Shoreline exposure controls teal carbon accumulation in boreal lakes

Abstract. Aquatic vegetated ecosystems play an important role in global carbon sequestration. While research on coastal marine environments has expanded in recent decades, freshwater vegetated shorelines remain understudied despite their potential for significant carbon burial. This is especially relevant in boreal landscapes with high numbers of small, shallow lakes. In this study, we quantify organic carbon stocks (mass of carbon per area) in boreal lacustrine vegetated shorelines, so-called teal carbon environments. Moreover, we identified the main environmental drivers of carbon storage in these areas. We took 27 sediment cores from three large lakes in Finland with available satellite data of macrophyte coverage. At each site, sediment cores were sampled along a depth transect through macrophyte zones, from the landside towards the waterside. Sedimentary organic carbon (SOC) stocks ranged from 0 to 40.8 kg m⁻², and showed a large spatial variability among lakes, zones and type of vegetation. We identified grain size as the most significant parameter explaining variability in the size of SOC stocks. Sites dominated by silts and with large SOC stocks were found in sheltered embayments, independent of proximity to rivers, density of vegetation or slope of the shoreline, implying a strong control of exposure on SOC accumulation. In more exposed areas, vegetation density might play an additional controlling role in SOC accumulation. Accounting for shoreline exposure is crucial for improving regional carbon budget estimates. This study highlights the central role of teal carbon ecosystems in carbon cycling in the boreal zone, often characterized by very high densities of lakes.