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

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

doi:10.5194/bg-15-3743-2018

Benjamin L Moore-Maley, Susan E Allen + Show 1 more

Open Access

https://doi.org/10.5194/bg-15-3743-2018

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Abstract

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.

Highlights

Estuaries support productive ecosystems (Cloern et al, 2014) and significant human populations (Cloern et al, 2015)
Critical trophic links within many of these ecosystems may be negatively impacted by increases in dissolved inorganic carbon (DIC) and reduced pH associated with ocean acidification (Haigh et al, 2015)
The Fraser River is a large, free-flowing, glacially fed river that exerts a strong physical and chemical influence on a neighboring, semi-enclosed estuarine sea, the Strait of Georgia, that is DIC-rich relative to the open ocean

Summary

Introduction

Estuaries support productive ecosystems (Cloern et al, 2014) and significant human populations (Cloern et al, 2015). Critical trophic links within many of these ecosystems may be negatively impacted by increases in dissolved inorganic carbon (DIC) and reduced pH associated with ocean acidification (Haigh et al, 2015). Those organisms using the calciumcarbonate mineral form aragonite in their external hard parts (e.g., some molluscs) are especially vulnerable since oceanic CO2 uptake lowers the aragonite saturation state ( A) of seawater (Waldbusser et al, 2015). The strength and location of these zones depend on the river endmember carbonate chemistry (Hofmann et al, 2009; Hu and Cai, 2013; Xue et al, 2017) This sensitivity is generally expected to increase as the ocean

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