Abstract
Bacterioplankton (bacteria and archaea) structure the base of marine food webs and play an essential role in global biogeochemical cycles. Up to 50% of the organic carbon that is fixed by marine primary producers is utilised by bacterioplankton where they facilitate carbon and energy transfer to higher trophic levels via the microbial loop. The provision of organic matter through primary production is unevenly distributed through space and time, following seasonal trends that vary with geographic location. Bacterioplankton community composition and successions are closely tied to these seasonal trends in surface waters. Seasonal fluctuations in organic matter export to the deep ocean via the biological carbon pump provide connections to deep bacterioplankton communities that also vary over time. Bacterioplankton community structure is closely linked to its function, thereby understanding the factors that influence community structure over space and time can help to inform their role in facilitating global biogeochemical cycling. This has been a core aim for many marine microbial studies, and the establishment of several long-term microbial observatory time-series programs around the world have made key insights into the seasonal and long-term trends in bacterioplankton communities across diverse oceanographic settings, although most are confined to the Northern Hemisphere with sampling limited to one water mass type.
In this thesis, three years (January 2014 to March 2017) of bacterioplankton community composition via 16S rRNA amplicon sequencing data, in addition to flow cytometric prokaryotic abundance counts and heterotrophic prokaryotic production, obtained from sampling across a subtropical front (STF) and into the deep ocean were examined to determine seasonal and interannual trends across the distinct water masses associated with the STF system, along with seasonal trends and connections from surface to mesopelagic communities. Each investigation revealed the strong influence of spatial structuring on bacterioplankton communities with distinct water masses harbouring unique communities both across the STF and between surface and mesopelagic waters. Seasons also played a key role in shaping bacterioplankton communities, with seasonal trends evident in each of the sampled water masses, along with seasonally changing connections between surface and mesopelagic communities. Some inconsistencies in seasonal trends over the different sampling years were observed and attributed to El Niño conditions that developed in the second sampling year, revealing the impact of climate cycles on bacterioplankton communities.
Collectively, this thesis provides key insights into seasonal and interannual trends in bacterioplankton community dynamics both across the STF and into the deep ocean. The findings contribute to our understanding of spatial and temporal bacterioplankton community dynamics, with new insights from an understudied region of the global ocean, utilising the longest running microbial observatory time-series in the Southern Hemisphere and across a frontal system.