Marine microbial ecology off East Antarctica (30 - 80°E): Rates of bacterial and phytoplankton growth and grazing by heterotrophic protists

Marine microbes (<200 μm) contribute most of the living matter and carbon flow in the Southern Ocean, yet the factors that control the composition and function of these microbial communities are not well understood. To determine the importance of microbial grazers in controlling microbial abundance, we determined microbial standing stocks and rates of herbivory and bacterivory in relation to the physical environment off East Antarctica during the Baseline Research on Oceanography, Krill and the Environment: West (BROKE-West) survey, which covered waters from the Polar Front to the coast between 30 and 80°E. Concentrations of heterotrophic nanoflagellates (HNF) (∼2 to 20 μm), microzooplankton (∼20 to 200 μm), bacteria, and chlorophyll a (Chl a) were determined and the growth and grazing mortality of phytoplankton and bacteria were estimated using the grazing dilution technique at 22 sites along the survey. Results showed that microzooplankton and HNF consumed on average 52% of bacterial production d−1 and 62% primary production d−1 but consumed >100% d−1 at the western ice-edge sites. Rates of bacterivory ranged from 0.4 - 2.6 d−1 and were correlated with bacterial concentrations, bacterial growth rates and longitude. Rates were highest in the eastern-most part of the survey, which was sampled last, reflecting the transition along the successional continuum toward a respiration-based, senescent, microbial community. Rates of herbivory ranged from 0.3 to 2.4 d−1 and were correlated with concentrations of microzooplankton and HNF combined, rates of phytoplankton growth, and latitude. Rates were highest at southern ice edge sites where concentrations of prey (as represented by Chl a) and microzooplankton were also highest. Cluster analysis of the concentrations of marine microbes and their rates of growth and grazing mortality identified 5 groups of sample sites that conveniently summarised the variability in the composition and function of the microbial community. Cluster groups differentiated between low Chl a (∼0.3 μg l−1) open ocean Antarctic Circumpolar Current (ACC) communities; and high Chl a (∼2.4 μg l−1) ice-associated coastal blooms at various stages between bloom formation and senescence. This partitioning of cluster groups can be used to determine spatial and temporal patterns of carbon transfer by the microbial loop within the BROKE-West survey area.