Abstract
Dissolved oxygen concentrations in the ocean are declining on a global scale. However, the impact of climate change on oxygen in shelf seas is not well understood. We investigate potential future changes in oxygen on the northwest European continental shelf under a business as usual greenhouse gas emissions scenario (Representative Concentration Pathway RCP8.5). Regions of the European shelf are thermally stratified from spring to autumn, which can cause oxygen depletion in sub-pycnocline waters. A transient climate-forced model simulation is used to study how the temperature, salinity and concentration of near bed dissolved oxygen change over the 21st century. In warming and freshening water, the oxygen concentration declines in all shelf regions. The climate change signal emerges first in salinity, then in temperature and finally in near bed oxygen. Regions that currently experience oxygen depletion (the eastern North Sea, Celtic Sea and Armorican shelf) become larger in the future scenario and oxygen depletion lasts longer. Solubility changes, caused by changes in temperature and salinity, are the dominant cause of reducing near bed oxygen concentrations in many regions. Until about 2040 the impact of solubility dominates over the effects of the evolving ecosystem. However, in the eastern North Sea by 2100, the effect of ecosystem change is generally larger than that of solubility. In the Armorican Shelf and Celtic Sea the ecosystem changes partially mitigate the oxygen decline caused by solubility changes. Over the 21st century the mean near bed oxygen concentration on the European shelf is projected to decrease by 6.3%, of which 73% is due to solubility changes and the remainder to changes in the ecosystem. For monthly minimum oxygen the decline is 7.7% with the solubility component being 50% of the total.
Highlights
Dissolved oxygen in the ocean is an indicator of water quality and low concentrations can be a threat to the health of aquatic life
The average regional warming over the 21st century of between 1.5 and 4.0 °C in near bed temperature (NBT) and between 2.7 and 4.0 °C in sea surface temperature (SST) is similar to other modelling studies of the European shelf. [Tinker et al, 2016] calculate shelf-wide increases of 2.71 ± 0.75 °C and 2.90 ± 0.82 °C respec tively from their model ensemble under the SRES A1B medium emis sions scenario, which is less severe than the RCP8.5 scenario used here
The freshening of Atlantic water affects much of the shelf and the impact is intensified by circulation changes in the North Sea, where a shutdown in Atlantic exchange leads to reduced circulation and decreased freshwater flushing from the North Sea [Holt et al, 2018]
Summary
Dissolved oxygen in the ocean is an indicator of water quality and low concentrations can be a threat to the health of aquatic life. Due to a combination of warmer temperatures, which reduce gas solubility, and an increase in nutrients discharged at the coast, dissolved oxygen concentrations have been declining globally since at least the middle of the 20th century [Breitburg et al, 2018]. Over the 21st century, the global oxygen content is “very likely” to decline [Bindoff et al, 2019; IPCC, 2019] by 3.2–3.7%, for RCP8.5 (Representative Concentration Pathway 8.5 [van Vuuren et al, 2011]) or by 1.6–2.0% for RCP2.6. Marine dissolved oxygen is produced by photosynthesis in surface waters; as phyto plankton sink out of the photic zone respiration exceeds photosynthesis and there is net phytoplankton consumption of oxygen. In the isolated part of the water column, biological processes may re duce oxygen concentrations to below healthy levels
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