- Carbon dioxide (CO₂) fluxes between the ocean and atmosphere on continental margins are difficult to diagnose because these regions experience large variability over spatial and temporal scales spanning meters to basins and hours to years, respectively. In a global sense, continental margins could represent a significant atmospheric CO₂ sink, equivalent to ~30% of the open ocean CO₂ uptake. However, many regions have inadequate data coverage to resolve the large inherent variability. The role of continental margins also becomes increasingly more complicated if nearshore waters, such as estuaries and salt marshes, are included in global or regional carbon budgets. This thesis addresses variability in surface seawater CO₂ partial pressure (pCO₂) and sea-air CO₂ flux across three sub-regions of the northeast Pacific continental margin: the Oregon shelf, the western Canadian coastal ocean, and the Columbia River Estuary and plume.
Chapter 1 describes the motivation for this work. Chapter 2 is an assessment of the seasonal cycle of pCO₂ on the Oregon shelf from both ship and mooring data. Results from this work demonstrate strong seasonality on the shelf, as well as interannual variability important for constraining regional flux estimates. The observations included the highest surface water pCO₂ reported for the region, and prolonged exposure of this water to the atmosphere had a large impact on the annual estimate of sea-air CO₂ flux.
Chapter 3 presents a new data set of ship-based pCO₂ observations made over a larger portion of the western Canadian coastal margin than had been previously sampled during winter, summer and autumn. Analyses of temporally well-resolved historical data collected on the southwest Vancouver Island shelf, combined with the spatially well-resolved new data showed that CO₂ outgassing during autumn is significantly greater than during winter over most of the margin, and both seasons play a key role in nearly balancing the influx of atmospheric CO₂ that occurs during the remainder of the year.
Chapter 4 reports water-air CO₂ fluxes from the Columbia River Estuary and plume across three seasons. The river plume is always a sink for atmospheric CO₂, likely from a combination of chemical and biological factors, while the estuary sink/source character is seasonally modified. During spring, the estuary acts as a sink for atmospheric CO₂, whereas it is a source during other seasons. Primary productivity and ecosystem respiration are important for determining the magnitude of fluxes in the estuary between seasons, and the spring freshet marks the transition between CO₂ sink and source functionality in the estuary.
Chapter 5 examines the weak phytoplankton response to strong and persistent upwelling on the Oregon shelf in July 2008 to understand the prolonged exposure of high-pCO₂ water. Using ship, mooring and satellite data, I argue the phytoplankton response time is long relative to the timescales of strong upwelling, cross shore transport, and subduction. The rapid physical processes are most likely responsible for maintaining low chlorophyll conditions when nutrients and pCO₂ are elevated. Chapter 6 is a discussion of the importance of the large flux events observed during these studies, and their relevance to regional and global CO₂ flux studies. In each region studied in this thesis there were occurrences of high-intensity pCO₂ conditions, which highlights the variable nature of the coastal ocean and the importance of resolving the temporal and spatial scales of variability in order to assess continental margin sea-air CO2 fluxes.