Lakes release large amounts of CO2, now we know why

A new study has uncovered the missing science behind why some lakes around the world emit such large amounts of carbon dioxide (CO₂).

Lakes are unlikely sources of substantial amounts of greenhouse gas emissions. But the exact mechanisms at work hadn’t been fully understood, hampering efforts to identify sources and sinks of carbon in the fight against global warming.

The answer seems to be the natural erosion of rocks in upstream basins.

The study was based around Lake Geneva, which borders France and Switzerland, and is one of western Europe’s largest lakes.

“Our results not only explain the carbon cycle in Lake Geneva, they also reveal a universal process that applies to several of the world’s great lakes,” says Marie-Elodie Perga, professor of limnology at Switzerland’s University of Lausanne and co-author of the study, published in Science Advances.

Previously, it was thought that lake CO₂ emissions were mostly due to the influx of organic matter from surrounding soils, which is broken down by microorganisms in the water where it releases CO₂ in a process known as respiration.

However, while this process may apply to some lakes, it doesn’t account for the substantial CO₂ emissions of Lake Geneva, which receives very little organic matter from its shores.

“This issue had been nagging at me since my thesis,” says Perga.

“Using a scientific infrastructure that is unique in the world – the LéXPLORE platform – we were able to observe, model and equate these processes on a very fine scale, providing the missing piece to traditional carbon cycle modelling.”

Perga and her colleagues simulated the lake’s carbon cycle over the past 40 years and validated their findings using data collected by the LéXPLORE platform – a 10m², floating research platform located almost 600m from the shore.

They found that when bedrock in Lake Geneva’s catchment is exposed to rainwater it releases bicarbonate and calcium ions, which flow into the lake.

In the summertime, heat and algal growth change the pH of the water and catalyse a chemical reaction which turns these ions into microparticles of calcium carbonate. This is known as calcite precipitation, and the reaction also releases CO₂.

Calcite precipitation is a common phenomenon in lakes and has been reported in Great Laurentian Lakes in the US, Great Lakes in Africa, and lakes in northern China, South America, and Europe.

“Assessments are carried out every year to identify the emitters (sources) and storages (sinks) of carbon on our planet,” says Perga.

“It’s very important to have in-depth knowledge of how CO₂ is naturally transported, stored and transformed between continents, water and the atmosphere. Only a global vision will enable us to take effective action to combat global warming.”