Introduction
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). Resolving the downward trajectories of seawater pH and carbonate ion () concentration that result from the uptake of anthropogenic CO2, termed ocean acidification (OA; Caldeira and Wickett, 2003; Feely et al., 2004; Orr et al., 2005), is of specific interest because of the anticipated impacts on marine ecosystems and the services they provide (Cooley et al., 2009; Doney et al., 2012), as well as downstream economic consequences by jeopardizing food security and fisheries revenue, and destabilizing coastal communities (Cooley and Doney, 2009; Ekstrom et al., 2015; Mathis et al., 2015; Seung et al., 2015). Understanding secular change in CO2 system parameters associated with OA is critical in order to forecast the ecological implications, however, the effort is significantly impaired in coastal settings that contain sparse CO2 system information in the context of large inherent dynamic variability.
The Salish Sea is one of the largest inland seas on the North American Pacific Coast, is bi-national with a shared border between the United States of America (Washington State) and Canada (British Columbia, BC), and consists of a collection of straits, sounds, and inlets; the largest waterways being: Strait of Juan de Fuca, Puget Sound, and the Strait of Georgia. 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 along-axis of the NSS is oriented ∼50° west of true north, and the distance spanned along this axis is ∼110 km. The NSS cross-axis is narrower and spans ∼50 km at its widest point (near Baynes Sound; 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). Deep water renewal occurs annually during summer and maximal surface water residence times are at most a few months (Masson, 2002; Pawlowicz et al., 2007). The NSS is bounded by coastal mountains on both Vancouver Island and mainland BC, such that wind patterns are highly channelized (i.e., oriented along channel; Figure 1) with predominantly northwesterly winds in summer and southeasterly winds in winter (Bakri et al., 2017). 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). Fraser River water flow-weighted mean total alkalinity (TA) is ∼700 μmol kg-1, with higher values during lower flow states (Voss et al., 2014). Numerous smaller river systems also likely play an important role, particularly 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 also 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).
Marine CO2 dynamics have not been well documented for the NSS beyond the information gained from periodic research cruises (Ianson et al., 2016) and limited underway observations from ships-of-opportunity (Evans et al., 2012; Tortell et al., 2012). A greater wealth of information exists for the central and southern Salish Sea, that collectively describe surface CO2 system variability that is driven by large seasonal oscillations in primary productivity, freshwater delivery, and localized zones of intense tidal mixing (Feely et al., 2010; Evans et al., 2012; Tortell et al., 2012; Ianson et al., 2016; Fassbender et al., 2018). In deep Salish Sea water, total dissolved inorganic carbon (TCO2) concentrations are high due to organic matter remineralization combined with restricted connectivity to the open continental shelf (Feely et al., 2010; Johannessen et al., 2014; Ianson et al., 2016). Nonetheless, as importantly noted by Fassbender et al. (2018), TA and TCO2 concentrations are lower within the Salish Sea relative to the open North Pacific, however, the ratio of these two parameters (i.e., TA:TCO2) is closer to 1, and therefore these waters are more weakly buffered (Egleston et al., 2010) than open North Pacific water. Weaker buffering capacity results in an amplified response of pH, CO2 partial pressure (pCO2), and aragonite saturation state (Ωarag; where Ωarag = [][Ca2+]/Ksp(arag), and Ksp(arag) is the aragonite-specific solubility product) to changes in TCO2. A TA:TCO2 ratio near 1 also sets the tendency for seawater to be corrosive to aragonite (i.e., Ωarag < 1). Seasonally, the entire Salish Sea water column has been observed to be corrosive to aragonite during winter, while during productive spring and summer months, Ωarag increases in the surface layer (<20 m) to saturated levels (Feely et al., 2010; Ianson et al., 2016; Fassbender et al., 2018).
A number of organisms residing in the Salish Sea may be sensitive to currently observed Ωarag and pHT (total scale) conditions. Research has shown that Ωarag during first shell development was a determining factor for growth and survival of larval Crassostrea gigas (Pacific oyster), and an Ωarag value of 1.7 was determined to be the “break-even” point for commercial larvae production (Barton et al., 2012; Barton et al., 2015). C. gigas larval biomass production within the Whiskey Creek Hatchery on the Oregon coast was zero below this value (Barton et al., 2012). More detailed experimental work has revealed acute stress in juvenile C. gigas, Mytilus galloprovincialis (Mediterranean mussel), and M. californianus (California mussel) at Ωarag values of 1.2 and 1.5, respectively (Waldbusser et al., 2014, 2015). Collectively, these studies highlight that Ωarag levels above the thermodynamic threshold of 1 can negatively impact vulnerable early life stages of these species, with the duration and frequency of exposure to such adverse conditions also being important factors (Waldbusser and Salisbury, 2014). These vulnerable species reside in the NSS with Ωarag values above these thresholds only in the surface layer during spring and summer months (Feely et al., 2010; Ianson et al., 2016; Fassbender et al., 2018). Similarly, pelagic species, such as Euphausia pacifica (krill; McLaskey et al., 2016) and Limacina helicina (thecosome pteropod; Bednarsek et al., 2017) may also be impacted by currently observed conditions. For instance, there is experimental evidence that E. pacifica larval development and survival is reduced at pHT levels of 7.69 (McLaskey et al., 2016), values already seen in the region (Feely et al., 2010; Ianson et al., 2016; Fassbender et al., 2018). Efforts to model this area have focused on the southern domain (Moore-Maley et al., 2016; Bianucci et al., 2018), but highlight that the stability of favorable surface layer CO2 conditions during summer can be disrupted by episodic variability.
In this paper, we will describe a growing NSS high-resolution dataset from the Hakai Institute’s Quadra Island Field Station (QIFS) that began in late 2014 in conjunction with other datasets collected by an array of observing activities that occurred during 2016 and 2017. These datasets collectively highlight important characteristics of the region, including perpetually low-Ωarag at depth and large surface ocean variability across seasonal and sub-seasonal timescales. We also present estimates of anthropogenic CO2 for NSS surface water spanning the start of the Industrial Revolution (e.g., 1765) to the end of the 21st century. Using this information, we assess the changes in Ωarag and pHT over ∼2.5 centuries and consider the passing of key biological thresholds during this period.
