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.
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Deans F. S. C., 2022. The influence of climate change on marine bacterioplankton communities and greenhouse gases in New Zealand waters. PhD thesis, University of Otago. Thesis (restricted access).
