Mitigation through restoration: reducing carbon gas emissions in alpine lakes

Lakes play a significant role in the global carbon cycle by transforming, burying, emitting, and transporting carbon from terrestrial ecosystems to the ocean. Carbon is emitted primarily as carbon dioxide (CO₂) and methane (CH₄), both potent greenhouse gases (GHGs), but it can be difficult to determine whether lakes are net sources or sinks of those GHGs. Alpine lakes, which are sentinel systems experiencing rapid and severe climate change-related impacts, are especially challenging. Anthropogenic activities such as fishing or intensive pasture might additionally impact their C balance forcing them into net C sources to the atmosphere. Restoring those lakes might lead not just to recover their threatened biodiversity, but also their GHG balance, constituting a great mitigation strategy. Here we aim to test the potential of ongoing restoration efforts based on the recuperation of biodiversity for reducing C gas emissions in alpine lakes.

To do so, we focused on two alpine lakes in the Pyrenees, as representative examples of broader trends in mountain lake ecosystems. Lake Tres Estanys de Baix (TEB) has been suffering a eutrophication process due to the introduction of invasive fish species in the past. Conversely, Naorte has undergone ecological restoration, with the near-total removal of invasive fish through two European LIFE projects. In 2023, sensor-equipped platforms were installed in both lakes to continuously gather data on weather, oxygen (O₂), and CO₂ at depths of 2 and 8 meters. To obtain additional information on nutrients and carbon concentrations and the quality of dissolved organic matter (DOM), weekly manual samples were taken. The restored lake, Naorte, demonstrated remarkable recovery, with significantly improved water clarity and reduced emissions of CO₂ and CH₄ compared to the impacted lake, TEB. In TEB, DOM was more colored, and concentrations of dissolved organic carbon and nitrogen were higher. This lake also exhibited signs of nutrient imbalance, with lower levels of inorganic nitrogen species, further showing a disrupted metabolism. The carbon gas balance based on coupled O2 and CO2 measurements, revealed a striking contrast: TEB functioned as a net carbon source, actively emitting GHGs, whereas Naorte acted as a net carbon sink, sequestering more carbon than it released. These findings validate our initial hypothesis that removing invasive species can restore alpine lake’s natural ability to act as carbon sinks by significantly reducing its GHGs emissions.

Beyond in-lake measurements, atmospheric CO₂ sensors were deployed in the surrounding basins to capture broader carbon dynamics. The data collected was made available online in real-time allowing for immediate access and integration into educational resources, bringing the project’s findings directly to local communities. Approximately 300 students and teachers from rural schools in the Pyrenees participated in an outreach program, engaging with the data and exploring its implications. This initiative highlighted the transformative potential of ecological restoration not only to mitigate greenhouse gas emissions but also to recover mountain ecosystems. By involving young minds in understanding and addressing these environmental challenges, we aim to inspire a new generation to contribute to a sustainable and resilient socio-ecological future.