An international research team has established a link between aquatic deoxygenation and the general stability of terrestrial systems, which should lead to the inclusion of deoxygenation as the tenth planetary boundary. This study also proposes to improve tools for monitoring and predicting aquatic systems, and oxygen in particular, by developing numerical models of improved accuracy and setting up a global database of the highest quality.
O
xygen is a fundamental element of life, and the loss of oxygen in water, known as aquatic deoxygenation, poses a threat to life at every level. In a study published in Nature Ecology and Evolution, a scientific team led by Prof. Kevin Rose of the Rensselaer Polytechnic Institute describes how ongoing deoxygenation represents a major threat to the stability of the planet as a whole.
Previous research has identified a series of global processes, known as planetary boundaries, that govern the overall habitability and stability of our planet. Planetary boundaries represent thresholds for certain processes that must not be exceeded for not compromising the stability of the Earth system," explains Marilaure Grégoire, an oceanologist at the University of Liège and co-author of the study. At present, nine planetary boundaries have been recognised for which maximum thresholds of change have been defined (i.e. climate change, loss of biodiversity, disruption of the nitrogen cycle, disruption of the phosphorus cycle, chemical pollution, destruction of the ozone layer, ocean acidification, the quantity of aerosols in the atmosphere, land use and freshwater). Exceeding these thresholds is likely to jeopardise the stability of our environment, with the emergence of feedback loops that will amplify the speed of change and thus risk taking the system towards a new state that is undesirable for humanity".
At present, six of the nine global boundaries are considered to have been crossed and for three of them we enter a high-risk zone. "It is important that aquatic deoxygenation is added to the list of global limits," says Kevin Rose. "This will help support and guide global monitoring, research and policy efforts to help our aquatic ecosystems and, by extension, society as a whole."
© Grégoire et al., 2023 & Huang et al., 2021
In all aquatic ecosystems, whether rivers, lakes, reservoirs, ponds, estuaries, coasts or the open sea, dissolved oxygen (DO) concentrations have declined rapidly and considerably over recent decades. Lakes and reservoirs have suffered oxygen losses of 5.5% and 18.6% respectively since 1980. The oceans have suffered oxygen losses of around 2% since 1960 and, although this figure is lower, it represents a larger mass in geographical and volumetric terms. Marine ecosystems have also experienced significant variability in oxygen depletion. For example, the middle waters off central California have lost 40% of their oxygen in recent decades. The volumes of aquatic ecosystems affected by oxygen depletion have increased dramatically for all types of ecosystem.
"Aquatic deoxygenation is closely linked to climate change and land use," continues Kevin Rose. "It is due to a reduction in the solubility of oxygen in water as a result of higher temperatures, a reduction in the ventilation of deep waters due to stronger and longer stratification, and an increase in oxygen-consuming respiration linked both to the rise in temperature and to the increase in the input of nutrients and organic matter. Global warming and pollutants are disrupting biogeochemical processes in aquatic ecosystems, with adverse effects on freshwater and marine organisms".
And Marilaure gégroire adds: "The past teaches us that there is a close link between biodiversity and the level of oxygenation of the Earth. The Great Oxygenation Event (GOE) ~2.5 billion years ago and the Paleozoic Oxygenation Event (POE) ~500 million years ago were bifurcation points for the Earth biology and chemistry, with the emergence of complex and diverse life. Furthermore, at least 3 of the 5 mass extinction events are associated with the deoxygenation of the oceans. The variation in oxygen levels in the oceans is therefore a process that needs to be quantified as accurately as possible, because the loss of oxygen can destabilise global biogeochemical balances. Deoxygenation of the oceans leads to changes in the phosphorus, nitrogen and carbon cycles. If these changes exceed a critical level, a runaway phenomenon can occur, causing the system to converge towards a new state of equilibrium via positive feedback loops (i.e. loops that amplify the cause that gave rise to them, thereby accelerating the destabilisation of the system). It is for this reason that deoxygenation should be considered as the tenth planetary limit, as deoxygenation that is too rapid can destabilise the system and also have an impact on the other planetary limits (nitrogen and phosphorus cycles)".
Kevin C. Rose, Erica M. Ferrer, Stephen R. Carpenter, Sean A. Crowe, Sarah C. Donelan, Véronique C. Garçon, Marilaure Grégoire, Stephen F. Jane, Peter R. Leavitt, Lisa A. Levin, Andreas Oschlies & Denise Breitburg, Aquatic deoxygenation as a planetary boundary and key regulator of Earth system stability, Nature Ecology & Evolution, 15 July 2024. https://doi.org/10.1038/s41559-024-02448-y
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