Ocean alkalinity enhancement (OAE) is a promising marine carbon dioxide removal strategy, yet its ecological consequences for coastal bacterial communities remain poorly understood, particularly in eutrophic coastal waters where nutrient availability and seasonal variability may shape microbial responses. Here, we investigated the interactive effects of OAE and nitrogen enrichment using 15-day seawater microcosm incubations in Daya Bay during contrasting summer and winter seasons. A full factorial experiment was conducted in summer with control, nitrogen enrichment (30 µM NaNO₃), OAE (500 µM kg⁻¹ NaOH), and combined treatments, while winter incubations included control and OAE treatments. The results showed that moderate OAE caused little change in bacterial diversity or overall community composition, indicating that community structure was largely maintained under elevated alkalinity. However, OAE consistently reshaped the responses of individual bacterial taxa. Synechococcus declined during early incubation, whereas several low abundance taxa, including Actinobacteriota and Planctomycetota, increased across both seasons. Ecological niche width remained unchanged, suggesting that community functional potential was maintained despite taxonomic reorganization. Nitrogen enrichment altered these responses by mitigating the decline of Synechococcus and redirecting community succession under OAE. Elevated alkalinity also improved the complexity of microbial co-occurrence networks without reducing network stability. Seasonal comparisons further revealed that bacterial communities responded more rapidly and strongly to OAE in summer, whereas responses developed more gradually in winter. Overall, our results show that moderate OAE does not destabilize coastal bacterial communities but promotes internal community reorganization. More importantly, the magnitude and trajectory of this reorganization depend on nutrient availability and seasonal conditions. These findings demonstrate that microbial responses to OAE are context dependent and highlight the need to incorporate environmental variability into ecological risk assessments and future coastal OAE deployment.

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