Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

Abstract

Abstract. The incomplete spatial coverage of CO 2 partial pressure (pCO 2 ) measurements across estuary types represents a significant knowledge gap in current regional-and global-scale estimates of estuarine CO 2 emissions. Given the limited research on CO 2 dynamics in large estuaries and bay systems, as well as the sources of error in the calculation of pCO 2 (carbonic acid dissociation constants, organic alkalinity), estimates of air-sea CO 2 fluxes in estuaries are subject to large uncertainties. The Estuary and Gulf of St. Lawrence (EGSL) at the lower limit of the subarctic region in eastern Canada is the world's largest estuarine system, and is characterized by an exceptional richness in environmental diversity. It is among the world's most intensively studied estuaries, yet there are no published data on its surface-water pCO 2 distribution. To fill this data gap, a comprehensive dataset was compiled from direct and indirect measurements of carbonate system parameters in the surface waters of the EGSL during the spring or summer of 2003-2016. The calculated surface-water pCO 2 ranged from 435 to 765 µatm in the shallow partially mixed upper estuary, 139-578 µatm in the deep stratified lower estuary, and 207-478 µatm along the Laurentian Channel in the Gulf of St. Lawrence. Overall, at the time of sampling, the St. Lawrence Estuary served as a very weak source of CO 2 to the atmosphere, with an area-averaged CO 2 degassing flux of 0.98 to 2.02 mmol C m −2 d −1 (0.36 to 0.74 mol C m −2 yr −1 ). A preliminary analysis revealed that respiration (upper estuary), photosynthesis (lower estuary), and temperature (Gulf of St. Lawrence) controlled the spatial variability in surface-water pCO 2 . Whereas we used the dissociation constants of Cai and Wang (1998)

Spatial variations in CO<sub>2</sub> fluxes in the Saguenay Fjord (Quebec, Canada) and results of a water mixing model

“…The original OMP algorithm is a linear inverse model that assumes all hydrographic tracers are conservative. The algorithm has since been modified to handle nonconservative properties such as DIC and nutrients by taking into consideration the stoichiometry of microbial respiration and photosynthesis (Dinauer and Mucci, 2018;Karstensen and Tomczak, 1998).…”

“…DIC rather than TA was used as an input parameter to CO2SYS, since the fjord surface waters are enriched in colored dissolved organic carbon (> 4 mg L −1 ) delivered by the Saguenay River and are characterized by a negative organic alkalinity (positive organic acidity) (see below). The carbonic acid dissociation constants (K * 1 and K * 2 ) of were used for the calculations, as the latter were found to be more suitable for the low-salinity waters encountered in estuarine environments, such as the Saguenay Fjord (S P < 20) (Dinauer and Mucci, 2017). pCO 2(SW-calc) values were computed for the surface mixed layer located above the sharp pycnocline (∼ 10 m), where most physical and chemical properties are directly impacted by biological activity (photosynthesis and respiration), as well as heat and gas exchange across the air-sea interface ( Table 2).…”

“…The uncertainty in Sc ranges from 3 % to 10 % and is mainly due to the imprecision of diffusion coefficients (Wanninkhof, 2014). Estimates of k, calculated at each sampling point using the equation of Wanninkhof (2014), ranged from 0.36 to 3.38 cm h −1 for the fjord, compared to 1.6 to 4.5 cm h −1 in the St. Lawrence Estuary (Dinauer and Mucci, 2017). Atmospheric pCO 2 values (pCO 2(air) ) were computed using the daily averages of measured mole fractions of CO 2 in dry air, obtained at the La Baie weather station and retrieved from the Climate Research Division at Environment and Climate Change Canada.…”

“…Overall, the total area-averaged degassing flux of the fjord adds up to 2.14±0.43 mmol m −2 d −1 or 0.78 ± 0.16 mol m −2 yr −1 . In comparison, the degassing flux in the adjacent St. Lawrence Estuary was estimated at between 0.36 and 0.74 mol m −2 yr −1 during the late spring and early summer (Dinauer and Mucci, 2017). This discrepancy can be explained by the low carbonate alkalin-ity (and buffer capacity) of the Saguenay River waters that flow through the Grenvillian metamorphic and igneous rocks of the Canadian Shield (Piper et al, 1990), as with most rivers on the north shore of the St. Lawrence Estuary (e.g., Betsiamites, Manicouagan, Romaine; Paul del Giorgio, personal communication, 2016), and the low productivity of the fjord surface waters because of very limited light penetration due to their high chromotrophic dissolved organic matter (CDOM) content (Tremblay and Gagné, 2009;Xie et al, 2012).…”

“…In contrast, waters of the St. Lawrence River have an elevated carbonate alkalinity (∼ 1200 µM), inherited from the Ottawa River that drains through limestone deposits (Telmer and Veizer, 1999). Furthermore, the estuary is host to multiple seasonal phytoplankton blooms (Levasseur and Therriault, 1987;Zakardjian et al, 2000;Annane et al, 2015) that strongly modulate its trophic status (Dinauer and Mucci, 2018).…”