The Strait of Georgia (SoG) is a large semienclosed estuary that spatially dominates the Salish Sea on the North American Pacific coast. The region is well populated, harbors significant aquaculture, and is vulnerable to climate change. We present the first inorganic carbon data collected in the SoG covering all seasons (2003 and 2010–2012) and put them into the context of local circulation and oxygen cycles. Results show that the SoG has a higher carbon content and lower pH than surrounding waters. Aragonite saturation horizons in the SoG do not become deeper than 20–30 m and shoal to the surface for extended periods. Furthermore, incoming upwelled “acidified” water from the outer coast actually increases local pH. Finally, intense mixing in the physically restricted channels connecting the SoG to the outer coast allows significant oxygen uptake but minimal CO2 out gassing, protecting the SoG from hypoxia but not from ocean acidification.

Abstract. Ocean acidification threatens to reduce pH and aragonite saturation state ( A ) in estuaries, potentially damaging their ecosystems. However, the impact of highly variable river total alkalinity (TA) and dissolved inorganic carbon (DIC) on pH and A in these estuaries is unknown. We assess the sensitivity of estuarine surface pH and A to river TA and DIC using a coupled biogeochemical model of the Strait of Georgia on the Canadian Pacific coast and place the results in the context of global rivers. The productive Strait of Georgia estuary has a large, seasonally variable freshwater input from the glacially fed, undammed Fraser River. Analyzing TA observations from this river plume and pH from the river mouth, we find that the Fraser is moderately alkaline (TA 500-1000 µmol kg −1 ) but relatively DICrich. Model results show that estuarine pH and A are sensitive to freshwater DIC and TA, but do not vary in synchrony except at high DIC : TA. The asynchrony occurs because increased freshwater TA is associated with increased DIC, which contributes to an increased estuarine DIC : TA and reduces pH, while the resulting higher carbonate ion concentration causes an increase in estuarine A . When freshwater DIC : TA increases (beyond ∼ 1.1), the shifting chemistry causes a paucity of the carbonate ion that overwhelms the simple dilution/enhancement effect. At this high DIC : TA ratio, estuarine sensitivity to river chemistry increases overall. Furthermore, this increased sensitivity extends to reduced flow regimes that are expected in future. Modulating these negative impacts is the seasonal productivity in the estuary which draws down DIC and reduces the sensitivity of estuarine pH to increasing DIC during the summer season.

Abstract.Ocean acidification threatens to reduce pH and aragonite saturation state (Ω A ) in estuaries, potentially damaging their ecosystems. However, the impact of highly variable river total alkalinity (TA) and dissolved inorganic carbon (DIC) on pHand Ω A in these estuaries is unknown. We assess the sensitivity of estuarine surface pH and Ω A to river chemistry using a 1-dimensional, biogeochemical-coupled model of the Strait of Georgia on the Canadian Pacific coast and generalize the results 5 in the context of global rivers. The productive Strait of Georgia estuary has a large, seasonally variable freshwater input from the glacially fed, undammed Fraser River. Analyzing TA and pH observations from this river and its estuary, we find that the Fraser is moderately alkaline (TA 500-1350 µmol kg −1 ) but relatively DIC-rich, especially during winter (low flow). Model results show that estuarine pH and Ω A , while sensitive to freshwater DIC and TA, do not vary in synchrony. Instead, rivers with high DIC and TA produce lower estuarine pH due to an increased estuarine DIC:TA ratio, but higher estuarine Ω A because of 10 DIC contributions to the carbonate ion. This estuarine pH sensitivity decreases with increasing mean river TA, but the zone of maximum pH sensitivity also moves to higher salinity which could impact a larger areal extent of the estuary. Many temperate rivers, such as the Fraser, are expected to experience weaker freshets and stronger winter flows under climate change, reducing the extent of the river plume and the impact of river chemistry in much of the estuary. However, increasing carbon in rivers will move the highest sensitivity zone to higher salinities that cover larger areas under present-day flow regimes.

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