In a groundbreaking study that challenges decades of prevailing assumptions, researchers have unveiled the pivotal role of lake littoral zones—the vegetated margins of lakes—in the global carbon cycle. Contrary to long-held beliefs that lakes predominantly act as net sources of atmospheric carbon, this new analysis reveals that when littoral zones are properly accounted for, lakes may in fact function as substantial long-term carbon sinks. This revelation not only revises our understanding of inland water carbon fluxes but also has profound implications for continental carbon budgeting and climate mitigation strategies.
Lakes have traditionally been studied with a focus on their pelagic, or open-water, zones. Previous global carbon budgets largely neglected or underestimated the littoral zones, which consist of shallow, plant-rich margins where aquatic macrophytes thrive. These zones facilitate significant carbon burial in sediments, counteracting the carbon dioxide and methane outgassed from lake surfaces. The latest estimates demonstrate that inclusion of littoral carbon turnover can shift the net carbon balance of lakes from positive sources to net sinks, altering the carbon dynamics at continental scales.
The research team meticulously estimated carbon fluxes, distinguishing between carbon burial and outgassing processes in both pelagic and littoral zones. Their findings indicate that the global net carbon burial by lakes, when including littoral contributions, ranges approximately from 0.28 ± 0.09 to 0.11 ± 0.02 petagrams of carbon per year (Pg C yr⁻¹), rivaling or surpassing net carbon outgassing, which they estimate between 0.24 ± 0.27 and 0.14 ± 0.11 Pg C yr⁻¹. This contrasts starkly with earlier models where lakes were predominantly viewed as net carbon emitters, mainly because pelagic zones were the primary focus.
A key driver of this paradigm shift lies in the macrophyte-driven carbon uptake within the littoral zones. Aquatic plants capture atmospheric CO₂ at rates sufficiently high to offset the terrestrial carbon released through outgassing in these zones. Specifically, the net atmospheric carbon uptake attributed to macrophytes is estimated between 0.11 and 0.26 Pg C yr⁻¹. This uptake not only balances but may surpass the rate of terrestrially derived carbon released as CO₂ in littoral areas, which ranges from 0.02 to 0.07 Pg C yr⁻¹.
The effect of including littoral zones on whole-lake carbon budgets is profound. Not only does it elevate the estimated net carbon burial, but it also reduces net atmospheric carbon emissions from lakes. Pelagic zones alone contribute to net carbon outgassing estimated between 0.23 and 0.43 Pg C yr⁻¹, while littoral zones present a net sink estimated between 0.09 and 0.19 Pg C yr⁻¹. Integrating these results reveals that the net carbon emissions of entire lakes decrease significantly when littoral carbon fluxes are included, potentially reversing the net carbon source paradigm.
This transformative insight hinges strongly on the extent of littoral zone coverage across lakes globally. The research estimates vegetated littoral zones cover between 13% and 33% of global lake surfaces, with more precise upper-bound estimates ranging between 23% and 33%, based on comprehensive datasets including GloWaBo and HydroLAKES. Notably, the model identifies a critical threshold: when vegetated littoral zone coverage exceeds approximately 19%, lakes transition from net carbon sources to net carbon sinks, profoundly impacting global carbon accounting.
The uncertainty around vegetated littoral coverage remains a critical challenge, especially because the smallest lakes—those under 0.1 km²—are often omitted from global datasets yet tend to have disproportionately larger littoral areas. This omission may mean that current estimates undervalue the importance of littoral zones and their carbon sequestration capacity, suggesting the critical threshold of 19% coverage could be attainable or even exceeded globally, reinforcing the case for lakes as net carbon sinks.
Beyond carbon burial and emissions of CO₂, the study also explores methane (CH₄) dynamics within littoral zones. Methane, a greenhouse gas with a global warming potential roughly 27 times that of CO₂, often originates from anoxic sediment conditions prevalent in vegetated littoral regions. Macrophyte-derived organic carbon is particularly susceptible to transformation into methane, which then predominantly escapes oxidation and is vented to the atmosphere via ebullition or plant-mediated pathways.
By integrating recent global estimates of plant-associated methane emissions, the study quantifies the increase in total lake methane emissions upon including littoral zones. This addition is estimated to enhance lake methane emissions by 7% to as much as 49%, depending on the dataset considered, consistent with observed increases of about 13% in northern lakes when aquatic vegetation is factored in. These figures underscore the littoral’s dualistic role as both a sink of carbon and a source of potent greenhouse gases.
Yet, the net climatic impact of littoral zones appears complex. When expressed in CO₂-equivalent terms by applying a conversion factor of 27.2 to methane emissions, littoral zones globally hover near neutral or slightly positive greenhouse gas sources with emissions ranging from −0.02 to 0.06 Pg C-CO₂eq yr⁻¹. Still, this small atmospheric GHG emission is outweighed by the substantial carbon sequestration occurring via sediment burial in these zones, which spans approximately 0.05 to 0.16 Pg C yr⁻¹.
One limitation of current assessments is the considerable uncertainty in methane emission measurements from littoral regions, compounded by the omission of nitrous oxide (N₂O) fluxes, another potent greenhouse gas. Such gaps highlight the urgent need for refined methodologies and expanded monitoring to accurately resolve the greenhouse gas balance of vegetated aquatic habitats and fully capture their climate footprints.
In addition to its carbon and greenhouse gas dynamics, including littoral fluxes in continental carbon budgets also refines estimates of terrestrial carbon leaching into standing waters. Accounting for littoral carbon processes suggests a 13% reduction in estimated carbon leaching from terrestrial ecosystems to lakes, shifting from about 0.60 ± 0.35 to 0.51 ± 0.29 Pg C yr⁻¹. This adjustment stems from recognizing that a significant proportion of carbon emitted in lake pelagic zones may originate from carbon fixed and processed in littoral macrophytes.
The implications of these findings extend well beyond carbon accounting. Understanding the littoral’s contribution to inland water carbon dynamics reveals a more nuanced picture of lakes as complex chemical reactors that integrate biological productivity, terrestrial inputs, and biogeochemical transformations. This improved comprehension transforms our conceptual models of lakes from net carbon emitters to vital sinks that influence continental carbon budgets and potentially mitigate climate change.
Moreover, the results underscore the importance of incorporating littoral zones in both observational studies and global carbon models. Historically overlooked vegetated margins may constitute one of the most significant lacustrine carbon reservoirs and flux regulators, capable of buffering atmospheric carbon inputs, sequestering organic carbon in sediments, and modulating greenhouse gas emissions. Future carbon budgets ignoring these contributions risk substantial inaccuracies and underestimations.
This paradigm shift also points towards promising nature-based climate solutions. By managing and restoring littoral habitats with abundant macrophyte growth, there exists potential to enhance carbon burial and reduce net greenhouse gas emissions from inland waters. Such strategies could leverage the natural productivity and carbon sequestration capacity of lake margins to complement terrestrial and marine carbon mitigation efforts.
Ultimately, this research makes a compelling case for expanding our focus beyond pelagic zones when studying freshwater ecosystems’ carbon cycling. The vegetated littoral zones, with their dynamic interplay of organic carbon uptake, burial, and emissions, reshape how scientists, policymakers, and environmental managers view lakes’ global carbon roles. Recognizing lakes as net carbon sinks rather than net sources could have transformative impact on global carbon budgets and climate change models.
Going forward, an interdisciplinary approach integrating remote sensing, field measurements, biogeochemical modeling, and ecosystem management practices is essential to reduce uncertainties surrounding littoral zones. Specifically, refining estimates of macrophyte coverage, sediment burial rates, greenhouse gas emissions—particularly methane and nitrous oxide—and their spatial-temporal variability will enhance predictive capacity and inform targeted conservation efforts.
In conclusion, by uncovering the critical yet previously underestimated role of lake littoral zones, this study represents a milestone in global carbon science. It demonstrates that lake margins, rich in aquatic plants and sediments, play an outsized role in sequestering carbon and mitigating atmospheric emissions, fundamentally revising our understanding of inland waters’ contributions to the continental carbon budget and climate regulation.
Subject of Research: The carbon budget dynamics of lake littoral zones and their impact on continental carbon cycling.
Article Title: Contribution of lake littoral zones to the continental carbon budget.
Article References:
Grasset, C., Mesman, J.P., Tranvik, L.J. et al. Contribution of lake littoral zones to the continental carbon budget. Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01739-8
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