Exploring contributions and responses of microbial quantity and diversity to coastal seasonal hypoxia

Dissolved oxygen (DO) concentrations in euphotic waters are governed by combined effects from air-sea exchange, photosynthesis, and aerobic respiration. Bacteria and microeukaryotes both respond to and shape these oxygen dynamics, yet the quantitative contributions of microbial traits versus physicochemical factors to DO variability remain unclear. Moreover, the extent to which DO, and other environmental parameters or inter-domain biotic interactions control microbial diversity and abundance is still unresolved. To fill these gaps, we conducted a two-month summer survey of a shallow coastal area off Shandong Peninsula, a region frequently affected by seasonal hypoxia. Modelling revealed a depth-dependent divergence in DO drivers: physicochemical variables dominated in surface and bottom waters, whereas microbial traits-particularly the abundance of high nucleic acid bacteria-outperformed environmental factors in the mid-depth layer. Under hypoxic conditions, microbial abundances declined and community structures shifted significantly, yet alpha diversity metrics remained unchanged. Inter-domain interactions were the primary force structuring bacterioplankton communities, exerting a stronger influence than environmental variables. Overall, bottom-water deoxygenation in this coastal system was predominantly by physiochemical processes, especially nutrients loading and stratification, rather than by direct microbial feedback. Non-DO environmental variables and inter-domain interactions emerge as the principal forces structuring planktonic communities, underscoring the need for nutrient-management strategies that account for these complex biotic-abiotic linkages.