Contrasting sediment carbon storage and turnover in vegetated littoral and open water habitats of boreal lakes

Abstract. In recent years, the research interest of aquatic systems as natural carbon sinks has grown rapidly. Diffuse organic carbon buried in lake sediments represents an important long-term carbon sink, with potential to mitigate climate change by removing carbon from active biogeochemical cycling on timescales of millennia. This sink may be particularly significant in boreal regions such as Finland, where lakes cover approximately 10 % of the land surface. However, major knowledge gaps limit our ability to assess the true sink potential of boreal lakes. Specifically, the spatial heterogeneity and balance between burial and remineralization of organic carbon, and the role of the vegetated littoral zone (“teal carbon habitat”) in carbon storage remain poorly constrained.

To address these gaps, we studied three boreal lakes along a latitudinal gradient across Finland, by sampling sediment, porewater, and surface water from three sites in the teal carbon habitat and three to four in the open water (limnetic) habitat. Our results reveal two contrasting carbon‑cycling regimes: teal carbon habitats might accumulate substantial organic carbon inventories in the top 10 cm (SOC₁₀) of the sediment, where turnover is negatively associated with inventory due to greater retention of fine-grained sediments, resulting in lower diffusive turnover efficiency. In open water habitats, total diffusive carbon flux is closely linked to organic matter quality, referring to its composition and reactivity, while SOC₁₀ remains consistent among sites. Turnover efficiency across all sites is low (< 5.5 %), implying only a small fraction of the near‑surface SOC pool is processed annually. These findings highlight the importance of protecting vegetated near‑shore belts from physical disturbance to safeguard SOC storage and managing catchment inputs to moderate open water carbon fluxes. Notably, not all teal carbon habitats are equal; sandy sites possess small SOC stocks below the top 10 cm and exhibit higher turnover rates, suggesting that interventions in these areas may carry lower risks.