The 11 largest lakes on the planet hold more than 50 percent of the surface freshwater that millions of people and many more living organisms require to survive. A new study highlights how these lakes absorb carbon dioxide from the atmosphere and how their ability to do so has changed over the last two decades.
The study used a combination of satellite observation and fieldwork to establish a key factor in the role of lakes as carbon sinks, places where carbon dioxide is absorbed from the atmosphere. Phytoplankton are tiny algae that photosynthesize or make energy from sunlight. Part of photosynthesis is carbon fixation, where inorganic carbon dioxide is converted into an organic compound. The amount of phytoplankton and the rate they photosynthesize give the carbon fix rate of a body of water like a lake. Estimating phytoplankton primary production can indicate how much carbon dioxide these algae pull from the air.
The study, published in the journal Water, is the first to use the same approach for the 11 largest lakes in the world, including the five Laurentian Great Lakes bordering the US and Canada; the Great Bear and Great Slave lakes in Canada; the three African Great Lakes, Tanganyika, Victoria, and Malawi; and Lake Baikal in Russia.
“The base of the food chain in these lakes is algal productivity. These lakes are oceanic in size, and are teeming with phytoplankton — small algae,” co-author Gary Fahnenstiel from Michigan Tech Research Institute said in a statement. “We measured the carbon fixation rate, which is the rate at which the algae photosynthesize in these lakes. As that rate changes, whether increasing or decreasing, it means the whole lake is changing, which has ramifications all the way up the food chain, from the zooplankton to the fish.”
Using NASA data from between 2003 and 2018, the researchers used the color of the lake from the satellite images to estimate algae productivity. Though the team found that most lake productivity did not change much over the years, for three lakes, the change was dramatic. The team was surprised at how fast the changes occurred. Great Bear and Great Slave lakes in northern Canada both saw a major increase in productivity. On the other hand, Lake Tanganyika in southeastern Africa has seen a significant decline.
“Three of the largest lakes in the world are showing major changes related to climate change, with a 20-25% change in overall biological productivity in just the past 16 years,” Fahnenstiel said.
The researchers say the changes in these three lakes are linked to increases in water temperature, solar radiation, and reduction in wind speed. “Temperature and solar radiation are factors of climate change,” lead author Michael Sayers added.
The factors that influence algae productivity are their abundance in a lake, how much sunlight a lake gets, how deep that light can penetrate, water temperature, and wind speed. All these factors are influenced by climate change. Understanding exactly how these vast bodies of water are changing is key to their future and the future of those who depend on them.