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Analysis of plankton data sets from the Bermuda Atlantic Time-series Study site (31 degrees 40'N, 64 degrees 10'W) demonstrates a long-term decreasing trend of marine heterotrophic bacterial production (BP) in winter (November-February) over the past 24 years (1989-2012). We propose winter extratropical cyclones (storms) as a potential driver of the observed BP trend. Stormy winters were characterized by significantly enhanced phytoplankton biomass and productivity as well as organic carbon pools, as storms induce deeper convective mixing and increased nitrate fluxes into upper mixed-layer. Counter to expectation, BP was lower by 24% due to storm-mediated entrainment of cold water into upper mixed-layer. The frequency of storm-impacted months has increased over time, driven by a decline of the North Atlantic Oscillation (NAO) index. Our findings suggest that a NAO-induced southward shift of winter storm tracks contributes to the decreasing BP trend, via temperature-dependent decoupling of bacteria from phytoplankton, ultimately impacting carbon fluxes and cycling near Bermuda.

Plain Language Summary Heterotrophic marine bacteria metabolize organic matter produced by phytoplankton in the ocean for their growth. In the Sargasso Sea near Bermuda, wintertime (November-February) bacterial production has decreased significantly over the past 24 years (1989-2012); however, this trend was observed without concurrent long-term changes in the availability of organic matter that bacteria feed on. Our analysis demonstrates that winter storms passing over the Bermuda region are responsible for the observed decline of bacterial production. By bringing strong wind mixing power, storms make deep water intrude into the upper water column. As a consequence, bacterial activity in the upper ocean is significantly suppressed due to cold temperature water that is being entrained from depth. Our study provides novel findings that storms possibly provide a mechanistic basis for the long-term bacterial trend in the ocean.