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A new study examines surveys of lakes conducted over the past eighty years finding freshwater bodies are losing dissolved oxygen more quickly than the oceans.
Earth’s temperate freshwater lakes are losing oxygen much more quickly than its oceans, new research has revealed. The trend is being driven predominantly by climate change and threatens not only our supply of clean drinking water but also the biodiversity of some of the planet’s most beautiful ecosystems.
In fact, though freshwater lakes only account for about 3% of Earth’s surface they are home to a disproportionately large amount of our planet’s animal and plant life. That means that these changes in oxygen levels should be considered concerning on two fronts: as a demonstration of the impact of ongoing climate change and for their impact on global biodiversity.
The concentration of oxygen dissolved in freshwater lakes also impacts greenhouse gas emissions, nutrient biochemistry, as well as human health.
All complex life depends on oxygen. It’s the support system for aquatic food webs. And when you start losing oxygen, you have the potential to lose species. Lakes are losing oxygen 2.75–9.3 times faster than the oceans, a decline that will have impacts throughout the ecosystem.
Kevin Rose, Professor, Rensselaer Polytechnic Institute
Alongside lead-author Stephen F. Jane, Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, Rose is one of the authors of a paper published in the journal Nature¹ that discusses the increasing de-oxygenation of freshwater lakes.
Examining 80 Years of Freshwater Lake Oxygenation and Temperature Profiles
The researchers reveal in the study that oxygen levels in surveyed lakes located in the temperate zone — an area of the world that spans 23 to 66 degrees north and south latitude and is characterized by having a climate that is warm in the summer, cold in the winter, and moderate in the spring and autumn — have declined by 5.5% at the surface and by 18.6% in deep waters over the last four decades.
The team took data from over 45 thousand dissolved oxygen and temperature profiles collected from around 400 lakes located across the world to conduct their research. Some of the profiles examined by the scientists date back as far as 1941, with long-term records originating from the temperate zone.
Though the researchers highlight climate change as one of the primary causes of the de-oxygenation of freshwater lakes, they point out that other mechanisms are also at play.
Though climate change is the primary driver of widespread losses of dissolved oxygen in the lakes studied by the researchers, the mechanisms at work differ between surface waters and deep waters.
World’s Lakes Losing Oxygen Rapidly as Planet Warms
What Factors are Driving Lake Surface De-Oxygenation?
Surface water de-oxygenation is primarily driven by increases in temperature. As surface water temperature has risen by around 0.38⁰C over the past few decades, surface water oxygen concentrations have fallen by 0.11 milligrams per liter per decade over the same period. The team found that lakes affected by agricultural runoff experienced an increased rate of de-oxygenation and increased temperatures. This is very probably because this form of pollution is rich in nutrients and thus causes an increase in algal blooms. The proliferation of these blooms, in turn, cause increased de-oxygenation.
“The fact that we’re seeing increasing dissolved oxygen in those types of lakes is potentially an indicator of widespread increases in algal blooms, some of which produce toxins and are harmful,” Rose explains. “Absent taxonomic data, however, we can’t say that definitively, but nothing else we’re aware of can explain this pattern.”
The loss of dissolved oxygen in deeper waters results from a more complex interplay between different factors, including the warming of surface waters.
A Deeper and More Complex Problem
Temperatures in deeper waters surveyed by the team have remained mostly stable and de-oxygenation at these depths is likely due to rising surface water temperature and the increased duration of warm periods each year.
As surface water temperatures increase, but deep water temperatures remain stable there is a more significant difference between the density of water in these areas. That means that the mixing of these waters is less likely to occur. As a result, the oxygen in deep water areas is also less likely to be replenished because oxygenation occurs at the surface of a body of water.
The decrease in oxygen levels across freshwater lakes has a number of knock-on effects. This includes encouraging the growth of bacteria populations, which results in the increased emission of methane — one of the most problematic greenhouse gases.
Additionally, low oxygen conditions cause sediments to release more phosphorous, which adds nutrients to bodies of water thus further driving burgeoning populations of algal blooms.
Ongoing research has shown that oxygen levels are declining rapidly in the world’s oceans. This study now proves that the problem is even more severe in freshwaters, threatening our drinking water supplies and the delicate balance that enables complex freshwater ecosystems to thrive. We hope this finding brings greater urgency to efforts to address the progressively detrimental effects of climate change.
Curt Breneman, Dean, Rensselaer Polytechnic Institute’s School of Science
Sources
Jane. S. F., Hansen. G. J. A., Rose. K. C., et al, [2021], ‘Widespread de-oxygenation of temperate lakes,’ Nature, [https://doi.org/10.1038/s41586-021-03550-y]
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