Sea-Air CO2 Fluxes in the Western Canadian Coastal Ocean

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/258707232 Sea-air CO2 fluxes in the western Canadian coastal ocean Article in Progress In Oceanography · August 2012 DOI: 10.1016/j.pocean.2012.01.003 CITATIONS READS 8 40 4 authors, including: Wiley Evans Burke Hales The Hakai Institute Oregon State University 33 PUBLICATIONS 249 CITATIONS 105 PUBLICATIONS 3,672 CITATIONS SEE PROFILE SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Wiley Evans letting you access and read them immediately. Retrieved on: 29 August 2016 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Progress in Oceanography 101 (2012) 78–91 Contents lists available at SciVerse ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean Sea-air CO2 fluxes in the western Canadian coastal ocean ⇑ Wiley Evans a, , Burke Hales a, Peter G. Strutton a,1, Debby Ianson b a College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA b Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, British Columbia, Canada article info abstract Article history: Sea-air carbon dioxide (CO2) fluxes have been analyzed from recently-collected winter, summer and Received 7 July 2011 autumn surface ocean CO2 partial pressure (pCO2) data spanning a large portion of the western Canadian Received in revised form 14 January 2012 coastal ocean, and historical underway pCO2 measurements from the southwest Vancouver Island shelf Accepted 16 January 2012 and the Strait of Juan de Fuca. Sea-air CO fluxes from the recent data for specific subregions of the coastal Available online 28 January 2012 2 ocean, selected based on geography or bathymetry, were used to make seasonal area-specific estimates of CO2 exchange. These show significant differences between subregions, which have important conse- quences for estimating seasonal area-weighted fluxes on the margin. Climatologies of sea-air CO2 flux were calculated from the historical data using two approaches: One based on fluxes calculated from temporally-averaged values of sea-air pCO2 differences, solubility and gas transfer velocities, and the other from temporally-averaged instantaneous flux estimates. Seasonal flux estimates from our recently-collected data are consistent with the climatological estimates, in that both show stronger outgassing of CO2 in autumn relative to winter, and both reveal straits as important atmospheric CO2 source regions. Taken together, both analyses of recent and historical data suggest that the transition seasons (spring and autumn) contain the largest (positive and negative) fluxes because of the coincidence of high gas transfer velocities and large surface seawater disequilibria with the atmosphere. By combining the results from these analyses, and making some assumptions where data are missing, we estimate moderate net À2 À1 annual sea-air CO2 influx in the western Canadian coastal ocean of À6 mmol m d . Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction exists for open waters of continental shelf systems, it may be more pronounced within inner waterways, such as straits, fjords, and It is difficult to constrain the role of the coastal ocean in the ex- estuarine environments (Laruelle et al., 2010; Cai, 2011). The com- change of CO2 with the atmosphere because continental margin plex coastline of western Canada (British Columbia; BC) epitomizes settings exhibit significant spatial and temporal variability that is this dilemma that occurs at the global scale, as the majority of challenging to adequately sample (Borges et al., 2005; Cai et al., measurements have been collected on the open shelf (Ianson 2006; Hales et al., 2008; Chen and Borges, 2009; Laruelle et al., et al., 2003; Nemcek et al., 2008; Wong et al., 2010) while few mea- 2010). Data compilations have been the best approach to charac- surements exist for the semi-enclosed sounds, fjords and inside terize and overcome the aliasing caused by large variability on con- passage areas (Nemcek et al., 2008). These inadequacies in existing tinental margins, and these have shown that most open shelves in data highlight the need to increase the temporal and spatial cover- the mid to high latitudes are sinks for atmospheric CO2 (Borges age of observations to better characterize continental margin et al., 2005; Cai et al., 2006; Laruelle et al., 2010), while inner settings on both regional and global scales. waterways and estuaries are sources (Borges et al., 2005; Cai The western Canadian coastal ocean is an important area of high et al., 2006; Chen and Borges, 2009; Cai, 2011). However, an issue primary productivity along the North American west coast (Denman common to many coastal sites is limited spatial and temporal data et al., 1981; Ware and Thomson, 2005), and observations to date coverage relative to the system variability, and while this problem have shown that drawdown of surface ocean pCO2 occurs on the shelf west of Vancouver Island in summer (Ianson et al., 2003; Nemcek et al., 2008; Wong et al., 2010). Data with the greatest ⇑ Corresponding author. Present address: Ocean Acidification Research Center, spatial coverage have existed only for summer (Ianson et al., 2003; School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, Nemcek et al., 2008), and the best temporally-resolved data are lim- AK, USA. Tel.: +1 541 207 5943. ited to subregions of the margin: the southwest (SW) Vancouver Is- E-mail address: [email protected] (W. Evans). land shelf and the Strait of Juan de Fuca (Chavez et al., 2007; Hales 1 Present address: Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia. et al., 2008; Wong et al., 2010). Excluding data from within these 0079-6611/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2012.01.003 Author's personal copy W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 79 subregions, published pCO2 data have not existed for non-summer July 20 to August 15, 2010. The intake depth for the seawater flow- seasons. pCO2 data in existence have shown that significant through system was 3 m on both ships. temporal and spatial variability exists on this margin. The western Even with our extensive field efforts, we were unable to sample Canadian coastal ocean houses the terminus of the California all areas of the study region with uniform spatial and temporal Current System (Pennington et al., 2010; Foreman et al., 2011), so coverage. We are thus unable to analyze these data by means of summer values on the shelf, at times, can greatly exceed atmo- simple averaging approaches, which would lead to disproportion- spheric levels as a result of wind-driven upwelling that brings ate weighting of the well-sampled areas that might not be repre- high-pCO2 water to the surface (Ianson et al., 2003; Nemcek et al., sentative of the region as a whole. As a result, we categorized 2008). High-pCO2 surface waters have also been observed during data as falling within specific subregions of the BC coastal ocean summer in some of the straits around Vancouver Island (Nemcek (Fig. 1), each of which we further parsed into seasonal temporal et al., 2008), and on the inner region of the SW Vancouver Island shelf intervals. These subregions were selected based on their geogra- where low salinity water from the Strait of Juan de Fuca flows north- phy, and bathymetry for the case of the subregions on the shelf ward in the Vancouver Island Coastal Current (Ianson et al., 2003). (<200 m) west of Vancouver Island (Fig. 1). The subregions com- There are limited measurements of surface seawater pCO2 in the prised two areas on the open Vancouver Island shelf (subregions other straits and fjords that make up the complex BC coastline, but 1 and 2) that are separated by a large promontory (Brooks Penin- these suggest that large heterogeneity in CO2 exchange with the sula); the large semi-enclosed sound north of Vancouver Island atmosphere exists in the western Canadian coastal ocean when (subregion 3; collectively referred to as Queen Charlotte Sound, the entire margin is considered (Nemcek et al., 2008). but is a combination of Queen Charlotte Sound and Hecate Strait); Thus far the region has been characterized as a weak to moderate the straits on the northern side of Vancouver Island (subregion 4; annual sink for atmospheric CO2 based on temporally well-resolved collectively referred to as Johnstone Strait, but is a combination but spatially-limited data (Chavez et al., 2007) and a subregion- of Queen Charlotte Strait, Broughton Strait, Johnstone Strait and specific biogeochemical model (Ianson and Allen, 2002). The annual Discovery Passage); the Strait of Georgia (subregion 5); and the flux of CO2 for this region has been estimated to be near Strait of Juan de Fuca (subregion 6). À2 À1 À1 mmol CO2 m d (negative fluxes are directed into the ocean; Ianson and Allen, 2002; Chavez et al., 2007). The estimate from Chavez et al. (2007) was based on an analysis of the Carbon Dioxide 2.1. OSU underway pCO2 system Information and Analysis Center (CDIAC) database of pCO2 measurements collected around North America from the coast to 1000 km The OSU underway pCO2 measurement system used a LI-COR LI- from shore.

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