Coastline of lakes an important part of global carbon cycle

Lakes have long been viewed as sources of carbon dioxide emissions, but new research suggests they may actually act as carbon sinks. A study led by Uppsala University reveals that lake shorelines store more carbon than previously believed, highlighting the need to include these littoral zones in calculations of the continental carbon balance.

The ‘coastlines’ of lakes, called littoral zones, are often surrounded by aquatic plants that are among the fastest growing plants in the world. They take up a large amount of carbon from the atmosphere and high quantities of carbon from remaining plant material are stored in the sediments. Although the total littoral zone of lakes around the world is four times longer than the coastline of the oceans, the role of lake shorelines has not been considered in global carbon budgets, until now.

A new publication by Uppsala University is the first to include littoral zone plants in lake carbon budgets and reveals that, rather than releasing carbon to the atmosphere, lakes may actually be a carbon sink.

“We were planning to write a conceptual paper about how aquatic plants in the littoral zone are overlooked in lake carbon cycling. But after doing some initial calculations quantifying the role of these plants, we quickly realised that littoral zones could be a significant player in the global carbon budget! So, our conceptual paper eventually turned into the first global carbon budget to include the critical contribution of aquatic plants from lake littoral zones,” says Charlotte Grasset, first author of the study and researcher at Uppsala University.

Lakes switched from net carbon source to net carbon sink

Using existing data and a simple model connecting littoral zones to the lake centre, the authors did the first global-scale quantification of the contribution of littoral zones to the carbon budget of lakes.

“We find that adding the littoral zone vegetation to the global carbon budget of lakes substantially changes the carbon accounting of lakes. Depending on the values used, we found that when littoral zones were included, lakes switched from a net carbon source to a net carbon sink; that is, the carbon stored annually in lake sediments was greater than the carbon released to the atmosphere,” says Grasset.

Restoring lakeshores for climate and biodiversity gain

The authors all conclude that there is still much work to be done. Although their estimates are based on the best data and information currently available, more measurements are required to improve these initial findings for lakes. This includes a better understanding of the areal extent of vegetated littoral zones in lakes, more refined estimates of carbon gas exchange between the aquatic plants, sediments, atmosphere and the lake’s centre.

“We hope this study will stimulate future research on the critical role of lake littoral zones in carbon budgets, and of the possible role of restoring the lake littoral as a nature-based solution,” says Grasset.

Given that plants in marine coastal habitats known as ‘blue carbon’ have been considered as a nature-based solution for over 15 years, the authors argue that it is time to start focusing some of that attention on lake littoral zones, not only for better carbon accounting, but to improve lake water quality and restore aquatic biodiversity.