Abstract
Subpolar gyre regions such as the Western Subarctic North Pacific (WSNP) contain sluggish, low-oxygen water, and are threatened by loss of oxygen (deoxygenation). Our simulations under RCP 8.5 emission scenario suggest that installing pipes to induce artificial downwelling and upwelling (AD and AU) provides short-term solutions to combat deoxygenation in the WSNP. With no engineering, the WSNP's subsurface oxygen decreases by 30–100 mmol/m3 by the year 2100. Continuous implementation of AD and AU instead counters this declining trend, and AD is more effective than AU. The oxygenation effect is primarily a consequence of how the two engineering schemes vertically redistribute oxygen via physical processes. AD directly improves oxygen at depth via advecting surface water toward the ocean interior and subsequent enhanced pycnocline mixing, and AU does so via generating compensatory downwelling outside of the pipes. Both schemes take near 40 years to complete the oxygenation. After that, oxygen reaches a new equilibrium state in the WSNP with no further improvement by the engineering. AD and AU both strongly increase primary production surrounding the deployment sites, but lead only to weak enhancement of aerobic respiration in subsurface water and thus a minor impact on the oxygenation. Other unwanted environmental side effects are negligible compared to those caused by rapid climate change within this century, including outgassing of carbon dioxide, pH decrease, and precipitation reduction.
Highlights
With climate warming and eutrophication, the global ocean is undergoing deoxygenation, resulting in loss of biomass and significant changes in biogeochemical cycles (Diaz and Rosenberg, 2008; Breitburg et al, 2018)
From 1950 to 2000, our model indicates oxygen in the 200–400 m range of the Western Subarctic North Pacific (WSNP) decreasing at a mean rate of 18 mmol/m3/yr, within the observed range of 15–21 mmol/m3/yr (Keeling et al, 2010)
Our model suggests that 22% increase of net primary production participates in nutrient recycling in the shallow water, 50% goes into the zooplankton community, and the remaining part is in the form of detritus
Summary
With climate warming and eutrophication, the global ocean is undergoing deoxygenation, resulting in loss of biomass and significant changes in biogeochemical cycles (Diaz and Rosenberg, 2008; Breitburg et al, 2018). Management plans focused on reducing nutrient loads have made significant progress in restoring oceanic oxygen, yet uncertainty about their immediate effectiveness leads to calls for short-term solutions (Stigebrandt and Gustafsson, 2007; Conley, 2012; Feng et al, 2020). The arguably most promising solution includes using pipes to transport surface oxygen-rich water into deep water. This approach, termed artificial downwelling (hereafter AD), has been tested in AD/AU Impact on the WSNP coastal waters with some success (Stigebrandt et al, 2015). The potential of AU to improve oxygen deserves further attention
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