Rapid Infill of Abandoned Tidal Channels Creates Hotspots for Blue-Carbon Accumulation in Coastal Wetlands

Abstract

Sinuous channel networks dissecting tidal wetlands are highly dynamic and are often abandoned as a result of channel captures and meander cutoffs. However, the effects of channel dynamics on blue carbon fluxes remain unclear. Analyses of abandoned tidal channels in the Venice Lagoon (Italy) demonstrate that they take up organic carbon at significantly faster rates than neighboring marshes. This is because, despite slightly lower sediment carbon density, abandoned tidal channels yield significantly higher rates of sediment vertical accretion owing to topographic accommodation and reduced flow velocities, which facilitate the deposition of particulate matter and debris. We estimate that abandoned tidal channels in Venice capture 17 tons of carbon annually, equivalent to 21 ha of marshes, despite covering only 3.5 ha in total. Hence we argue that abandoned tidal channels serve as hotspots for blue-carbon accumulation and should be considered to improve estimates of carbon fluxes in coastal wetlands.