Warmer earth means less oxygen, more greenhouse gases for lakes 

Enterprise file photo — Noah Zweifel

Thompsons Lake, in East Berne, in the summer collects oxygen from the wind in its top layer, which is warmed by the sun. Lakes around the world are experiencing the effects of climate change, which is lowering their oxygen levels and potentially creating ecologically destructive feedback loops.

ALBANY COUNTY — As the earth has warmed, so have its lakes, researchers have found, and the consequences of this could be dire for wildlife in New York State and beyond. 

A study conducted by researchers from Rensselaer Polytechnic Institute and Cornell University, who analyzed historic data from 429 lakes, has found that many of these lakes are experiencing extended stratification periods in the summer due to rising average temperatures. 

Lake stratification is essentially the horizontal layering of lake water based on temperature and other factors, with warmer, less dense waters flowing on the top, and colder, denser water resting on the bottom, according to the International Institute for Sustainable Development. 

In temperate climates like New York’s, this layering holds steady during the summer when the sun is continually heating the top layer, the institute explains, and in that time the warm surface-layer collects oxygen from wind — oxygen being “the most fundamental parameter of aquatic ecosystems apart from water itself,” according to the authors of the recent study — while the deepest layer loses dissolved oxygen, sometimes completely. 

The temperature boundaries in lakes tend to weaken in cooler seasons, at which point the oxygen-rich top layer sinks down and delivers its oxygen to the lower layers, which also experience warming as winds move the water all around. This process, known as turnover, is how lakes maintain homeostasis, the institute explains. 

What the researchers found is that deep lake waters are largely spending more time with less oxygen, and that the proportion of lake water that was below “ecologically important” oxygen-level thresholds was increasing by a rate of 0.9 percent to 1.7 percent per decade. 

Altogether, the total volume of low-oxygen lake water has increased by 50 percent, the study says.

The potential consequences of this are numerous.

For one thing, certain fish populations, such as salmonids, may be pushed out of their usual habitat because they can’t tolerate low oxygen levels, the study says through reference to earlier studies. 

One paper out of the Mediterranean Institute for Advanced Studies states that mobility of various wildlife plays a role in how different species handle hypoxia, or low oxygen levels, with fish generally being able to climb to shallower layers more easily than crustaceans.

Regardless of their ability to attain oxygen, though, certain species may be more vulnerable to predators in less-familiar water layers, which has ripple effects on the ecosystem, particularly on biodiversity. 

The Cornell/RPI study also points out that phosphorous levels can increase in deoxygenated water, which in turn can lead to toxic algal blooms. These blooms further pull down oxygen levels, creating a feedback loop, the study says, but points out that this has not been observed yet. 

Finally, because deoxygenated water also stores methane, when that water eventually comes to the surface, it releases that methane into the atmosphere, contributing to climate change and creating yet another dangerous feedback loop, the study suggests.

“Ultimately,” the researchers say, “changes in overall release of methane from lakes will depend on changing mixing processes or overturns during spring and fall.”

They reference a high greenhouse gas emission model of the year 2100 in which the stratification period would increase by over a month for “many lakes.”

“Our results indicate that continued lengthening of stratification will have substantial negative impacts on lake DO concentrations,” the study says of dissolved oxygen, known as DO. “Thus, active management to maintain deep water DO above critical biological and ecologically important thresholds may be required even to maintain current DO concentrations.”

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