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Temporal trends and biogeochemical controls on methane and nitrous-oxide distributions in coastal waters of the subarctic Pacific Ocean Capelle, David


This PhD thesis examines the marine cycling of the greenhouse gases methane (CH₄) and nitrous-oxide (N₂O) in coastal British Columbia waters. The primary objectives of the work were to increase spatial and temporal data availability in an under-sampled coastal region, and to examine the processes responsible for CH₄ and N₂O distributions, and their sensitivity to changing environmental conditions (e.g. O₂-availability). Using a novel high-throughput analytical system, based on purge and trap gas chromatography-mass spectrometry (GC-MS), we measured a 6 year time-series of monthly water column CH₄ and N₂O profiles from Saanich Inlet, British Columbia, as well as three years of water column profiles and surface measurements along the West coast of Vancouver Island (WCVI). The physical and biological processes responsible for the observed CH₄ and N₂O distributions were investigated using relationships with ancillary physical data and biological data, including recently available meta-genomic information. The results presented in this thesis document a dominant role for O₂ concentrations in driving spatial and temporal variability in CH₄ and N₂O concentrations over a range of scales. In Saanich Inlet, the seasonal cycle of anoxia and deep water renewal exerts a primary control on water column N₂O and CH₄ accumulation, with additional likely contributions from sedimentary processes and in situ cycling of various nitrogen species and methylated compounds in the upper water column. In both Saanich Inlet and the WCVI, inter-annual variability and longer-term trends are associated with changes in upwelling intensity and El Nino events, and these changes are set against a background of declining O₂ concentrations across the Subarctic Pacific. Results from our work suggest that coastal CH₄ and N₂O concentrations may be responding significantly to these long-term declines in O₂ levels, with significant implications for regional sea-air fluxes of climate-active trace gases.

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