Assessing the impacts of hydrological alterations due to anthropogenic ditching on salt marsh ecosystem services and sustainability

Abstract:

Coastal resource managers are tasked with making decisions that must simultaneously address the welfare of local communities, sustainability of salt marshes, and valuable ecosystem services they support. In New England, anthropogenic ditches were dug to reduce shallow ponds on the marsh platform where mosquitos breed, although this may have led to unintended consequences such as lower elevations and increased susceptibility to sea-level rise. In order to evaluate the net impacts of ditching to carbon storage services and elevation resilience, we collected 1 m soil cores along 100 m transects from the tidal creekbank to the marsh interior within the ditched (60% of the marsh platform) and adjacent unditched sections of Great Barnstable Marsh (MA, USA). We measured ²¹⁰Pb-based accretion rates, bulk physical and elemental soil properties, and soil organic carbon (SOC) sources and ages. Overall, vegetated platform elevation and accretion rates were similar closest to the creekbank in both sections but diverged 100 m away from the tidal creek, with lower elevations and accretion rates since the 1940s within the ditched marsh interior. A geomorphic model calibrated with field data predicted similar soil physical properties and organic content in the ditched and unditched marsh sections, though dry bulk density decreased and organic matter increased with distance from the tidal creek. Carbon isotopic analysis (¹³C and ¹⁴C) of SOC pools with different thermal reactivities revealed that SOC largely derived from salt marsh peat and suggest that erosion and redeposition of older material contributes to accretion. Our findings suggest that while reduced pond densities may alter salt marsh carbon storage services, ditching impacts overall marsh sustainability to future sea-level rise due to lower marsh platform elevations and accretion rates.