Articles | Volume 15, issue 12
Research article
20 Jun 2018
Research article |  | 20 Jun 2018

The sensitivity of estuarine aragonite saturation state and pH to the carbonate chemistry of a freshet-dominated river

Benjamin L. Moore-Maley, Debby Ianson, and Susan E. Allen

Abstract. Ocean acidification threatens to reduce pH and aragonite saturation state (ΩA) in estuaries, potentially damaging their ecosystems. However, the impact of highly variable river total alkalinity (TA) and dissolved inorganic carbon (DIC) on pH and ΩA in these estuaries is unknown. We assess the sensitivity of estuarine surface pH and ΩA 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 DIC-rich. Model results show that estuarine pH and ΩA 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 ΩA. 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.

Short summary
Estuaries are vulnerable to ocean acidification, but present-day estuarine pH and aragonite saturation state variability are larger than in the open ocean. Using a numerical model of a large estuary and data from its primary river, we find that changes in river alkalinity relative to river carbon may determine a small but significant portion of this variability, while the majority is controlled by photosynthesis/respiration. Future watershed changes may shift the river alkalinity–carbon balance.
Final-revised paper