Abstract
Marine carbon dioxide (CO2) system data has been collected from December 2014 to June 2018 in the northern Salish Sea (NSS; British Columbia, Canada) and consisted of continuous measurements at two sites as well as spatially- and seasonally-distributed discrete seawater samples. The array of CO2 observing activities included high-resolution CO2 partial pressure (pCO2) and pHT (total scale) measurements made at the Hakai Institute’s Quadra Island Field Station (QIFS) and from an Environment Canada weather buoy, respectively, as well as discrete seawater measurements of pCO2 and total dissolved inorganic carbon (TCO2) obtained during a number of field campaigns. A relationship between NSS alkalinity and salinity was developed with the discrete datasets and used with the continuous measurements to highly resolve the marine CO2 system. Collectively, these datasets provided insights into the seasonality in this historically under-sampled region and detail the area’s tendency for aragonite saturation state () to be at non-corrosive levels (i.e. > 1) only in the upper water column during spring and summer months. This depth zone and time period of reprieve can be periodically interrupted by strong northwesterly winds that drive short-lived (~1 week) episodes of high-pCO2, low-pH, and low- conditions throughout the region. Interannual variability in summertime conditions was evident and linked to reduced northwesterly winds and increased stratification. Anthropogenic CO2 in NSS surface water was estimated using data from 2017 combined with the global atmospheric CO2 forcing for the period 1765 to 2100, and projected a mean value of 49 ± 5 µmol kg-1 for 2018. The estimated trend in anthropogenic CO2 was further used to assess the evolution of and pHT levels in NSS surface water, and revealed that wintertime corrosive conditions were likely absent pre-1900. The percent of the year spent above = 1 has dropped from ~98% in 1900 to ~60% by 2018. Over the coming decades, winter pHT and spring and summer are projected to decline to conditions below identified biological thresholds for select vulnerable species.
Highlights
The marine carbon dioxide (CO2) system in coastal settings is influenced by a host of processes that are unique to the land-ocean boundary and create a complicated mosaic of spatially and temporally varying seawater CO2 conditions (Feely et al, 2016; Chan et al, 2017) that hinders long-term trend detection in the absence of lengthy observational records (Sutton et al, 2018)
Understanding secular change in CO2 system parameters associated with ocean acidification (OA) is critical in order to forecast the ecological implications, the effort is significantly impaired in coastal settings that contain sparse CO2 system information in the context of large inherent dynamic variability
This relationship was coupled with highresolution surface measurements of salinity, temperature, and pCO2 or pHT collected at Quadra Island Field Station (QIFS) or from the Environment Canada weather buoy 46131, respectively (Figure 1), to characterize surface CO2 system variability over a 3.5-year period
Summary
The marine carbon dioxide (CO2) system in coastal settings is influenced by a host of processes that are unique to the land-ocean boundary (e.g., freshwater inputs, coastal upwelling and downwelling circulations, benthic-pelagic coupling, eutrophication, and the uptake of anthropogenic CO2) and create a complicated mosaic of spatially and temporally varying seawater CO2 conditions (Feely et al, 2016; Chan et al, 2017) that hinders long-term trend detection in the absence of lengthy observational records (Sutton et al, 2018). The Northern Salish Sea (NSS) is defined here as the Strait of Georgia and peripheral waterways north of Lasqueti Island and south of Quadra Island (Figure 1). The maximum depth of the NSS is ∼350 m and deep water exchange with the open shelf is thought to mainly occur via a flow pathway over ∼100 m sills in the southern Strait of Georgia (Masson, 2002; Johannessen et al, 2014). The Fraser River is the dominant freshwater source to the southern Salish Sea, with peak discharge near 10,000 m3 s−1 during the summer snow/ice melt freshet (Masson and Cummins, 2004). Numerous smaller river systems likely play an important role, in the NSS, that collectively stratify the upper water column and allow for high spring and summer phytoplankton biomass accumulation and rates of primary production (Masson and Peña, 2009). The NSS houses the majority of the shellfish aquaculture industry lease sites in BC (Haigh et al, 2015), and serves as an important region for migrating salmon traveling through the Discovery Islands to the open North Pacific (Journey et al, 2018)
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