Abstract
Bacterioplankton communities play a fundamental role in the cycling of carbon and nitrogen in the oceans. Cycling of these nutrients by bacterioplankton also contributes to the production of nitrous oxide and methane, resulting in the oceans being a net source of both these greenhouse gases. Climate change is impacting the oceans through warming and acidification resulting in alteration of planktonic ecosystems, via changes in productivity, biomass, and species composition. The response of marine bacterioplankton communities to the direct effects of ocean warming and lowered pH, and to the indirect effects of changes in phytoplankton and zooplankton, has implications for biogeochemical cycling and therefore the production of nitrous oxide and methane. This thesis investigates the impact of both direct and indirect climate pressures by determining the influence of ocean warming and lowered pH on bacterioplankton and the production of methane and nitrous oxide in New Zealand coastal waters. It also assesses how open ocean bacterioplankton communities and dissolved methane and nitrous oxide are influenced by water mass properties and, in particular, how they may be affected by climate-induced changes in the distribution and abundance of salps, a dominant group of zooplankton.
To determine the impact of lower pH and warming on bacterioplankton community, production and abundance, coastal water was manipulated in three mesocosm experiments to projected future ocean temperature and pH. The experiments ran for 18-21 days using 4000-Litre mesocosms filled with coastal water and associated plankton communities, with pH and temperature continuously regulated. High-throughput sequencing of the 16S rRNA gene was used to determine bacterioplankton community composition and leucine incorporation was used to measure bacterial production during the experiments. Minor but significant increases in alpha diversity were seen under low pH and warming. However, overall results from the mesocosm experiments indicate resilience to ocean warming and low pH in coastal bacterioplankton communities, with no significant impacts on production, abundance or beta-diversity found. Bacterioplankton communities in coastal sites are likely to experience high natural variability, which may result in lack of sensitivity to projected climate change.
The coastal mesocosms were also utilised to test the impacts of projected ocean warming and lowered pH on dissolved methane and nitrous oxide. However, the high initial methane concentration and subsequent loss to the atmosphere required generation of a gas transfer factor to enable determination of trace gas production. Whereas there was no impact on nitrous oxide saturation, warming and low pH combined resulted in higher methane production and saturation. Pathways to methane production in aerobic waters is a rapidly evolving area of research, with the production of methane in these aerobic coastal waters of uncertain origin. The potential for methane production to increase under ocean warming and increased CO2 highlights the importance of further research on aerobic methane production pathways to determine climate feedbacks to oceanic methane emissions.
Warming temperatures and changes to phytoplankton blooms has increased the frequency of blooms of the Antarctic salp, Salpa thompsoni, in the Southern Ocean. To investigate potential influences on bacterioplankton and trace gases, a semi-Lagrangian design was used to follow surface ocean water parcels with and without blooms of S. thompsoni in the Chatham Rise region, east of New Zealand. Subantarctic, subtropical, and frontal waters were sampled, with bacterioplankton community composition, abundance and production compared to S. thompsoni biomass, non-salp zooplankton biomass, and other environmental parameters. Again, 16S rRNA gene sequencing was used to determine bacterioplankton community composition. The presence of S. thompsoni resulted in higher bacterial abundance and production, as well as shifts in the relative abundance of several taxa. Some specialist heterotrophic bacteria showed significantly higher, or lower, relative abundance where salps were present. Additionally, some ammonia-oxidising archaea showed a decrease in relative abundance where salp blooms were present. Overall, water mass was a stronger driver of bacterial community composition than salp presence, with NMDS analyses showing clustering by water mass rather than salp presence. As salp bloom presence influenced bacterioplankton production and the abundance of certain taxa, further study is required into the potential effects of increasing salp abundance in the Southern Ocean.
During the same research voyage, the saturation of methane and nitrous oxide in surface waters to 500m depth was measured and compared to environmental parameters, with a focus on salp and other zooplankton biomass. The presence of salps in subantarctic waters resulted in an increase in methane saturation, whereas in subtropical waters the salp bloom site was characterised by lower nitrous oxide and methane concentrations than the non-salp site. Salp bloom sites exhibited higher ammonium concentrations, but there was no evidence that this increased nitrification rate and associated nitrous oxide production. Overall, oxygen was the primary driver of dissolved nitrous oxide saturation, with increased nitrous oxide in deeper waters. Although methane saturation showed responses to salp presence, it was more strongly correlated to non-salp mesozooplankton biomass. This may have been due to ‘sloppy’ grazing by zooplankton releasing DMSP and DOM and so providing substrate for bacterial methane production. The mechanisms by which salps and other zooplankton influence methane production requires further study as shifts in grazer dominance occur in the Southern Ocean.
The present thesis explores marine bacterioplankton communities, as well as greenhouse gas saturation, production, and flux, in both current and future New Zealand waters. This investigation expands the knowledge of influences on bacterioplankton in future oceans. Low pH and warming did not directly impact bacterioplankton to the same extent as potential climate-driven changes to plankton food webs, although this may have been due to coastal bacterioplankton communities exhibiting more resilience compared to open ocean communities. This thesis shows that determining how bacterioplankton will respond to future oceans requires the inclusion of trophic interactions with both phytoplankton and zooplankton, as these groups will also change under climate change. The findings from this thesis also demonstrate that marine methane production may respond to both the direct effects of warming and low pH, as well as shifts in pelagic food webs. Much of the current oceanic greenhouse gas emission research focuses on nitrous oxide due to the oceans being a greater source of this gas than of methane. However, potential changes to marine methane emissions in the future ocean highlights the importance of verifying aerobic methane production pathways and establishing how these pathways will alter with climate change.