Scientists have warned that dissolved oxygen in oceans, lakes and rivers is decreasing at alarming rates due to human activities, threatening the stability of aquatic systems and the entire biosphere.
A study published in the journal Limnology and Oceanography called for the inclusion of "aquatic oxygen depletion" within the framework of the "planetary boundary," a system developed by 28 scientists in 2009 to identify nine vital processes that humans have already surpassed, such as climate change, loss of biodiversity, ocean acidification, and chemical pollution.
Lead author Erica Ferrer from the University of California explains that the health of the planet depends on the health of water systems that need oxygen to function, adding that the study aims to highlight the depletion of water oxygen as a global threat that does not operate in isolation.
The truth is that dissolved oxygen in water differs from oxygen bound to H2O molecules, as living organisms rely on dissolved oxygen for respiration. There are two main factors behind its declining levels.
First: rising water temperatures. As carbon emissions increase and the average global temperature rises, water bodies heat up, and warmer water holds less dissolved oxygen than colder water. Furthermore, warmer surface layers prevent oxygen from mixing with deeper waters.
Second: Nutrient pollution. Agricultural, sewage, and industrial practices release large quantities of nitrogen and phosphorus into the water, fueling massive algal blooms. As these algae decompose, they consume large amounts of oxygen. Furthermore, heat and excess nutrients stimulate microbial oxygen consumption, exacerbating the oxygen deficiency.
Researchers warn that the depletion of water oxygen is approaching a "danger zone," with effects that could be irreversible within our lifetimes.
According to the European Copernicus Initiative, the global ocean has lost about 2% of its dissolved oxygen since the 1950s, and is expected to lose an additional 1% to 7% by the end of the century. While these percentages may seem small, even a slight decrease is enough to deprive living organisms of their basic needs and jeopardize the delicate ecological balance.
In their review, the researchers analyzed the interactions between oxygen depletion and the nine known planetary boundaries, and found that it interacts with climate change, nutrient loading, biodiversity loss, and aerosol loading, making it a crisis intertwined with the rest of the environmental challenges.
The researchers proposed four indicators to assess the state of aquatic oxygen depletion and determine a global limit for it: the concentration of dissolved oxygen and the extent of the spread of very low levels, how close the water is to its oxygen saturation capacity, changes in oxygen-sensitive organisms, and a metabolic index that compares the organisms’ oxygen needs with the amount available.
Researchers hope that including the depletion of water oxygen within the framework of planetary boundaries will galvanize global attention to this crisis and provide a roadmap for addressing it.
