The concept of “Planetary Boundaries” (PB) outlines nine critical ecological thresholds that, if crossed, could trigger irreversible damage to ecosystems and jeopardise humanity’s future on Earth. Alarmingly, we have already surpassed six of these nine vital boundaries. Now, scientists are raising concerns about a potential tenth, previously unrecognised, boundary: the widespread deoxygenation of our planet’s aquatic environments, including lakes, reservoirs, oceans, and other water bodies.
While “climate change” is often used as a broad term for human-induced environmental degradation, it represents only one facet of the numerous threats our planet faces. The Planetary Boundary framework, first introduced in 2009, identifies nine distinct thresholds that, if breached, could lead to catastrophic outcomes. Climate change is indeed one of these boundaries, alongside others such as biosphere integrity, ozone depletion, ocean acidification, and freshwater change. It’s worth noting that climate change exacerbates all of these issues, making it a primary concern.
A New Threat: Aquatic Deoxygenation
Scientists are now suggesting that a tenth boundary, aquatic deoxygenation, warrants inclusion in this critical list. While some water bodies, like the Black Sea, the Baltic Sea, and certain fjords, are naturally anoxic (lacking oxygen), widespread deoxygenation refers to the significant depletion of oxygen in previously oxygenated waters across the globe, occurring to varying degrees.
Recent research indicates a concerning trend:
* Lakes have experienced oxygen losses of approximately 5.5 percent over the past 45 years.
* Reservoirs have seen even more dramatic oxygen declines, losing around 18.6 percent.
* Oceans, despite their immense volume, have experienced a 2 percent drop in dissolved oxygen, a significant figure when considering their vast scale.
One particularly stark example of this phenomenon is observed in the midwaters off the coast of California, where oxygen levels have plummeted by a staggering 40 percent since 1960. These findings were recently published in the esteemed journal Nature Ecology & Evolution.
Understanding the Causes
Professor Andreas Oschlies, a co-author of the study and a specialist in Marine Biogeochemical Modelling at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, elaborated on the drivers behind this rapid deoxygenation. He explained that two primary factors are at play:
Global Warming: Rising global temperatures, driven by greenhouse gas emissions, directly impact water’s ability to hold oxygen. As water warms, its solubility for oxygen decreases. Furthermore, global warming intensifies stratification within the water column. This phenomenon occurs when warmer, less dense freshwater sits atop colder, denser saltwater, creating a barrier that hinders the vital exchange of oxygen-poor deep water with oxygen-rich surface water.
Land Use and Nutrient Runoff: Human activities, particularly land use practices, contribute significantly to nutrient pollution. Excess nutrients, often from agricultural runoff and sewage, fuel excessive algal blooms. When these algae die and decompose, microbes consume vast amounts of oxygen in the process, further depleting dissolved oxygen levels, especially in deeper waters.
The Far-Reaching Consequences
The implications of widespread aquatic deoxygenation are profound and multifaceted.
- Impact on Marine Life: Aquatic animals, from fish and mussels to crustaceans, are critically dependent on oxygenated water for survival. Deoxygenated zones create “dead zones” where these organisms cannot live, leading to significant disruptions in marine and freshwater ecosystems.
- Food Chain Collapse: The decline in oxygen levels directly threatens marine populations, which in turn reverberates up the food chain, potentially leading to ecological collapse.
- Greenhouse Gas Production: Deoxygenated waters can become breeding grounds for specific microbial processes that produce potent greenhouse gases, namely nitrous oxide and methane. This creates a dangerous feedback loop, exacerbating climate change.
Addressing the Crisis
Kevin Rose, a professor at Rensselaer Polytechnic Institute in New York and lead author of the study, emphasised the interconnectedness of these issues. He stated, “Dissolved oxygen regulates the role of marine and freshwater in modulating Earth’s climate. Improving oxygen concentrations depends on addressing the root causes, including climate warming and runoff from developed landscapes.”
The consequences of failing to address aquatic deoxygenation extend beyond ecological concerns, impacting economic activities and societal well-being on a global scale. As humanity has already crossed six of the nine established Planetary Boundaries, the addition of a tenth, aquatic deoxygenation, serves as a stark reminder of the urgent need for action. Fortunately, the solution remains clear and has been understood for over a century: a drastic reduction in emissions is paramount to safeguarding our planet’s future.
