The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

The Ross Sea is an important source region of dense shelf water and Antarctic Bottom Water (AABW). High-Salinity Shelf Water (HSSW), formed in the Terra Nova Bay Polynya (TNBP) through brine rejection during sea-ice formation, can be transported northward through the Drygalski Trough and exported toward the continental slope. However, how this transport pathway influences bacterial communities and associated biogeochemical processes remains poorly understood. Here, we investigated spatial, vertical, and seasonal variations in bacterial communities across the TNBP, J1, R1, J2, and J5 sections during autumn, together with a summer–autumn comparison at R1. Bacterial community composition varied significantly among sections and depth layers, with particularly pronounced differences between TNBP and J5. J5 exhibited higher relative abundances of Marinicauda, Idiomarina, Muricauda, Joostella, Aequorivita, and Aurantiacibacter, whereas Nitrosomonas and Magnetospira were relatively enriched in TNBP. Constrained ordination and variation partitioning indicated that community variation was jointly associated with water-mass characteristics, nutrient availability, salinity, temperature, and depth, rather than being controlled by a single environmental factor. Seasonal changes were also observed at R1. Summer communities contained a higher relative abundance of Gammaproteobacteria but lower relative abundances of Alphaproteobacteria and Bacteroidota than autumn communities. Summer was more strongly associated with bloom-related genera such as Pseudoalteromonas and Leeuwenhoekiella, whereas autumn exhibited a broader range of indicator taxa, including Acinetobacter, OM60(NOR5), Pelagibacterium, and Methylobacterium. Along the J1 Trough, a substantial proportion of TNBP-associated bacterial signatures persisted during downstream transport, although their relative contributions varied among stations and depths. Taxa such as Candidatus Nitrosoarchaeum and Algiphilus were relatively abundant in deep and bottom waters. Community similarity to TNBP increased with the estimated HSSW contribution, particularly in deep and bottom layers. Ammonium concentrations declined with increasing HSSW contribution, while putative nitrifiers were negatively associated with ammonium and nitrite, suggesting that dense-water formation and transport may be linked to nitrogen transformation processes. Overall, bacterial communities provided complementary biological signatures of water-mass transport across the Ross Sea continental shelf. These findings reveal close coupling among physical circulation, dense-water formation, microbial dispersal, and nitrogen cycling, and demonstrate the potential of bacterial communities for tracing shelf-water export and its biogeochemical consequences.