A new study published on Wednesday in the journal Nature shows that oxygen levels in hundreds of lakes in Europe and the United States have dropped over the last four decades.
This disturbing condition may lead to more fish kills, algae blooms and even emissions of methane gas, the authors warn.
Researchers studied the temperature and amounts of dissolved oxygen in the waters of over 400 lakes and came to the dismaying conclusion that the decline in oxygen was widespread throughout the European continent and the US.
The amount of dissolved oxygen in lakes, including Europe’s most well-known bodies of water, Lake Zurich, declined 5.5% in surface waters — and as much as 18.6% in deeper waters.
The authors state that their conclusions suggest that warming temperatures and lessened water clarity stemming from human activity are the culprits in the oxygen decline over the years.
Oxygen levels tied directly to health of ecosystems
“Oxygen is one of the best indicators of ecosystem health, and changes in this study reflect a pronounced human footprint,” stated co-author Craig Williamson, who is a professor of biology at Miami University in Ohio.
That unfortunate footprint is part of the gradual warming trend that is a part of climate change, they posit, as well as increased runoff from sewage, fertilizers, cars and power plants, they state.
Dissolved oxygen in the world’s oceans has also been the target of scientific studies in the past, with a study from 2017 finding that there had been a decline of 2% in such oxygen levels since the year 1960.
However, less information had been in the database regarding the lakes of the world, which over the same period of time has lost two to nine times the amount of oxygen over the same period, according to the new study.
Up until now, other scientists had already reported declines in oxygen levels in certain lakes over periods of time — but no one study has looked at this many lakes over different continents at the same time.
Samuel B. Fey, a Reed College biology professor who studies lakes — but who was not involved in this most recent study — agrees with the findings. “I think one of the really interesting endings here is that the authors were able to show that there’s this pretty pronounced decline in dissolved oxygen concentrations in both the surface and (deep) parts of the lake,” he states in an interview with the Associated Press.
The steep drop in oxygenation in the deeper regions of lakes is especially concerning, since they are home to aquatic organisms that are already inclined to be more sensitive to temperature fluctuations, such as cold water fish. During the hot Summer months, they must have the cooler temperatures found only in deep water — but of course if this water contains less oxygen, that directly threatens their survival.
“Those are the conditions that sometimes lead to fish kills in water bodies,” states co-author Kevin C. Rose, who is a biology professor at Rensselaer Polytechnic Institute. “It really means that a lot of habitats for cold water fish could become inhospitable.”
A glimmer of hope remains, however, in the study as searchers concluded that fish that live closer to the surface of lakes, which are more tolerant of warmer water, can always take in the oxygen they need by spending more time near the surface, where there is much more oxygen saturation.
Approximately one quarter of the lakes studied recently actually demonstrated an increase in oxygen levels in surface waters — but Rose explains this is a negative development because it is most likely due to increased blooms of blue-green algae.
Dissolved oxygen in these lakes, Rose said is actually “very low” in deeper sections, to the point that they have become unlivable for many species.
In addition, the sediments found in such lakes even gives off methane as well, which is a potent greenhouse gas contributing to global warming.
All the lakes in the study were in Europe or the United States with the exception of one in Japan and several others in New Zealand. The authors of the study stated that there was not sufficient data to incorporate findings from other areas of the world.