4.7 Article

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

Journal

BIOGEOSCIENCES
Volume 15, Issue 12, Pages 3743-3760

Publisher

COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/bg-15-3743-2018

Keywords

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Funding

  1. Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery
  2. Marine Environmental Observation, Prediction, and Response (MEOPAR) Network of Centres of Excellence of Canada
  3. Fisheries and Oceans Canada's Climate Change Science Initiative and International Governance Strategy programs

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Ocean acidification threatens to reduce pH and aragonite saturation state (Omega(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 Omega(A) in these estuaries is unknown. We assess the sensitivity of estuarine surface pH and Omega(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 mu mol kg(-1)) but relatively DIC-rich. Model results show that estuarine pH and Omega(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 Omega(A). When freshwater DIC : TA increases (beyond similar to 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.

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