| Summary: | Changes in oceanic circulation and physiochemical parameters due to climate change may alter the distribution, structure and function of marine microbial communities, thereby altering the action of the biological carbon pump. One area of current and predicted future change is the sub-Antarctic zone (SAZ) to the southeast of Tasmania, Australia, where a southward shift in westerly winds appears to be forcing warmer and macronutrient-poor subtropical waters into the sub-Antarctic zone (SAZ). We investigated the impact of these subtropical waters on the microbial community of the SAZ on the SAZ-Sense cruise during the austral summer of 2007. The abundance of pico- and nanoeukaryotic algae, cyanobacteria, heterotrophic nanoflagellates, bacteria and viruses was determined by flow cytometry at stations in the Polar Frontal Zone (PFZ), the SAZ and in Subtropical Zone (STZ). Using cluster and similarity profile analyses on integrated microbial abundances over the top 200 m, we found that microbial communities located in the potential future SAZ to the southeast of Tasmania formed two distinct groups from those of the remainder of the SAZ and the PFZ. In the waters of the potential future SAZ, shallow mixed layers and increased iron concentrations elevated cyanobacterial, bacterial and viral abundances and increased percentage high DNA bacteria, resulting in communities similar to those of subtropical waters. Conversely, waters of the PFZ exhibited relatively low concentrations of autotrophic and heterotrophic microbes and viruses, indicative of the iron limitation in this region. A Distance Based Linear Model determined that salinity and nitrogen availability (nitrate, nitrite and ammonia concentrations) were the most influential environmental parameters over the survey, explaining 72% of the variation in microbial community structure. The microbial community of the potential future SAZ showed a shift away from particulate carbon export from the photic zone towards increased production by smaller cells, increased ...
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