The “coastline” of lakes: a key part of the global carbon cycle

The study — the first to quantify the contribution of littoral zones to the global carbon budget of lakes — was published in August in Nature Geoscience.

"Our discovery quantifies the role of aquatic plants in the littoral zones of lakes in the global carbon cycle, and clearly demonstrates the importance of these plants in the net carbon balance of lakes," said lead author Charlotte Grasset, a researcher at Uppsala University (UU).

Although the total littoral zone of lakes around the world is four times longer than the coastline of the oceans, the role of vegetated lake shorelines has not been considered in global carbon budgets, she said. Instead, measurements of carbon are usually taken in the middle of lakes, away from shore.

"Including the potential of these shoreline plants reverses the role of lakes in the global carbon cycle," said Maranger, who collaborated in the research as a visiting professor at UU since 2023 and who, at UdeM, holds a Tier 1 Canada Research Chair in aquatic ecosystem science and sustainability.

"When the growth rate and extent of these plants are included in calculations, lakes become a net carbon sink instead of an atmospheric source — that is, the carbon stored annually in lake sediments is greater than the carbon released into the atmosphere thanks to these plants,” said Maranger.

"This changes the current paradigm and highlights the potential of restoring lake littoral zones as a nature-based solution, while also having implications not only for carbon storage to mitigate against climate change, but also for improved water quality and conservation of biodiversity," she added.

At the UU Department of Ecology and Genetics, Maranger led a group of scientists in discussion about littoral zone plants and lake carbon cycling. One of those scientists was Charlotte Grasset, who wound up leading the study.

“We were planning to write a conceptual paper about how aquatic plants in the littoral zones are overlooked in lake carbon cycling, " Grasset recalled.

“But after we did some initial calculations quantifying the potential role of these plants, we quickly realized that littoral zones could be a significant player in the global carbon budget.”

The scientists used existing global data and a simple model connecting littoral zones to the lake centre to quantify how much carbon is actually stored in lakes.

“Depending on the values used, we found that when plants in the littoral zone were included, lakes switch from a net carbon source to a net carbon sink," said Maranger.

“Our findings challenge the current paradigm of how lakes are viewed in global budgets because we argue that more carbon is produced within the lake rather than exported from land.”

The authors all conclude that there is still much work to be done. Although their estimates are currently based on the best available data and information, more measurements are needed to improve these initial findings for lakes.

This includes a better understanding of the real extent of vegetated littoral zones in lakes globally as well as more refined estimates of carbon exchange between the aquatic plants, the atmosphere, the lake’s center and sediment.

“Given that plants in marine coastal habitats, called “blue carbon”, have been considered as a nature-based solution for over 15 years, it is time to start focusing some of that attention on lake littoral zones,” said Maranger, a member of UdeM's Groupe de Recherche Interuniversitaire en Limnologie (GRIL).