Abstract
Estimating sea–air CO2 fluxes in coastal seas remains a source of uncertainty in global carbon budgets because processes like primary production, upwelling, water mixing, and freshwater inputs produce high spatial and temporal variability of CO2 partial pressure (pCO2). As a result, improving our pCO2 baseline observations in these regions is important, especially in sub-Arctic and Arctic seas that are experiencing strong impacts of climate change. Here, we show the patterns and main processes controlling seawater pCO2 and sea–air CO2 fluxes in Hudson Bay during the 2018 spring and early summer seasons. We observed spatially limited pCO2 supersaturation (relative to the atmosphere) near river mouths and beneath sea ice and widespread undersaturated pCO2 in offshore and ice-melt-influenced waters. pCO2 was highly correlated with salinity and temperature, with a limited but statistically significant relationship with chlorophyll a and fluorescent dissolved organic matter. Hudson Bay on average was undersaturated with respect to atmospheric CO2, which we attribute mainly to the dominance of sea-ice meltwater. We calculated an average net CO2 flux of about –5mmol CO2 m–2 day–1 (–3.3 Tg C) during the spring and early summer seasons (92 days). Combining this result with extrapolated estimates for late summer and fall seasons, we estimate the annual CO2 flux of Hudson Bay during the open water season (184 days) to be –7.2 Tg C. Our findings indicate that the bay on average is a weaker CO2 sink than most other Arctic seas, emphasizing the importance of properly accounting for seasonal variability in the Arctic coastal shelves to obtain reliable sea–air CO2 exchange budgets.
Highlights
Global oceans slow the effects of climate change by absorbing anthropogenic carbon dioxide (CO2) emitted by the burning of fossil fuels, land-use change, and cement production
Results from past studies in Hudson Bay are consistent with our observations, with low dissolved inorganic carbon associated with sea-ice melt (Burt et al, 2016), which typically results in low pCO2 values (e.g., Ahmed et al, 2020)
Our results suggest that Hudson Bay was undersaturated in pCO2 relative to the atmosphere during our study time as a result of the dominance of low-pCO2 ice-melt waters, with few observations of supersaturated pCO2
Summary
Global oceans slow the effects of climate change by absorbing anthropogenic carbon dioxide (CO2) emitted by the burning of fossil fuels, land-use change, and cement production. The ocean’s biological pump operates more efficiently in coastal shelves relative to the open ocean as a result of high terrestrial inputs and efficient use of nutrients (Gattuso et al, 1998; Chavez et al, 1999). The eventual outflow of CO2-enriched deep water from coastal shelf regions to the subsurface waters of the adjacent deep oceans constitutes “the continental shelf pump,” which is thought to contribute significantly to the global ocean’s atmospheric CO2 uptake (Tsunogai et al, 1999). The coastal shelves contribute 10%–30% of global marine primary production, 80% of organic carbon, and 30%–50% of inorganic carbon burial in sediments, and about 50% of the organic carbon supplied to the deep open ocean (Wollast, 1998; Liu et al, 2010). The sea–air CO2 exchanges are expected to be more intense in these waters than in the open ocean
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