Increasing the protection of coastal vegetated ecosystems has been suggested as one strategy to compensate for increasing carbon dioxide (CO2) in the atmosphere as the capacity of these habitats to sequester and store carbon exceeds that of terrestrial habitats. Seagrasses are a group of foundation species that grow in shallow coastal and estuarine systems and have an exceptional ability to sequester and store large quantities of carbon in biomass and, particularly, in sediments. However, carbon stocks (Corg stocks) and carbon accumulation rates (Corg accumulation) in seagrass meadows are highly variable both spatially and temporally, making it difficult to extrapolate this strategy to areas where information is lacking. In this study, Corg stocks and Corg accumulation were determined at 11 eelgrass meadows across New England, representing a range of eutrophication and exposure conditions. In addition, the environmental factors and structural characteristics of meadows related to variation in Corg stocks were identified. The objectives were accomplished by assessing stable isotopes of δ13C and δ15N as well as %C and %N in plant tissues and sediments, measuring grain size and 210Pb of sediment cores, and through assessing site exposure. Variability in Corg stocks in seagrass meadows is well predicted using commonly measured environmental variables such as grain size distribution. This study allows incorporation of data and insights for the northwest Atlantic, where few studies on carbon sequestration by seagrasses have been conducted.

With Arctic amplification (enhanced polar warming) possibly increasing periods of intense winter freezing and global warming producing more powerful extratropical storms, winter sedimentation on northern salt marshes may likely increase in the future. Here, we show that a large ice-rafting event in northern Massachusetts delivered the equivalent of 15 years of mineral deposition to the marsh surface in a single storm. During an intense extratropical cyclone in January 2018, sediment-laden ice was rafted onto the Great Marsh, Massachusetts, by an~1-m storm surge coinciding with high astronomical tides. The muddy sand content combined with abundant shells indicates that the sediment originated from proximal bays and tidal flats. Sediment layers delivered by individual ice rafts averaged 3.19 cm in thickness, 12 times the combined organic and inorganic yearly vertical accretion rate on high marshes. This previously underappreciated vector for sediment deposition is likely to help marsh resiliency to sea-level rise.Plain Language Summary We found that a single storm in 2018 moved ice onto a New England salt marsh from nearby bays and creeks, carrying the equivalent of 15 years of average mineral sediment deposition across the entire marsh. Salt marshes maintain elevation with rising sea level, in part, by trapping mud and sand. The storm deposit that we observed was a consequence of prolonged ice formation followed by a strong nor'easter coinciding with a very high tide. During the period of extended low temperatures, ice formed in shallow water, and mud and sand froze to its base at low tide. The cold spell combined with very low tides meant that the ice was especially thick and laden with sediment when the storm moved it on to the marsh. Recent studies suggest that climate change may increase the likelihood of extreme cold spells in winter in the northern hemisphere and nor'easters may strengthen. If these conditions persist, the amount of sediment added during winter to salt marshes in northern high latitudes (which is almost half of global marshes) may increase, helping salt marshes to be more resilient to sea-level rise.

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