Estimation of air-sea CO2 flux in Hudson Bay during the ice-free season using a combination of field data and satellite remote sensing products

Abstract

The lack of baseline estimates of air-sea CO₂ exchange in Arctic and sub-Arctic regions represents a major shortfall in our ability to understand how climate change may affect CO₂ fluxes at high latitude. The 2005 ArcticNet cruise of Hudson Bay provided a rare comprehensive oceanographic survey of one such region. Ship-based observations of sea­surface fugacity of CO₂ (fCO₂sw) were made at 56 locations between September 15 and October 26, and were found to range from 259µ atm in Hudson Strait to 425µ atm at the entrance to James Bay. Strong relationships between fCO₂sw and river discharge were identified, with coastal waters observed to be supersaturated in fCO₂sw with respect to the atmosphere (and thus a source of CO₂), while off-shore waters were undersaturated (thus a sink). High correlation of fCO₂sw with salinity, sea surface temperature (SST), and colored dissolved organic matter (CDOM) suggest that thermodynamic effects and possibly the oxidation of riverine carbon were driving supersaturation in the coastal zone. To expand the spatial and temporal domain of the study, a remote sensing approach was applied. Predictive algorithms for fCO₂sw were created from relationships with CDOM and SST; variables which can be measured from satellite remote sensing platforms. A SST­ fCO₂sw algorithm was used with monthly maps of SST obtained from the MODIS aqua sensor to extrapolate fCO₂sw in Hudson Bay for the 2005 ice-free season (August-October). Gas transfer velocities were estimated using twice-daily QuikSCAT wind retrievals, and by using a bulk aerodynamic approach the monthly flux of CO₂ in Hudson Bay was calculated. The results of these calculations revealed that Hudson Bay acts as a source of CO₂ during August and September (4.10 and 5.29 mmol m⁻² day⁻¹, respectively), but reverts to a sink of CO₂ in October as the water temperature cools (-5.83 mmol⁻² day⁻¹). By integrating over the spatial extent of Hudson Bay and the 92 day open-water season, a positive flux of 0.93 TgC was estimated. This result is in contrast to most Arctic or sub-Arctic continental shelf seas, where usually strong absorptions of CO₂ are observed. It is hypothesized that this difference is related to the strong influence of river discharge. Future studies are necessary to constrain fluxes in Hudson Bay over an annual cycle and to determine the oceanographic controls on CO₂ flux. However, this study is significant since it represents the first-ever examination of air-sea CO₂ exchange in Hudson Bay.