Northern freshwater lakes are turning brown as permafrost thaws and introduces more organic carbon into the water, according to a new study published in the journal Limnology and Oceanography Letters.

The study says the "browning" of lakes is a global phenomenon, but calls the rate at which it's happening in the North an "extreme version."

The study sampled 253 ponds in Arctic and subarctic regions across the world over a span of 15 years, from 2002 to 2016.

It noted organic carbon is particularly good at absorbing sunlight, which is not a good thing for aquatic ecosystems.

If sunlight can't penetrate the water, then phytoplankton, which uses photosynthesis, can't propagate. In turn, insects and fish will have nothing to eat, says Maxime Wauthy, one of the authors of the study.

"For the local people, browner lakes mean less fish, less good quality fish, and also more difficulties to have water to drink or at least the need to spend more money to have drinkable water," Wauthy said.

While fish would not contain additional contaminants as a result of these environmental changes, they would contain less of the nutritious omega-3 fatty acids, he said.

A subarctic pond with very clear water and no influence of permafrost thaw near Kuujjuarapik, Nunavik, with Hudson Bay behind. (Submitted by Maxime Wauthy)

The changing composition of the lakes caused by the added carbon also means less oxygen in the water, which Wauthy said can make fish less nutritious.

Emitting greenhouse gases

Frozen tundra soils are one of the largest pools of organic carbon on Earth, according to the study. It states climate warming has increased the chances that a large percentage of that carbon could be released into the atmosphere as methane.

This happens when carbon from thawing permafrost enters a lake's ecosystem and is turned into methane by microbes that live in the water. Since these lakes and ponds are relatively shallow, the methane gas easily makes its way to the water's surface.

Past studies have identified ponds affected by thawing permafrost as strong emitters of greenhouse gases.

Wauthy's study concludes that increased rainfall and extreme weather events may further increase the browning of northern lakes.