Planetary boundaries: Vital also in water

Around the world, the oxygen content of seas, lakes and rivers is decreasing rapidly. One points to this Community study from various institutes in the USA, Canada, France, Belgium and Germany, which appeared in the journal “Nature Ecology & Evolution” in mid-July. The finding is not necessarily new, but the authors were primarily concerned with the significance of this development for the Earth system and for human society.

According to the thesis of their study, there is a critical value for the oxygen content in the waters and oceans, below which various other subsystems are also affected. It represents the greatest threat to aquatic ecosystems worldwide. The authors advocate for the oxygen content of marine and freshwater ecosystems to be added to the list of planetary boundaries. “This will help support and focus global monitoring, research and policy efforts to protect our aquatic ecosystems and therefore society as a whole,” says Kevin Rose, who teaches at Rensselaer Polytechnic Institute in Troy, USA and lead author the study was.

Earth’s carrying limits

In 2009, a study presented for the first time resilience limits for nine central biophysical systems and processes on earth, which are now generally accepted. Planetary boundaries therefore exist, among other things, for climate change, the depletion of the ozone layer, changes in land use and the introduction of new substances into the environment. Six of the nine planetary boundaries are now considered to have been exceeded.

With regard to the possible tenth limit – the oxygen content in water – the scientists speak of a tipping point that should be urgently observed and further investigated. The oxygen content is not only important for the animals for whose existence and growth the element is essential, but also for various biogeochemical processes. Among other things, it influences the storage of carbon in water and sediments and thus indirectly also on the concentration of greenhouse gases in the atmosphere.

Oxygen content of water varies

Meanwhile, the oxygen is very evenly distributed there due to good mixing. After the photosynthesis of early plants in the early history of our planet ensured that more and more of this chemically aggressive but indispensable element for life was in the air, its concentration is now almost 21 percent.

In water, however, the concentration varies greatly in space and time. On the one hand, oxygen is absorbed through gas exchange on the surface and dissolved in the water, and on the other hand, it is produced by the algae via photosynthesis. However, this only happens in the upper layers into which sunlight can penetrate. It only gets to deeper altitudes if the water is mixed in one way or another.

Decline observed before 1900

But unlike the atmosphere, bodies of water are usually very stable. Warm water initially remains in the upper layers because it is less dense than the water below. Currents, storms or even large marine animals must first disrupt this stable layering. In this respect, anoxic zones, i.e. those without oxygen, or areas with dangerously low concentrations are nothing unusual in lakes and seas.

However, in recent decades, a rapid decline in dissolved oxygen in water has been observed worldwide in lakes and rivers as well as in the seas. According to the authors, the process was recorded in lakes before 1900, and in coastal waters it began primarily in the middle of the 20th century. A broader scientific awareness of the loss of oxygen in the open sea only developed in this century. The volume of anoxic water there has quadrupled since 1960.

Positive interactions are to blame for the decline in oxygen in the sea because, on the one hand, the ability to absorb oxygen and carbon dioxide decreases as the water temperature increases. On the other hand, it means expansion
oxygen-free zones on the seabed mean that microorganisms increasingly produce the greenhouse gases methane and nitrous oxide.