Coastal wetlands have sediments that contain organic matter preserved against decomposition for timespans that can range up to millennia. This “blue carbon” in wetland sediments has been proposed as a sink for atmospheric carbon dioxide and a potential source of greenhouse gases if coastal habitats are lost. A missing gap in the role of coastal habitats in the global carbon cycle is elucidating the fate of wetland sediment carbon following disturbance events, such as erosion, that can liberate organic matter to an oxygenated environment where decomposition can more readily occur. Here, we track the fate of previously stored salt marsh sediment by measuring the production of carbon dioxide (CO2) and methane (CH4) during an oxygenated incubation. Sediments from two depth horizons (5–10 cm and 20–25 cm) were incubated at two temperatures (20 and 30°C) for 161 days. Q10 of the decomposition process over the entire course of the experiment was 2.0 ± 0.1 and 2.2 ± 0.2 for shallow and deep horizons, respectively. Activation energy for the decomposition reaction (49.7 kJ ⋅ mol–1 and 58.8 kJ ⋅ mol–1 for shallow and deep sediment horizons, respectively) was used to calculate temperature-specific decomposition rates that could be applied to environmental data. Using high-frequency water temperature data, this strategy was applied to coastal states in the conterminous United States (CONUS) where we estimated annual in situ decomposition of eroded salt marsh organic matter as 7–24% loss per year. We estimate 62.90 ± 2.81 Gg C ⋅ yr–1 is emitted from eroded salt marsh sediment decomposition in the CONUS.

Dispersal sets the fundamental scales of ecological and evolutionary dynamics and has important implications for population persistence. Patterns of marine dispersal remain poorly understood, partly because dispersal may vary through time and often homogenizes allele frequencies. However, combining multiple types of natural tags can provide more precise dispersal estimates, and biological collections can help to reconstruct dispersal patterns through time. We used single nucleotide polymorphism genotypes and otolith core microchemistry from archived collections of larval summer flounder (Paralichthys dentatus, n = 411) captured between 1989 and 2012 at five locations along the US East coast to reconstruct dispersal patterns through time. Neither genotypes nor otolith microchemistry alone were sufficient to identify the source of larval fish. However, microchemistry identified clusters of larvae (n = 3-33 larvae per cluster) that originated in the same location, and genetic assignment of clusters could be made with substantially more confidence. We found that most larvae probably originated near a biogeographical break (Cape Hatteras) and that larvae were transported in both directions across this break. Larval sources did not shift north through time, despite the northward shift of adult populations in recent decades. Our novel approach demonstrates that summer flounder dispersal is widespread throughout their range, on both intra-and intergenerational timescales, and may be a particularly important process for synchronizing population dynamics and maintaining genetic diversity during an era of rapid environmental change. Broadly, our results reveal the value of archived collections and of combining multiple natural tags to understand the magnitude and directionality of dispersal in species with extensive gene flow.