Sea-Air CO2 Fluxes in the Western Canadian Coastal Ocean

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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

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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

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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 tem-

porally-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 2 1 annual sea-air CO2 inﬂux in the western Canadian coastal ocean of 6 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

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). 2 1 1 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. Their estimate was produced from a monthly-resolved 840 infrared (IR) CO2 analyzer to detect the CO2 content (±1 ppm at 370 ppm) of a gas stream equilibrated with flowing seawater using composite year of pCO2 data with 1° latitude by 1° longitude pixels. Hales et al. (2008) subsequently showed that most of the pixels in a miniature membrane contactor (Liqui-Cel 1x5.5). Prefiltration was achieved using a custom 8-lm tangential flow filter, in which 1–10% the western Canadian coastal ocean had observations of pCO2 in fewer than 2 months of the composite year. The majority of BC of the main flow was directed tangentially to the membrane contac- margin observations in the CDIAC data set are on the SW Vancouver tor. Sample liquid flow through the contactor was typically 1 Island shelf and Strait of Juan de Fuca because of the route ships-of- 300 ml min , while the atmospheric air carrier flow rate was 1 opportunity travel between the strait and the open North Pacific. fixed at 30 ml min . The carrier gas consisted of marine air, and Here we present recently-collected data from a large portion of no drying of the sample (seawater and atmospheric) or standard the BC margin during winter, summer and autumn. We use our data set to make seasonal estimates of the sea-air CO2 fluxes for six distinct subregions of the BC margin. Average area-specific fluxes for each subregion are then combined to create seasonal area-weighted fluxes representative of the western Canadian coastal ocean. These estimates are placed in context with analyses of the historical (1995–2001) CDIAC underway pCO2 measurements from the SW Vancouver Island shelf and the Strait of Juan de Fuca. The results from these approaches provide estimates of

CO2 exchange that span a broad area of the BC margin, and a framework for discussing the importance of temporal and spatial data coverage in the estimation of sea-air CO2 exchange in this geographically complex continental margin setting.

2. Site and methods

To examine sea-air CO2 fluxes in the western Canadian coastal ocean, we built a pCO2 data set that covered a large portion of the Fig. 1. Map showing the study area in the western Canadian (British Columbia) margin using two different ships during three seasons, and analyzed coastal ocean. Vancouver Island, the Fraser River mouth, and Brooks Peninsula are highlighted. The black enclosures represent the subregions where cruise data were historical measurements of pCO2 from the CDIAC repository. The Canadian Coast Guard ship (CCGS) W.E. Ricker was equipped with selected for calculating sea-air CO2 fluxes (subregion 1 = southwest Vancouver Island shelf; subregion 2 = northwest Vancouver Island shelf; subregion 3 = Queen an underway pCO2 measurement system (referred to as the OSU Charlotte Sound; subregion 4 = Johnstone Strait; subregion 5 = Strait of Georgia; underway pCO2 system; Hales et al., 2004, as modified by Evans subregion 6 = Strait of Juan de Fuca). White stars in subregions 1, 3 and 5 are et al. (2011)) for cruises in autumn (October 8 to November 14, Environment Canada buoys 46206, 46204 and 46131, respectively. The white star in 2008) and winter (February 25 to March 14, 2009). Summer data subregion 4 is the Port Hardy Airport on Vancouver Island, and the white star in subregion 6 is the NOAA NDBC buoy 46088. Bathymetry (color bar, m) was provided were collected using the Fisheries and Oceans Canada, Institute of by the National Geophysical Data Center (http://www.ngdc.noaa.gov/mgg/global/ Ocean Sciences’ (IOS) pCO2 system (Wong et al., 2010) during the global.html). (For interpretation of the references to color in this figure legend, the West Coast Ocean Acidification cruise aboard the CCGS J.P. Tully from reader is referred to the web version of this article.) Author's personal copy

80 W. Evans et al. / Progress in Oceanography 101 (2012) 78–91

gas streams was done. Data were collected at 1 Hz, and standard FCO2 ¼ kSST KCO2 DpCO2 ð1Þ sequences using gases of known CO mixing ratio (xCO , ppm; 2 2 where k is the gas exchange coefficient at the in situ SST (m d1; 148, 454 and 758 ppm) were run every 2 h and used to correct for SST corrected from k as described below), K is the solubility of CO IR analyzer inaccuracy. Atmospheric samples were also run with 600 CO2 2 (mmol m3 latm1; Weiss, 1974) and DpCO is the sea-air pCO every standard sequence. Calibrated seawater xCO data were ad- 2 2 2 difference (latm). Gas transfer velocity expressed as k is the justed to pCO using the measured total pressure in the equilibrator. 600 2 gas exchange rate for CO at a Schmidt number (Sc) of 600 (the ratio The calibrated atmospheric xCO data were converted to pCO using 2 2 2 of the kinematic viscosity of water to the diffusion coefficient of CO atmospheric pressure measured in the LI-COR cell, which has pro- 2 in freshwater at 20 °C), and was calculated using the following qua- ven to be an accurate measurement of the ambient total pressure dratic wind speed dependency from Ho et al. (2006): when vented to the atmosphere. The pCO2 system was integrated with a Seabird SBE45 for temperature and salinity, and a probe for 2 k600 ¼ 0:266 U10 ð2Þ monitoring temperature in the equilibrator. The ship provided surface seawater temperature at the flow-through system intake. where U10 is the wind speed referenced to 10 m. All wind speeds used in this manuscript were adjusted to a reference height of Equilibrator-temperature pCO2 was then corrected to pCO2 at sea surface temperature (SST) using the difference between ship intake 10 m using the power law relationship described by Hsu et al. and equilibrator temperatures following Takahashi et al. (1993) as (1994). kSST was converted from k600 using the following relation- recommended in Dickson et al. (2007), after accounting for the ship described by Wanninkhof (1992): flow-based lag time (50 s) between those two temperature sensor 0:5 k ¼ k ðSc =600Þ ð3Þ locations. Lag time was estimated by cross-correlation, with the SST 600 SST lag equal to the time step corresponding to the maximum correla- where ScSST represents Sc for in situ conditions. ScSST was calculated tion coefficient multiplied by the sample interval. Seawater flow using the freshwater CO2 coefficients for the least squares third-or- rate was also monitored downstream of the pCO2 equilibrator and der polynomial fit of Sc number versus temperature with the rela- used for data quality control; data from all measurements were tionship also described by Wanninkhof (1992). A salinity 1 removed during periods of low flow (<100 ml min ) to the correction of kSST data was not conducted here because the differ- equilibrator. ence between kSST in freshwater versus saltwater (S = 35) is very A computer and hard drive theft following the winter cruise small, on the order of 4%, and all our measurements fell in a rela- resulted in the partial loss of pCO2 data and all GPS information. tively narrow salinity range of 15–33. The DpCO2 for each cruise Therefore we were forced to present only the measurements that was calculated as the difference between the seawater pCO2 mea- were collected near CTD casts, where we could interpolate the posi- surements and the mean atmospheric pCO2 from each cruise. Mean tion and time information between adjacent cast locations. We were atmospheric pCO2 was used because, once measurements contami- also required to correct the pCO2 at the equilibrator temperature to nated by ship exhaust were removed, the variability (indicated by pCO2 at SST using a constant temperature offset of 0.5 °C, which was the standard deviation) in the remaining cruise data was small. the average temperature offset observed during the autumn cruise. Our recently-collected data lacked ship-based measurement of winds, therefore we used wind speeds from the Port Hardy Airport

2.2. IOS underway pCO2 system on Vancouver Island, Environment Canada buoys 46206, 46204, and 46131, and the National Oceanic and Atmospheric Administra-

Summer cruise pCO2 data collected aboard the CCGS J.P. Tully tion (NOAA) National Data Buoy Center (NDBC) buoy 46088 (posi- were obtained using the IOS underway pCO2 system described by tions shown in Fig. 1) to calculate kSST for the time period that each Wong et al. (2010). This system used a showerhead equilibrator that subregion was occupied. These wind data were hourly (with the has a pressure compensator to maintain the equilibrator at atmo- exception of NOAA NDBC buoy 46088 record that was 10 min spheric pressure. Inflow and equilibrator temperatures were mea- winds) and were interpolated to the temporal resolution of the sured continuously. xCO2 was measured every 2.5 min in the pCO2 data. This treatment inherently makes the assumption of spa- equilibrated air with a LI-COR LI-6262 IR CO2 sensor (±1 ppm at tial and temporal coherence in the wind fields across each subre- 350 ppm). A series of 20 consecutive measurements were made. At gion, which introduces a source of uncertainty. While the winds the end of the seawater sample series, gases of known xCO2 (0, are probably not perfectly coherent over such large distances, 250, 450 and 800 ppm) were sampled to correct for IR analyzer inac- any lack of wind coherence is not likely to significantly affect our curacy. An atmospheric sample was run following the gas standards. CO2 flux calculations. That is, surface seawater pCO2 does not cor- The entire sequence of seawater, standard gas and atmospheric sam- relate with the wind speed (Takahashi et al., 2009). pling took approximately 70 min. No drying was conducted of the Subregion 2 lacked a wind record (Fig. 1). A cross-correlation sample, atmospheric or standard gas streams. Calibrated seawater analysis was conducted on the wind records from buoys 46206 and atmospheric xCO2 measurements were adjusted to pCO2 using and 46204 over a 6-month period (August 1, 2009 to March 1, atmospheric pressure measured in line between the equilibrator 2010) to determine the coherence of the records between these and the LI-COR cell. Measurement-temperature pCO2 was then cor- buoys. These winds were most coherent at a 1-h lag (r = 0.66), rected to pCO2 at SST using the difference between inflow and equil- and there was no significant offset in wind speed between these ibrator temperatures, after accounting for the flow-based lag time two records after accounting for the lag. Because the winds were between those two temperature sensor locations, and the relation- not largely different between the two buoy locations, and because ship for the temperature effect on isochemical seawater described of the proximity of subregion 2 to buoy 46204, wind speeds from by Takahashi et al. (1993). Seawater temperature and salinity were 46204 were used to compute gas transfer velocity in subregion 2. monitored continuously throughout the cruise using a Seabird For the case of our historical flux analyses for subregions 1 and

SBE45. All salinity data are reported in this manuscript using the 6, wind data were not present in the CDIAC underway pCO2 data Practical Salinity Scale (PSS-78, dimensionless). repository, so measurements from the Environment Canada buoy 46206 (Fig. 1) and the NOAA NDBC Smith Island C-MAN station

2.3. Sea-air CO2 ﬂuxes (adjacent to buoy 46088; Fig. 1) were used to estimate gas transfer velocity. k600 was calculated using Eq. (2) with hourly wind speed

Sea-air CO2 ﬂux (FCO2 ) was calculated using the following data from both locations collected between January 1995 and equation: August 2001. Weekly running average k600 was then calculated Author's personal copy

W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 81 to reduce some of the high-frequency variability in the winds, but from subregion 1 using 30-yearday running averages made at maintain variability relevant to surface seawater pCO2, such as on weekly intervals. This averaging scheme was used in order to max- the time scale of phytoplankton bloom formation (days) and sur- imize the amount of pCO2 data incorporated into each yearday face water mass residence time (days to weeks). The conversion average, yet retain some intra-seasonal variability. This averaging of k600 to kSST from the weekly running average k600 data, and the approach was made over broader time intervals to achieve this calculation of climatological sea-air CO2 ﬂuxes from the historical requirement for the subregion 6 data because the data density data for the two subregions are discussed below. was much reduced relative to that in subregion 1. For subregion 6, 90-yearday running averages were made at biweekly intervals. 2.4. Seasonal area-weighted BC margin ﬂuxes In order to maintain continuity between the beginning and end of each climatology, a parcel of data (15 and 45 yeardays for sub-

The sea-air CO2 fluxes computed from our recently-collected regions 1 and 6, respectively) from the end of each record was ap- data in each subregion in Fig. 1 were used to calculate the seasonal pended to the beginning of each yearday record. Similarly, parcels area-weighted fluxes. Seasonal means and standard deviations for from the beginning of each yearday record were appended to the each subregion were computed from the instantaneous fluxes. end of each record. k600 climatologies were converted to climato- Area-weighted seasonal fluxes for the BC margin were calculated logical kSST using climatologies of the historical SST data, which using these means with the following equation: were computed in the manner described above, with the Wannink- hof (1992) relationship for the Sc number dependence of k Eq. Xn Xn SST (3). The climatological sea-air CO2 fluxes were then calculated as FCO2ðawÞ ¼ ðFCO2ðiÞ AiÞ Ai ð4Þ i¼1 i¼1 the product of the DpCO2, solubility and kSST climatologies Eq. (1). This approach is subsequently referred to as the climatology where F is the area-weighted flux, F is the mean flux in CO2 ðawÞ CO2 ðiÞ approach. The flux variability of the climatology approach for both each subregion, Ai is the area of the subregion and n is the number subregions was estimated as ±the product of the standard devia- of subregions (6 in this case). tions from the yearday running averaged DpCO2 and kSST multiplied by the solubility climatology. 2.5. Historical underway DpCO data 2 In the second approach, climatologies of sea-air CO2 flux were calculated from individual fluxes computed by time-syncing the We conducted analyses of the historical CDIAC underway pCO 2 DpCO2 and solubility with the weekly running average k600 record. measurements (http://cdiac3.ornl.gov/waves/underway/) from Prior to calculating instantaneous fluxes, k600 was converted to kSST. subregions 1 and 6 shown in Fig. 1. These regions are the most Parcels of flux data were appended to the beginning and end of each temporally well-resolved in the CDIAC data set for the BC margin, record in the same manner as previously described. Yearday run- with the majority of measurements collected between January ning averages of the instantaneous fluxes were made over the same 1995 and August 2001, so our analysis focused on this period. time windows described above at weekly and biweekly intervals for Accompanying the underway pCO2 data in the CDIAC repository subregions 1 and 6, respectively. The second approach is subse- were coincident measurements of sea surface temperature and quently referred to as the instantaneous approach. salinity. To calculate the sea-air pCO2 difference from these seawa- Average fluxes from both approaches were then computed to ter measurements, atmospheric CO data were acquired from 2 produce seasonal and annual sea-air CO2 flux estimates for the GLOBALVIEW-CO2, a product that is maintained by the NOAA SW Vancouver Island shelf and the Strait of Juan de Fuca (subre- Earth System Research Laboratory (GLOBALVIEW-CO2, 2011). gions 1 and 6; Fig. 1). The seasons are delineated following the sea- These data were obtained for the latitude of Vancouver Island sonal cycle in the northern North Pacific described by Zeng et al. (49.58°N) as mole fractions in dry air with a weekly sample inter- (2002), with winter as January 1 to March 31, spring as April 1 to val. Average atmospheric values were calculated for each month June 30, summer as July 1 to September 30, and autumn as October spanning the 1995–2001 time period. Hourly atmospheric pres- 1 to December 31. This seasonal demarcation also generally fol- sure measurements were acquired from the Environment Canada lows the harmonic-fit seasonal cycles of wind stress, sea level buoy 46206 and the NOAA NDBC Smith Island C-MAN station, and temperature for this latitude described by Strub et al. (1987). and averaged into monthly values. Hourly air temperature data Thomson and Ware (1996) proposed a different demarcation of were also acquired from both sites, converted to estimates of water the seasons, which, when employed, had a negligible effect on vapor pressure at 100% humidity using the relationship described the seasonal fluxes, and no impact on the annual fluxes. by Buck (1981), and averaged into monthly values. Monthly atmospheric pCO2 was then calculated using the equation: 3. Results pCO2ðmonthlyÞ ¼ xCO2ðmonthlyÞ ðPbðmonthlyÞ PwðmonthlyÞÞð5Þ where pCO2(monthly) and xCO2(monthly) are the monthly average pCO2 3.1. Seasonal and spatial pCO2 variability on the BC margin from (latm) and xCO2 (ppm) values, and Pb(monthly) and Pw(monthly) are the recent data monthly average barometric (atm) and water vapor (atm) pres- sures. The sea-air pCO2 difference from the CDIAC data was then The three cruise tracks varied in the coverage of the BC margin; calculated as the difference between the seawater pCO2 measure- therefore we limit our analysis to only the data collected within ments and the corresponding monthly atmospheric pCO2. each subregion (Fig. 1; dark gray enclosures in Fig. 2) in order to facilitate regional comparison between the cruise observations. It

2.6. Sea-air CO2 ﬂux climatologies from historical data should be noted, however, that this data set includes observations from several smaller sounds, open ocean waters and inside pas-

We calculated sea-air CO2 ﬂux climatologies from the historical sages, and the data from some of these areas are shown in Fig. 2. DpCO2 data from both subregions 1 and 6 using two approaches in The winter cruise began February 25 and ended March 14, 2009. order to determine differences in using long-term averaged versus The SST, salinity and pCO2 measurements from the winter cruise instantaneous measurements. In the ﬁrst approach, seasonally-re- covered a large range; from 4.5 °C to 8.9 °C, 20 to 32.3 and solved climatologies of DpCO2, solubility, and k600 were calculated 259 latm to 864 latm, respectively (Fig. 2). Owing to the data loss from the 81-month records of the historical data described above described previously, we present measurements that were Author's personal copy

82 W. Evans et al. / Progress in Oceanography 101 (2012) 78–91

Fig. 2. SST (top row; °C), salinity (middle row) and pCO2 (lower row; latm) for the winter (left column; February 25 to March 14, 2009), summer (middle column; July 20 to August 15, 2010) and autumn (right column; October 8 to November 14, 2008) cruises. Dark gray enclosures represent the subregions shown in Fig. 1. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

collected in the vicinity of CTD casts where it was possible to inter- Charlotte Sound and on the SW Vancouver Island shelf. The mean polate between closely neighboring CTD stations. This data loss atmospheric pCO2 during this cruise was 388 ± 3 latm. meant that the entire Johnstone Strait region was not included in The autumn cruise occurred in two legs and began October 8 this season’s analysis. pCO2 was most variable within the Strait and ended November 14, 2008. Sea surface temperature (SST), of Georgia, and most of these data indicated that the strait was salinity, and pCO2 ranged from 7.9 °C to 13.5 °C, 15 to 31.9, and functioning as a source of CO2 to the atmosphere (Fig. 2). However, 365 latm to 867 latm, respectively (Fig. 2). The majority of obser- there were a number of observations of pCO2 below atmospheric vations were above atmospheric levels, except within Queen Char- levels in the vicinity of the Fraser River (location shown in Fig. 1; lotte Sound. Surface waters within the subregions around Fig. 2) that demonstrate large variability can exist within this strait Vancouver Island, including Johnstone Strait, the Strait of Georgia, during winter. The measurements made on the Vancouver Island the Strait of Juan de Fuca and the west coast of Vancouver Island, shelf were more homogenous with values between 350 and were signiﬁcantly oversaturated with respect to atmospheric

400 latm. The waters in Queen Charlotte Sound were mostly CO2. The mean atmospheric pCO2 measured during this cruise above saturation with respect to the atmosphere, and values ran- was 388 ± 5 latm. ged from 385 to 450 latm. The mean atmospheric pCO2 measured during this cruise was 394 ± 4 latm. 3.2. Seasonal and spatial BC margin sea-air CO2 ﬂux variability from Our summer cruise began July 20 and ended August 15, 2010. recent data

The SST, salinity and pCO2 data from this cruise again covered a large range; from 9.6 °C to 20.7 °C, 19.3 to 33.0 and 145 latm to Sea-air CO2 fluxes from the winter, summer and autumn cruise 794 latm, respectively (Fig. 2). Despite some high values, most of data show that, in any season, almost all regions experienced some the observations during this cruise indicated that surface waters level of both influx and efflux conditions (Fig. 3). In winter, the were strongly undersaturated with respect to atmospheric CO2. Re- Strait of Georgia experienced one instance of CO2 outgassing of 2 1 gions where pCO2 levels were above atmospheric were on the shelf 48 mmol m d , while the SW Vancouver Island shelf experi- off the northwestern portion of Vancouver Island (subregion 2) enced a maximum influx of 12 mmol m2 d1 (Fig. 3; Table 1). during a period of upwelling, as evidenced by the occurrence of In summer, the Strait of Georgia experienced maximum efflux of cool SSTs (10 °C) with high salinity (32.5), and in Johnstone 37 mmol m2 d1 and maximum influx of 32 mmol m2 d1

Strait and the Strait of Juan de Fuca (Fig. 2). Excluding these re- (Fig. 3; Table 1). In autumn, the efﬂux of CO2 peaked at gions, the margin was potentially acting as a strong sink for atmo- 141 mmol m2 d1 in the Strait of Juan de Fuca, while the inﬂux 2 1 spheric CO2, with the lowest pCO2 values observed within Queen was greatest in Queen Charlotte Sound at 7 mmol m d Author's personal copy

W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 83

(Fig. 3; Table 1). These instantaneous ﬂuxes demonstrate that the the largest mean ﬂuxes for the majority of subregions were during

BC margin experiences a large dynamic range in sea-air CO2 fluxes autumn, and directed out of the ocean (Table 1). But autumn also both within and between seasons, and across subregions. In gen- contained the largest variability (indicated by the standard devia- eral, influxes were greatest and more broadly distributed during tion and range of flux values in Table 1), and the means for most summer, and effluxes dominated during autumn in most subre- subregions were impacted by large flux events (Fig. 3). gions (Fig. 3). Atmospheric CO2 influx was not observed within Using the mean fluxes from the subregions and Eq. (4), the sea- the Strait of Juan de Fuca or Johnstone Strait (Fig. 3). sonal area-weighted sea-air CO2 fluxes for the BC margin were 2.4, 2 1 Means and standard deviations of the sea-air CO2 fluxes from 6.6 and 4.6 mmol m d for winter, summer and autumn, each subregion (Table 1) represent seasonal estimates for winter, respectively (Table 1). Note that the area-weighted flux for winter summer and autumn over a broad spatial range (Fig. 3). Large stan- does not include Johnstone Strait (subregion 4) because of a lack of dard deviations of the mean fluxes are caused by variability in sur- data, and therefore, the flux estimate for this season may be under- face water pCO2, wind speed and solubility in each subregion, and estimated because it does not include this areally small, albeit reflect system variability as opposed to measurement inaccuracy. potentially strong atmospheric CO2 source region. These area- These broad spatial averages are snapshots in time, and can include weighted seasonal margin-wide averages show largest CO2 efflux extreme flux events. For instance, most of the subregions on the in autumn, and strong uptake during summer. The significant spa- margin during winter acted on average as sources of atmospheric tial variability discussed above is averaged into these seasonal esti-

CO2, however there was large variability factored into these means. mates, and is important to consider when examining seasonal The Strait of Georgia had the largest mean winter CO2 efflux fluxes based on snapshots of spatial data. (6 ± 10 mmol m2 d1; Table 1), but Fig. 3 shows that this was dominated by the occurrence of large positive fluxes on the north- 3.3. Seasonality in historical data from subregions 1 and 6 ern portion of the strait and that there were several instances of in-

flux. In summer, Johnstone Strait had the largest mean CO2 efflux 3.3.1. Subregion 1: SW Vancouver Island shelf 2 1 (16 ± 6 mmol m d ; Table 1), while the Strait of Georgia was a Historical underway pCO2 data contained a clear seasonal pat- 2 1 region of strong atmospheric CO2 uptake (18 ± 10 mmol m d ; tern in subregion 1, with values predominantly exceeding Table 1). However, the Strait of Georgia had the largest instanta- 400 latm during autumn and winter, while being largely below neous summer efflux of any subregion (Fig. 3; Table 1). Finally, 350 latm during spring and summer (Fig. 4). Our observations are strongly consistent with these historical patterns. This trend

was reﬂected in DpCO2, and is nearly opposite to the SST trend on the SW Vancouver Island shelf (Fig. 5). During winter, surface waters are mostly oversaturated with respect to the atmospheric

pCO2, and SSTs are cool (9 °C; Fig. 5). DpCO2 trends towards more negative values as SST increases from late February through May

(Fig. 5), with the lowest DpCO2 in mid-spring (May; 220 latm). All 7 years, 1995–2001, showed consistently negative mid-spring

DpCO2 values with no evidence of oversaturation until late spring (June; Fig. 5). We assume that the positive late-spring values were caused by the initiation of upwelling because this was when large variability in SST ﬁrst appeared (Fig. 5). Undersaturated conditions persisted in summer, but to a lesser degree than during spring, and

variability in DpCO2 increased (Fig. 5). DpCO2 values ranged between 200 and +300 latm, which is the largest dynamic range

of any season. This large summer range of DpCO2 resulted from upwelling, as evidenced by the variability in SST at this time

(Fig. 5), that drove elevated surface water pCO2 interposed with low pCO2 conditions that were caused by strong biological CO2 drawdown. During autumn, SSTs decreased and DpCO2 was generally well above equilibrium with the atmosphere (Fig. 5). Large po-

sitive DpCO2 values are prevalent during this season, and 5 of the 7 years of observations during mid-autumn (November) show the dominance of oversaturated conditions. Overall, these data indi-

cated the potential for CO2 uptake from the atmosphere on the SW Vancouver Island shelf was highest in spring. Conversely, the

potential for CO2 outgassing was greatest in autumn.

3.3.2. Subregion 6: Strait of Juan de Fuca

The historical underway pCO2 data from subregion 6 (Fig. 6) showed a different seasonal pattern than seen in subregion 1

(Fig. 4). Surface water pCO2 values well above atmospheric dominated during nearly every calendar month, however there were in-

stances of surface seawater pCO2 undersaturation with respect to the atmosphere. Also distinct from the historical data from subregion 1 was the lower and more variable density of observations (Fig. 6). Most noteworthy is the extremely limited historical au-

2 1 tumn data in this subregion. As was the case for subregion 1, our Fig. 3. Sea-air CO2 ﬂuxes (mmol m d ) from the winter (top), summer (middle) and autumn (lower) cruises calculated from data collected within the subregions recent observations are broadly consistent with the historical re- shown in Fig. 1. cords. The record of subregion 6 historical DpCO2 data shows large Author's personal copy

84 W. Evans et al. / Progress in Oceanography 101 (2012) 78–91

Table 1 2 1 Seasonal means, standard deviations, minima and maxima of the sea-air CO2 fluxes (mmol m d ) from the data shown in Fig. 3 for each subregion defined in Fig. 1. Seasonal 2 1 margin-wide sea-air CO2 fluxes (mmol m d ) for the western Canadian coastal ocean are also shown. Note that the margin-wide flux estimate for winter excluded subregion 4 due to a lack of data.

Subregion 1 2 3 4 5 6 Margin-wide Area (km2) 3979 1357 39,945 3463 7036 3316 Winter mean 3.9 0.3 2.9 No data 5.5 0.4 2.4 Winter std 3.1 0.7 3.1 No data 9.7 0.5 Winter min 11.9 0.7 2.0 No data 9.7 0.0 Winter max 5.0 2.7 19.1 No data 48.5 4.1 Summer mean 3.2 2.9 8.0 15.6 17.5 2.0 6.6 Summer std 2.9 6.0 5.3 5.9 10.4 1.1 Summer min 14.8 10.4 26.8 2.5 31.8 0.0 Summer max 1.8 23.0 0.0 28.8 37.0 4.9 Autumn mean 17.4 21.3 0.4 7.1 10.0 28.4 4.6 Autumn std 18.4 15.8 2.7 13.1 6.4 41.1 Autumn min 5.7 3.0 7.4 0.0 0.1 0.0 Autumn max 64.6 44.1 7.9 66.0 26.7 141.3

Fig. 4. Monthly distribution of CDIAC underway pCO2 (latm) measurements for the SW Vancouver Island shelf (subregion 1; black polygon) from 1995 to 2001.

variability exists within every season (Fig. 7). The large dynamic autumn. Both seasons experience similar kSST (Fig. 8) but different range of DpCO2 values observed during the summer in subregion pCO2 conditions (Fig. 5), with autumn surface water containing the 1 is characteristic of subregion 6 throughout the year, with the highest levels of pCO2 (Fig. 5) and the largest instantaneous fluxes exception that this range is shifted to mostly positive values. The (Fig. 8). Conversely, the largest CO2 influx from both approaches majority of negative DpCO2 observations in the strait were ob- was in spring, with the most negative fluxes observed between served during spring, but these had little effect on the climatolog- mid-April and mid-May. Even though DpCO2 on the SW Vancouver ical DpCO2 (Fig. 7). In general, the Strait of Juan de Fuca functioned Island shelf showed large departures from atmospheric values dur- as a potential CO2 source to the atmosphere throughout the year ing summer (Fig. 5), winds and therefore kSST were weakest during (Fig. 7). this season so sea-air CO2 fluxes were reduced (Fig. 8). Also, the large variability in summer DpCO2 described above (Fig. 5) caused positive and negative instantaneous fluxes that resulted in lower 3.4. Sea-air CO2 flux climatologies from historical data in subregions 1 and 6 mean fluxes (Fig. 8). The strongest outgassing of CO2 occurred in autumn in both climatologies, which resulted from the larger posi-

tive DpCO2 and higher gas transfer velocities during that season (Fig. 8). 3.4.1. Subregion 1: SW Vancouver Island shelf

Sea-air CO2 flux climatologies calculated from the historical data using the climatological and instantaneous approaches 3.4.2. Subregion 6: Strait of Juan de Fuca showed consistent results for subregion 1 (Fig. 8), and the observa- The climatological sea-air CO2 fluxes for subregion 6 show that tions from our cruises are well within the patterns shown in the this subregion acts as a strong source for atmospheric CO2 historical data. Outgassing occurred in winter and autumn, and throughout the year, however there were differences between atmospheric CO2 uptake in spring and summer. Both flux estimates the results from the approaches used to construct the climatologies showed that winter CO2 efflux was lower than outgassing during (Fig. 9). The combination of climatological DpCO2 (Fig. 7) and kSST Author's personal copy

W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 85

Fig. 5. Seasonal cycles of SST (top; °C) and underway DpCO2 (bottom; latm) collected on the SW Vancouver Island shelf (Fig. 1; subregion 1) from the CDIAC database (1995–

2001; shown by year as colored dots). The blue lines are the climatological SST and DpCO2, calculated as weekly-resolved running averages. The averages and ranges of subregion 1 DpCO2 from our cruises are shown as gray diamonds and vertical bars, respectively. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 6. Monthly distribution of CDIAC underway pCO2 (latm) measurements for the Strait of Juan de Fuca (subregion 6; black polygon) from 1995 to 2001.

produced higher seasonality than seen in the smoothed instanta- should produce highly variable instantaneous sea-air CO2 fluxes, neous values (Fig. 9), with lower spring and summer fluxes which they do on sub-seasonal intervals (Fig. 9). The lower number followed be a large autumn peak and a subsequent drop through of observations within subregion 6 relative to subregion 1, how- winter to the spring/summer conditions. Somewhat counter-intu- ever, illustrates the pitfalls of the instantaneous approach. Surface itively, the seasonality of the smoothed instantaneous flux calcula- ocean pCO2 responds slowly to gas exchange, and should have no tion showed only about half the seasonal variability compared to short-term wind speed dependence. The seasonal patterns of kSST the flux calculated with the climatology approach (Fig. 9). The large and DpCO2 are reasonably coherent and offer no evidence of a ro- dynamic range of DpCO2 values observed during every season bust seasonal anti-correlation that should reduce the seasonality in within subregion 6, combined with the time-matched kSST data, sea-air CO2 exchange. The smoothed instantaneous approach is Author's personal copy

86 W. Evans et al. / Progress in Oceanography 101 (2012) 78–91

Fig. 7. Seasonal cycles of SST (top; °C) and underway DpCO2 (bottom; latm) collected in the Strait of Juan de Fuca (Fig. 1; subregion 6) from the CDIAC database (1995–2001; shown by year as colored dots). The blue lines are the climatological SST and DpCO2, calculated as biweekly-resolved running averages. The averages and ranges of subregion

6 DpCO2 from our cruises are shown as gray diamonds and vertical bars, respectively. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

thus likely skewed by fortuitous combinations of kSST and DpCO2 across subregions and over historical data records. From the histor- conditions that are not likely to be persistent. Our own observa- ical data records, spring is identified as the time of lowest sea-air tions lend credence to this interpretation. Although our DpCO2 CO2 efflux, as shown by the strongest seasonal sink in subregion observations are within the observed variability of the historical 1 and the weakest seasonal source in subregion 6. The historical data, our flux estimates from summer and winter are both near analysis suggests summer and winter are intermediate between zero and do not show variability that encompasses the smoothed spring and autumn, and, with the exception of the large opposing instantaneous flux. This is a result of anomalous low wind speeds fluxes in subregions 4 and 5, summer subregional fluxes are rela- during our transects of the Strait of Juan de Fuca. Regardless of ap- tively low as well. proach, the average net annual efflux from subregion 6 is esti- The seasonal cycle of sea-air CO2 flux on the SW Vancouver Is- mated at 17 mmol m2 d1; however, we believe the land shelf had previously been estimated using a biogeochemical climatological approach to flux estimation provides the more real- model (Ianson and Allen, 2002), which showed the region was a istic picture of the seasonality in sea-air CO2 exchange. moderate annual atmospheric CO2 sink because outgassing mostly The sea-air CO2 flux climatologies from both approaches occurred in winter, with influx the rest of the year. The conclusion showed that the net annual CO2 exchange between the coastal drawn from this modeling approach is that outgassing during win- ocean and atmosphere within subregion 1 is near zero, while this ter almost counterbalances pCO2 drawdown during other seasons is far from the case within subregion 6 (Table 2). On an annual ba- (Ianson and Allen, 2002; Ianson et al., 2009). The analyses we pres- sis, subregion 6 is an atmospheric CO2 source of at least ent here suggest that, while winter fluxes do appear to be positive 2 1 17 mmol m d . This subregion emits CO2 to the atmosphere and summer fluxes negative, outgassing is actually much greater during every season, while subregion 1 experiences influx of atmo- during autumn and encompasses more of the margin than just spheric CO2 during spring and summer that is largely offset by the SW Vancouver Island shelf (Figs. 3 and 8), and CO2 uptake is strong autumn and moderate winter outgassing (Table 2). The greatest during spring (Fig. 8). Our results suggest the periods with analyses of historical data from these subregions are consistent the greatest fluxes (positive or negative) are the transition seasons with our broader analysis of the BC margin from recent data, in when high gas transfer velocities and large surface seawater dis- that autumn outgassing dominates on a seasonal sense and fluxes equilibria with the atmosphere coincide. The periods of greatest in the straits were mostly directed toward the atmosphere. exchange with the atmosphere, and with the largest impact on net annual fluxes, are, therefore, spring (ocean sink) and autumn (ocean source). For the SW Vancouver Island shelf, these large flux 4. Discussion periods nearly cancel each other out; resulting in close to balanced

net annual sea-air CO2 exchange (Table 2).

The margin-wide seasonal sea-air CO2 ﬂuxes show autumn as A unique aspect of our results is that they indicate that the sea- the largest outgassing season (Table 1). With only the exception sonal sea-air CO2 ﬂuxes from most of the straits do not share the of Johnstone Strait, where we had no winter data and summer ef- same pattern discussed above for the open shelf, and, also, that

ﬂux was larger than autumn, and Queen Charlotte Sound, where not all of these straits function similarly in terms of sea-air CO2 ex- estimated autumn ﬂux was near zero, this observation was true change. The analysis of our results and historical data revealed that Author's personal copy

W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 87

1 Fig. 8. Calculation of climatological sea-air CO2 fluxes for the SW Vancouver Island shelf (subregion 1). Panel (A) is the weekly running average k600 (m d ; black dots) from the Environment Canada buoy 46206 for the 1995–2001 period. The blue line is the climatological k600, and the red line is climatological kSST calculated using the SST climatology in Fig. 5. Averages and ranges of kSST from our cruise periods are also shown (gray diamonds and vertical bars). Panel (B) is the sea-air CO2 flux (climatology 2 1 approach; mmol m d ), and the red area is the one standard deviation variability of the flux (described in Section 2.6). Panel (C) is the sea-air CO2 flux (instantaneous approach; mmol m2 d1; shown by year as colored dots), and the average and range of flux values from our cruises are shown as gray diamonds and vertical bars. Panel (D) is 2 1 the sea-air CO2 fluxes (mmol m d ) from the climatology (black) and instantaneous (blue) approaches with the y-axis rescaled. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the Strait of Juan de Fuca is a seasonally persistent significant other times. However, the Strait of Georgia transitioned to be, on atmospheric CO2 source (Fig. 9). This trait was also shown by the average, a large atmospheric CO2 sink during the summer from a analysis of our recent data that revealed Johnstone Strait was a source for atmospheric CO2 during winter and autumn (Table 1). substantial atmospheric CO2 source during summer and autumn The processes leading to these site-specific differences are difficult as well (Fig. 3 and Table 1). We expect high-pCO2 conditions and to diagnose with the present data set, but possible causes include high sea-air CO2 fluxes are also prevalent in Johnstone Strait at tidal energy dissipation and the volume and source of fresh water. Author's personal copy

88 W. Evans et al. / Progress in Oceanography 101 (2012) 78–91

1 Fig. 9. Calculation of climatological sea-air CO2 fluxes for the Strait of Juan de Fuca (subregion 6). Panel (A) is the weekly running average k600 (m d ; black dots) calculated from hourly wind speeds recorded at the NOAA NDBC Smith Island C-MAN station (adjacent to buoy 46088; Fig. 1) for the 1995–2001 period. The blue line is the climatological k600, and the red line is climatological kSST calculated using the SST climatology in Fig. 8. Averages and ranges of kSST from our cruise periods are also shown 2 1 (gray diamonds and vertical bars). Panel (B) is the sea-air CO2 flux (climatology approach; mmol m d ), and the red area is the one standard deviation variability of the flux 2 1 (described in Section 2.6). Panel (C) is the sea-air CO2 flux (instantaneous approach; mmol m d ; shown by year as colored dots), and the average and range of flux values 2 1 from our cruises are shown as gray diamonds and vertical bars. Panel (D) is the sea-air CO2 fluxes (mmol m d ) from the climatology (black) and instantaneous (blue) approaches with the y-axis rescaled. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Tidal mixing has been suggested as a driver of enhanced nutrients summer primary productivity (Harrison et al., 1983; Yin et al.,

(Masson, 2006) and pCO2 (Ianson et al., 2003; Nemcek et al., 2008) 1997; Masson and Peña, 2009). Entrainment and stratiﬁcation in the Strait of Juan de Fuca and Johnstone Strait, and this is likely caused by the river plume are drivers for the observed high rates responsible for the persistence of these regions as sources for of primary production (Yin et al., 1997; Masson and Peña, 2009), atmospheric CO2 between seasons. and this may have been at least partially responsible for the Conversely, a major feature of the Strait of Georgia is the Fraser occurrence of atmospheric CO2 inﬂux seen in the Strait of Georgia. River plume, which has been characterized by high spring and During the winter and summer cruises, instances occurred where Author's personal copy

W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 89

Table 2 2 1 Seasonal and annual sea-air CO2 ﬂuxes (mmol m d ) from the two approaches used on the CDIAC data to construct the climatologies for the SW Vancouver Island shelf (subregion 1) and the Strait of Juan de Fuca (subregion 6).

Subregion 1: Western Vancouver Island shelf Subregion 6: Strait of Juan de Fuca mmol m2 d1 Climatology approach Instantaneous approach Climatology approach Instantaneous approach Winter 3.7 3.2 17.1 11.4 Spring 7.5 7.4 9.3 18.0 Summer 2.1 2.3 12.6 20.9 Autumn 7.4 7.9 32.3 19.5 Annual 0.20 0.35 18.00 17.35

surface seawater was observed with pCO2 below equilibrium with flux estimates with the summer and autumn estimates in Table 1, 2 1 the atmosphere coincident with low salinity (<28; Fig. 2) in the we approximate the net annual flux is 6 mmol CO2 m d for vicinity of the Fraser River (Fig. 1). It is clear from our recent data, the entire BC margin. This estimate is considerably different from however, that the undersaturated condition of these waters was our result for the SW Vancouver Island shelf, and more negative seasonal, as low salinity waters within the Strait of Georgia in au- than previous flux estimates, even with the inclusion of large tumn had pCO2 above atmospheric levels (>400 latm; Fig. 2). To source regions in some of the straits. This is because, although sub- illustrate the importance of the straits as atmospheric CO2 source regions 4 and 6 impacted the seasonal area-weighted fluxes, the regions, the combined area of Johnstone Strait and the Strait of potentially large spring and summer uptake in the Strait of Georgia Juan de Fuca in this study was only 10% of the total margin area (17.5 mmol m2 d1; Table 1) and the moderate influx over the considered and yet these two subregions greatly influenced the vast area of Queen Charlotte Sound (Table 1) had an overbearing margin-wide summer area-weighted fluxes. Excluding these sub- influence on the estimate of net annual CO2 exchange. This exercise regions from the area-weighted flux calculation resulted in a 30% highlights a few important aspects of the BC margin. First, the mar- increase in the summer influx. The fluxes from these straits there- gin, as a whole, is likely a stronger net annual sink for atmospheric fore play an important role in the seasonal exchange of CO2 on the CO2 than previously thought. Second, and perhaps most important, margin and should be included in estimates of margin-wide net there are radically different area-specific seasonal fluxes in this annual sea-air CO2 flux. geographically complex coastal ocean, and these all need to be in- Given the observed seasonal and subregional variability of sea- cluded to achieve the most accurate estimation of net annual air CO2 exchange discussed above, does the near zero net annual fluxes. sea-air CO2 flux seen on the SW Vancouver Island shelf hold for Through our analyses, seasonal sea-air CO2 flux estimates were the entire BC margin, particularly given that the straits have a large produced using both a recently-collected, spatially-expansive set effect on seasonal area-weighted exchanges? Combining the re- of underway pCO2 measurements and temporally well-resolved sults from our analyses of historical and recent data, we can make historical data sets from specific subregions of the BC margin. In some assumptions that allow a best guess of the margin-wide net one case we have flux estimates from data well resolved in time, annual CO2 exchange to be made. First, our analysis of historical but less so in space. In the other case we have flux estimates better data from subregion 1 showed strong springtime atmospheric resolved in space, but not in time. The comparison of these esti-

CO2 uptake, and we expect that all open shelf subregions function mates reveals the biases from different observational strategies in this way at this time because this period precedes the occur- that are important to consider in any field program focused on rence of upwelling-favorable winds (Foreman et al., 2011), and in diagnosing net sea-air CO2 exchange, and in a global context for the absence of upwelling conditions, summertime open shelf better constraining coastal fluxes. For instance, the autumn fluxes fluxes are directed into the ocean (Table 1). Therefore, we assume from the climatologies suggest the SW Vancouver Island shelf 2 1 spring sea-air CO2 fluxes in subregions 1 and 2 are equal to the emits between 7 and 8 mmol CO2 m d to the atmosphere (Ta- subregion 1 climatological spring flux (Table 2). The large semi-en- ble 2), while the area-weighted autumn flux for the entire BC mar- 2 1 closed Queen Charlotte Sound was a strong sink for atmospheric gin is 4.6 mmol CO2 m d (Table 1). The difference here CO2 during summer, and we anticipate similar conditions during (neglecting the difference between the climatologies) is caused spring, so we set the spring flux in this subregion equal to the sum- by the disparity in temporal and spatial data coverage. The area mer flux (Table 1). There was no wintertime estimate of sea-air CO2 estimates are based on snapshots in time, and do not capture flux for Johnstone Strait, but the subregion was a strong source important time variability in this system, but do capture significant during summer and autumn, and we anticipate this subregion spatial variability. That is, the margin-wide autumn flux included functions similarly to the Strait of Juan de Fuca. We therefore as- not only source regions along the margin, but also the large Queen sume that the winter and spring fluxes in subregion 4 are equiva- Charlotte Sound that was, at the time, functioning as an atmo- lent to either the summer or autumn fluxes (Table 1). We expect spheric CO2 sink (Fig. 3 and Table 1). Without including this large that the Strait of Georgia is a strong sink during spring, as it was sink region, autumn fluxes for the BC margin would have been during summer, due to the influence of the Fraser River plume more positive and overestimated. In the opposite sense, the clima- and the ensuing spring phytoplankton bloom (Masson and Peña, tological fluxes for subregion 1 will inaccurately represent the 2009). Therefore, we set the springtime flux in subregion 5 equal margin-wide fluxes because they only capture important time var- to the summertime flux. Finally we set the spring flux from the iability within one portion of the margin, and it is clear from our Strait of Juan de Fuca equal to the seasonal estimate from the his- results that not all subregions on this margin function in the same torical data that was produced from our climatological approach manner as the SW Vancouver Island shelf. Sampling strategies that (Table 2). have a limited ability to resolve temporal and spatial variability Based on these assumptions, the estimated area-weighted can lead to substantial differences in even the sign of the flux 2 1 spring flux for the entire study area is about 7 mmol CO2 m d . (e.g. winter subregion 1 fluxes compared to climatological winter Updating our winter area-weighted flux with the assumed John- fluxes; Tables 1 and 2). This point concerning the importance of stone Strait fluxes raised that seasonal estimate to about adequate temporal and spatial data coverage has been demon- 2 1 3 mmol CO2 m d . Combining these area-weighted seasonal strated in the past (Jiang et al., 2008; Schiettecatte et al., 2007), Author's personal copy

90 W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 however fluxes on the BC margin have previously been assessed on but did, however, for fluxes in the Strait of Juan de Fuca (Table 2). mainly the open shelf alone (Chavez et al., 2007; Ianson and Allen, This was likely because the density of historical data on the SW 2002; Ianson et al., 2009) and increased temporal and spatial data Vancouver Island shelf is substantially greater than that in the coverage over the subregions of this margin is clearly important. It Strait of Juan de Fuca (Figs. 4 and 6). The greater coverage of data is apparent that the most robust estimate of net annual sea-air CO2 on the SW Vancouver Island shelf permitted a climatology to be exchange within such dynamic coastal settings requires the simul- constructed from instantaneous measurements that was not taneous use of data well resolved in time and space. biased by extreme flux events, whereas this was not the case for Numerous studies have documented the importance of interan- the Strait of Juan de Fuca. The Strait of Juan de Fuca climatology nual variability in net annual sea-air CO2 fluxes (Friederich et al., calculated using the instantaneous approach was heavily impacted 2002; Fransson et al., 2006; McNeil, 2010; Evans et al., 2011). by extreme flux events. This illustrates the necessity for constant The historical analyses presented here spans almost 7 years of monitoring in order to ensure that the variability of instantaneous observation across both strong El Niño and La Niña events (i.e. fluxes is appropriately handled. Properly resolving the time and 1998 and 1999, respectively), and therefore we assume has little space scales of variability is critical because not doing so can result bias towards El Niño or La Niña. This is perhaps not the case for in missing or improperly extrapolating important high-intensity lower frequency climate variability such as the Pacific Decadal events, which can have regional impacts. This requires exhaustive Oscillation (PDO; Mantua et al., 1997) and the North Pacific Gyre sampling, which presents a challenge for future studies because it Oscillation (Di Lorenzo et al., 2008). Feely et al. (2006) described is likely impossible to simply make direct measurements every- in detail how the 1997–1998 shift in PDO had increased outgassing where all the time, perpetually. The existing approach of opportu- flux in the equatorial Pacific by 27%. Because the climate variability nistic and dedicated sampling must be augmented with synthetic associated with both the PDO and NPGO is centered in the North techniques combining in-water measurements, remote-sensing, Pacific (Mantua et al., 1997; Di Lorenzo et al., 2008), it is very prob- and modeling approaches (e.g. Hales et al., submitted for able that these climate modes would be strong sources of variabil- publication). ity unaccounted for in current sea-air CO2 flux estimates for the BC Our margin-wide net annual estimates are more negative than, margin. Long periods of observation are necessary to resolve the but within the variance of (reported as the standard deviation), the impact of these climate modes on sea-air CO2 exchange in the BC net annual flux reported by Chavez et al. (2007). Chavez et al. coastal ocean. (2007) describe the region as a small net annual sink (approxi- 2 1 There are other sources of uncertainty in our flux estimates mately 1.1 ± 18 mmol CO2 m d ) and we estimate that the en- associated with (1) the physical decoupling between our in-water tire BC coastal ocean is a moderate sink of 6 mmol m2 d1. The measurements (SST, salinity and pCO2) and the wind data, (2) dif- difference between their work and ours is (1) we used recently-col- ferences in the current gas transfer parameterizations, and (3) lected data that spans more of the BC coastal ocean than just the combining average versus instantaneous winds with pCO2 data. open shelves, (2) they used a different quadratic wind speed To consider point (1), the cross-correlation between buoy 46206 parameterization to estimate the gas transfer velocity, and (3) we and 46204 (positions shown in Fig. 1) revealed that the winds were used buoy winds whereas their analysis used coarse-resolution Na- not substantially different between the two locations, making the tional Centers for Environmental Prediction (NCEP) winds. We use of buoy winds instead of ship winds in open water less prob- examined the monthly fluxes specifically for the SW Vancouver Is- lematic than in confined waters. Within the confined waters of land shelf that went into the Chavez et al. (2007) estimate for the the straits, using a fixed wind record instead of ship winds may BC margin, and the trends in their flux estimates are the same introduce more uncertainty because the winds in these locations those presented here for subregion 1; strong to moderate efflux are probably not as spatially coherent over such large distances. in autumn and winter, and strong to moderate CO2 uptake in It is likely this uncertainty has its strongest effect in the most con- spring and summer. Their estimate and ours are consistent in that

fined waters of Johnstone Strait, and it is strongly recommended the margin acts as a sink for atmospheric CO2 and the transition that future studies aimed at estimating sea-air CO2 fluxes in these seasons have the largest (positive and negative) fluxes. However, regions rigorously pursue ship-based winds. Regarding point (2), the estimates presented here are based on data that spans much the different gas transfer parameterizations can result in large flux more of the margin, and illustrates how sensitive the diagnosis of differences (as much as 19% as shown by Jiang et al. (2008) who net annual sea-air CO2 flux is to time and space variability in such considered 6 versions of gas transfer parameterizations for esti- dynamic coastal ocean settings. mating fluxes in the US South Atlantic Bight). The BC margin at times experiences high wind conditions that could result in large differences in the fluxes computed from the various wind-based 5. Conclusions parameterizations of the gas transfer velocity, because they di- verge most significantly at high wind speeds (i.e. compare cubic We have presented analyses from both a recently-collected set and quadratic parameterizations; Jiang et al., 2008). Confounding of underway pCO2 measurements that span a large spatial area of this point is that gas transfer in these highly turbulent settings the BC margin and historical pCO2 measurements from the CDIAC may be inadequately parameterized by wind speed because turbu- data set for the SW Vancouver Island shelf and the Strait of Juan lence driven by high tidal velocities in restricted passages may add de Fuca. We have used these observations to produce sea-air CO2 substantially to the gas exchange rate (e.g. Orton et al., 2010). Fi- flux estimates that show considerable time and space variability. nally, to address point (3), the combination of different wind and Three important features of the BC margin have been identified: pCO2 products can have large implications on the fluxes. Lüger (1) the largest fluxes (positive and negative) are during the transi- et al. (2006) observed a 47% difference between fluxes calculated tion seasons when high gas transfer velocities are coincident with from grid-averaged monthly winds versus collocated winds. large surface seawater disequilibria with the atmosphere, (2) there

Takahashi et al. (2009) report no significant correlation between is large subregional heterogeneity in sea-air CO2 fluxes on the BC individual DpCO2 measurements and 6-h mean wind speeds, margin that is important to include in estimates of net annual ex- which supported their decision to use monthly means to approxi- change on this margin, (3) factoring in this variability, and making mate sea-air CO2 flux in their global climatology. We did not ob- some assumptions about conditions where data are lacking, the serve large differences between the fluxes for SW Vancouver margin-wide net annual sea-air CO2 flux for the western Canadian Island shelf calculated using averaged versus instantaneous values, coastal ocean is likely near 6 mmol m2 d1. Author's personal copy

W. Evans et al. / Progress in Oceanography 101 (2012) 78–91 91

Acknowledgements implications for global parameterizations. Geophysical Research Letters 33, L16611, doi:16610.11029/12006GL026817. Hsu, S.A., Meindl, E.A., Gilhousen, D.B., 1994. Determining the power-law wind- We would like to express our gratitude to Marc Trudel and the profile exponent under near-neutral stability conditions at sea. Journal of Department of Fisheries and Oceans Canada High Seas Salmon Pro- Applied Meteorology 33, 757–765. gram for allowing our participation in cruises aboard the CCGS Ianson, D., Allen, S.E., 2002. A two-dimensional nitrogen and carbon flux model in a coastal upwelling region. Global Biogeochemical Cycles 16. doi:10.1029/ W.E. Ricker. We also thank Marc for valued discussions of this work GB001451. throughout its development. We thank Marty Davelaar for his Ianson, D., Allen, S.E., Harris, S.L., Orians, K.J., Varela, D.E., Wong, C.S., 2003. The assistance with the IOS pCO system. We also thank the captains inorganic carbon system in the coastal upwelling region west of Vancouver 2 Island, Canada. Deep-Sea Research I 50, 1023–1042. and crews of the CCGS W.E. Ricker and J.P. Tully. We are grateful Ianson, D., Feely, R.A., Sabine, C.L., Juranek, L.W., 2009. Features of coastal upwelling for the comments from two anonymous reviewers that greatly im- regions that determine net air-sea CO2 flux. Journal of Oceanography 65, 677– proved this manuscript. This work was supported by NSF Chemical 687. Jiang, L.-Q., Cai, W.-J., Wanninkhof, R., Wang, Y., Lüger, H., 2008. Air-sea CO2 fluxes Oceanography award OCE-0752576. on the US South Atlantic Bight: spatial and seasonal variability. Journal of Geophysical Research 113, C07019, doi:07010.01029/02007JC004366. References Laruelle, G.G., Dürr, H.H., Slomp, C.P., Borges, A.V., 2010. Evaluation of sinks and sources of CO2 in the global coastal ocean using a spatially-explicit typology of estuaries and continental shelves. Geophysical Research Letters 37. Borges, A.V., Delille, B., Frankignoulle, M., 2005. Budgeting sinks and sources of CO 2 doi:10.1029/2010GL043691. in the coastal ocean: diversity of ecosystems counts. Geophysical Research Lüger, H., Wanninkhof, R., Wallace, D.W.R., Körtzinger, A., 2006. CO fluxes in the Letters 32, L14601, doi:14610.11029/12005GL023053. 2 subropical and subarctic North Atlantic based on measurements from a Buck, A.L., 1981. New equations for computing vapor pressure and enhancement volunteer observing ship. Journal of Geophysical Research 111. doi:10.1029/ factor. Journal of Applied Meteorology 20, 1527–1532. 2005JC003101. Cai, W.-J., 2011. Estuarine and coastal ocean carbon paradox: CO sinks or sites of 2 Mantua, N.J., Hare, S.R., Zhang, Y., Wallace, J.M., Francis, R.C., 1997. A Pacific terrestrial carbon incineration? Annual Review of Marine Science 3, 123–145. interdecadal climate oscillation with impacts on salmon production. Bulletin of Cai, W.-J., Dai, M., Wang, Y., 2006. Air-sea exchange of carbon dioxide in ocean the American Meteorological Society 78, 1069–1079. margins: a province-based synthesis. Geophysical Research Letters 33. Masson, D., 2006. Seasonal water mass analysis for the Straits of Juan de Fuca and doi:10.1029/2006GL026219. Georgia. Atmosphere-Ocean 44, 1–15. Chavez, F.P., Takahashi, T., Cai, W.-J., Friederich, G.E., Hales, B., Wanninkhof, R., Masson, D., Peña, A., 2009. Chlorophyll distribution in a temperature estuary: the Feely, R.A., 2007. Coastal oceans. In: King, A.W., Dilling, L., Zimmerman, G.P., Strait of Georgia and Juan de Fuca Striat. Estuarine, Coastal and Shelf Science 82, Fairman, D.M., Houghton, R.A., Marland, G., Rose, A.Z., Wilbanks, T.J. (Eds.), The 19–28. First State of the Carbon Cycle Report (SOCCR): The North American Carbon McNeil, B.I., 2010. Diagnosing coastal ocean CO interannual variability from a 40 Budget and Implications for the Global Carbon Cycle. US Climate Change 2 year hydrographic time series station off the east coast of Australia. Global Science Program, Washington, DC, pp. 157–166. Biogeochemical Cycles 24. doi:10.1029/2010GB003870. Chen, C.-T.A., Borges, A.V., 2009. Reconciling opposing views on carbon cycling in Nemcek, N., Ianson, D., Tortell, P.D., 2008. A high-resolution survey of DMS, CO , and the coastal ocean: continental shelves as sinks and near-shore ecosystems as 2 O /Ar distributions in productive coastal waters. Global Biogeochemical Cycles sources of atmospheric CO . Deep-Sea Research II 56, 579–590. 2 2 22, GB2009, doi:2010.1029/2006GB002879. Denman, K.L., Mackas, D.L., Freeland, H.J., Austin, M.J., Hill, S.H., 1981. Persistent Orton, P.M., Zappa, C.J., McGillis, W.R., 2010. Tidal and atmospheric influences on upwelling and meso-scale zones of high productivity off the west coast of near-surface turbulence in an estuary. Journal of Geophysical Research 115. Vancouver Island, Canada. In: Richards, F. (Ed.), Coastal Upwelling. American doi:10.1029/2010JC006312. Geophysical Union, Washington, DC, pp. 514–521. Pennington, J.T., Friederich, G.E., Castro, C.G., Collins, C.A., Evans, W., Chavez, F.P., Di Lorenzo, E., Schneider, N., Cobb, K.M., Franks, P.J.S., Chhak, K., Miller, A.J., 2010. The northern and central California coastal upwelling system. In: Liu, K.- McWilliams, J.C., Bograd, S.J., Arango, H., Curchitser, E., Powell, T.M., Rivière, P., K., Atkinson, L., Quiñones, R.A., Talaue-McManus, L. (Eds.), Carbon and Nutrient 2008. North Pacific Gyre Oscillation links ocean climate and ecosystem change. Fluxes in Continental Margins. Springer, Berlin. Geophysical Research Letters 35. doi:10.1029/2007GL032838. Schiettecatte, L.-S., Thomas, H., Bozec, Y., Borges, A., 2007. High temporal coverage Dickson, A.G., Sabine, C.L., Christian, J.R. (Eds.), 2007. Guide to Best Practices for of carbon dioxide measurements in the Southern Bight of the North Sea. Marine Ocean CO Measurements. North Pacific Marine Science Organization. 2 Chemistry 106, 161–173. Evans, W., Hales, B., Strutton, P.G., 2011. The seasonal cycle of surface ocean pCO on 2 Strub, P.T., Allen, J.S., Huyer, A., Smith, R.L., 1987. Seasonal cycles of currents, the Oregon shelf. Journal of Geophysical Research 116. doi:10.1029/ temperatures, winds, and sea level over the northeast Pacific continental Shelf: 2010JC006625. 35°Nto48°N. Journal of Geophysical Research 92, 1507–1526. Feely, R.A., Takahashi, T., Wanninkhof, R., McPhaden, M.J., Cosca, C.E., Sutherland, Takahashi, T., Olafsson, J., Goddard, J.G., Chipman, D.W., Sutherland, S.C., 1993. S.C., Carr, M.-E., 2006. Decadal variability of the air-sea CO fluxes in the 2 Seasonal variation of CO and nutrients in the high-latitude surface oceans: a equatorial Pacific Ocean. Journal of Geophysical Research 111. doi:10.1029/ 2 comparative study. Global Biogeochemical Cycles 7, 843–878, doi:810.1029/ 2005JC003129. 1093GB02263. Foreman, M.G.G., Pal, B., Merryfield, W.J., 2011. Trends in upwelling and Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D., downwelling winds along the British Columbia shelf. Journal of Geophysical Hales, B., Friederich, G.E., Chavez, F.P., Sabine, C.L., Watson, A., Bakker, D.C.E., Research 116. doi:10.1029/2011JC006995. Schuster, U., Metzl, N., Yoshikawa-Inoue, H., Ishii, M., Midorikawa, T., Nojiri, Y., Fransson, A., Chierici, M., Nojiri, Y., 2006. Increased net CO outgassing in the 2 Körtzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Arnarson, T.S., Tilbrook, upwelling region of the southern Bering Sea in a period of variable marine B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C.S., Delille, B., Bates, N.R., de climate between 1995 and 2001. Journal of Geophysical Research 111. Baar, H.J.W., 2009. Climatological mean and decadal change in surface ocean doi:10.1029/2004JC002759. pCO and net sea-air CO flux over the global oceans. Deep-Sea Research II 56, Friederich, G.E., Walz, P.M., Burczynski, M.G., Chavez, F.P., 2002. Inorganic carbon in 2 2 554–577. the central California upwelling system during the 1997–1999 El Niño-La Niña Thomson, R.E., Ware, D.M., 1996. A current velocity index of ocean variability. event. Progress in Oceanography 54, 185–203. Journal of Geophysical Research 101, 14297–14310. GLOBALVIEW-CO2: Cooperative Atmospheric Data Integration Project – Carbon Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the Dioxide, 2011. NOAA ESRL, Boulder, Colorado, Also Available on Internet via ocean. Journal of Geophysical Research 97, 7373–7382. Anonymous FTP to ftp.cmdl.noaa.gov, Path: ccg/co2/GLOBALVIEW (CD-ROM). Ware, D.M., Thomson, R.E., 2005. Bottom-up ecosystem trophic dynamics Hales, B., Chipman, D., Takahashi, T., 2004. High-frequency measurements of partial determine fish production in the Northeast Pacific. Science 308, 1280–1284. pressure and total concentration of carbon dioxide in seawater using Weiss, R.F., 1974. Carbon dioxide in water and seawater: the solubility of a non- microporous hydrophobic membrane contactors. Limnology and ideal gas. Marine Chemistry 2, 203–215. Oceanography: Methods 2, 356–364. Wong, C.S., Christian, J.R., Wong, S.-K.E., Page, J., Xie, L., Johnannessen, S., 2010. Hales, B., Cai, W.-J., Mitchell, B.G., Sabine, C.L., Schofield, O., 2008. North American Carbon dioxide in surface seawater of the eastern North Pacific Ocean (Line P), Continental Margins: A Synthesis and Planning Workshop. US Carbon Cycle 1973–2005. Deep-Sea Research I 57, 687–695. Science Program, Washington, DC. Yin, K., Harrison, P.J., Beamish, R.J., 1997. Effects of a fluctuation in Fraser River Hales, B., Strutton, P.G., Saraceno, M., Letelier, R., Takahashi, T., Feely, R.A., Sabine, discharge on primary production in the central Strait of Georgia, British C.L., Chavez, F.P., submitted for publication. Satellite-based prediction of pCO 2 Columbia, Canada. Canadian Journal of Fisheries and Aquatic Sciences 54, 1015– in coastal waters. Progress in Oceanography. 1024. Harrison, P.J., Fulton, J.D., Taylor, J.R., Parsons, T.R., 1983. Review of the biological Zeng, J., Yukihiro, N., Murphy, P.P., Wong, C.S., Fujinima, Y., 2002. A comparison of oceanography of the Strait of Georgia: pelagic environment. Canadian Journal of DpCO distributions in the northern North Pacific using results from a Fisheries and Aquatic Sciences 40, 1064–1094. 2 commercial vessel in 1995–1999. Deep-Sea Research II 49, 5303–5315. Ho, D.T., Law, C.S., Smith, M.J., Schlosser, P., Harvey, M., Hill, P., 2006. Measurements of air-sea gas exchange at high wind speeds in the Southern Ocean: