Summer comes to the Southern Ocean: how phytoplankton shape bacterioplankton communities far into the deep dark sea

During austral spring and summer, the coastal Antarctic experiences a sharp increase in primary production and a steepening of biotic and abiotic gradients that result from increased solar radiation and retreating sea ice. In one of the largest seasonally ice‐free regions, the Amundsen Sea Polynya, pelagic samples were collected from 15 sites during a massive Phaeocystis antarctica bloom in 2010/2011. Along with a suite of other biotic and abiotic measurements, bacterioplankton were collected and analyzed for community structure by pyrosequencing of the 16S rRNA gene. The aims were to identify patterns in diversity and composition of heterotrophic bacterioplankton and to test mechanistic hypotheses for explaining these differences along variations in depth, water mass, phytoplankton biomass, and organic and inorganic nutrients. The overall goal was to clarify the relationship between primary producers and bacterioplankton community structure in the Southern Ocean. Results suggested that both epipelagic and mesopelagic bacterioplankton communities were structured by phytoplankton blooming in the euphotic zone. As chlorophyll a (chl‐a) increased in surface waters, the abundance of surface bacterioplankton increased, but their diversity decreased. Similarity in bacterioplankton community composition between surface‐water sites increased as the bloom progressed, suggesting that algal blooms may homogenize surface‐water bacterioplankton communities at larger spatial scales. Below the euphotic zone, the opposite relationship was found. Mesopelagic bacterioplankton diversity increased with increasing chl‐a in the overlying surface waters. This shift may be promoted by several factors including local increase in organic and inorganic nutrients from particles sinking out of the euphotic zone, an increase in niche differentiation associated with the particle flux, interactions with deep‐dwelling macrozooplankton, and release from competition with primary producers. Additional multivariate analyses of bacterioplankton community structure and nutrient concentrations revealed distinct depth horizons, with bacterioplankton communities having maximum alpha and beta diversity just below the euphotic zone, while nutrient composition gradually homogenized with increasing depth. Our results provide evidence for bloom‐driven (bottom‐up) control of bacterioplankton community diversity in the coastal Southern Ocean and suggest mechanisms whereby surface processes can shape the diversity of bacterioplankton communities at great depth.