimed to disentangle the mechanisms driving DPR and investigate how estuarine plumes modulate these processes. Our results show that the substantial enhancement of phytoplankton production by the Pearl River plume increased bacterial production, abundance, and cell-specific production. From a metacommunity perspective, phylogenetic diversity, rather than species diversity, significantly enhanced productivity. The plume reduced positive species interactions and complementarity but amplified the selection effect, where increased phylogenetic diversity raised the likelihood of including highly productive species. In plume-impacted communities, distantly related and highly productive clades dominated the DPR. Phylogenetically diverse assemblages exhibited enhanced niche differentiation that facilitated the stable coexistence of productive clades by mitigating exclusion. We also delineated how the negative selection effect and increased species exclusion contributed to the decoupling of species diversity from productivity in communities unaffected and affected by the plume, respectively. These findings highlighted the pivotal role of estuarine plumes in enhancing productivity via increased phylogenetic diversity and in eliciting complex adaptive responses within bacterioplankton communities. Future comprehensive assessments will be needed to elucidate the implications of these dynamics on marine ecosystem services.