SPATIAL VARIABILITY IN COASTAL SALTMARSH RESILIENCE TO SEA-LEVEL RISE NEAR WESTPORT AND SLOCUM RIVERS, MASSACHUSETTS

Wetlands provide carbon storage, protection against flooding and wave action, and support diverse ecological communities. They are also among the ecosystems most threatened by rising sea level. However, the roles of various factors that influence saltmarsh response to sea-level rise remain largely under-constrained. This study aims to add to the database of marsh resilience by quantifying the historical vertical accretion and organic-matter content over the last 70-90 years of saltmarshes at the mouths of two modest-size rivers near Westport, Massachusetts.

We collected 12 large (25 cm) diameter sediment cores 60–80 cm long at the lower reaches of the Westport and Slocum rivers and within a small lagoonal-marsh system at Allens Pond. Down-core analysis of ²¹⁰Pb and ¹³⁷Cs concentrations, soil bulk density, and loss-on-ignition (LOI) were used to calculate multi-decadal accretion rates and changes in the organic-matter composition of the marshes through time. Values of accretion excess (AE_C)—a dimensionless measure of marsh vertical accretion rate normalized by the sea-level rise rate contemporaneous with the accretion period—were calculated to access the relative “health” of the marsh; that is, whether the marsh was accreting at a rate that kept pace with sea-level rise or was losing vertical resilience over the measured timeframe.

Our resulting accretion rates range from 1.14 to 3.78 mm yr^-1 (AE_C values: 0.38–1.29) with an average of 2.33 mm yr^-1 (AE_C: 0.81), suggesting these marshes are struggling to keep pace with sea-level rise. There is no correlation between accretion rates and marsh elevation, but marshes closer to river mouths generally grew faster than those in more protected locations. These marshes are highly organic-rich, as revealed by high LOI (median down-core average: 38.8 %) with low soil bulk density (median down-core average: 0.27 g cm^-3) values, suggesting that marsh accretion in this region is dependent on below ground biomass production rather than deposition of allochthonous sediment input for survival. These results contribute evidence that New England saltmarshes are receiving low terrestrial sediment inputs, and thus, are more prone to drowning.