Winter Extratropical Cyclones as a Potential Driver of a Long-Term Decline of Bacterial Production in the Sargasso Sea Near Bermuda

Abstract

Analysis of plankton data sets from the Bermuda Atlantic Time-series Study site (31°40′N, 64°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.