QUANTIFYING CLIMATE-DRIVEN SALTMARSH PLATFORM DEGRADATION THROUGH THE FORMATION OF POTHOLES IN THE GREAT MARSH, MA

Saltmarshes are threatened by accelerating sea level rise, with marsh loss occurring through multiple mechanisms such as submergence, edge erosion, pond expansion, and channel widening. This study examines marsh platform degradation through the formation of sparsely vegetated “potholes” at four high marsh sites in the Great Marsh, MA. Since 2018, qualitative observations suggest that these potholes are deepening, expanding, and merging over time, causing rapid high marsh loss. The objectives of this study are 1) to quantify the rate and extent of pothole development, and 2) to characterize the biogeochemical processes driving pothole development.

To quantify the rate and extent of pothole development, we take a two-pronged approach. First, we use handheld structure-from-motion (SfM) photogrammetry to quantify three-dimensional pothole growth rates for ~20m² study areas at the four sites. Second, we use aerial imagery to identify areas of marsh containing these potholes, thus monitoring the ratio of potholed:unpotholed marsh through time. To characterize the biogeochemical factors contributing to pothole development, we measure soil shear strength, bulk density, and organic carbon content.

Field observations indicate that pothole formation and expansion are leading to rapid high marsh loss with increased waterlogging and vegetation die-off. Preliminary data suggest that soil shear strength, bulk density, and organic carbon content differ between the insides of the potholes, their immediate surroundings, and distal high marsh control sites. The combination of significantly decreased soil shear strength and increased bulk density inside the potholes is indicative of the lack of vegetation within these areas. Furthermore, the significantly decreased organic carbon content inside the potholes supports this lack of vegetation and suggests the decomposition of below-ground biomass in association with this degradation. This comprehensive study will characterize the response of complex, heterogeneous saltmarsh environments to current and future threats, contributing to marsh vulnerability assessments and actions toward resiliency, thus protecting critical ecosystem services.