A Salt Marsh Erosion Model: Interplay Between Biotic and Physical Factors at the Seaward Edge

Abstract:

We present a new model to monitor the cycle of erosion occurring on the seaward edge of salt marshes as sea level rises. In our model, a southern New England salt marsh edge is stable when the bank edge exhibits a normal slope, is fringed by the low-marsh grass Spartina alterniflora, and the ribbed mussel Guekensia demissa is abundant. As erosion proceeds, the seaward bank becomes vertical (Stage 1), then undercut (Stage 2), then slumped (Stage 3), and finally a detached island (Stage 4) to expose a new vertical bank. If erosion progresses relatively slowly, S. alterniflora will dominate and G. demissa will be abundant.

We applied this model to four sites at the Barn Island Salt Marsh in southeastern Connecticut. The central headland of the heavily mosquito-ditched Headquarters Marsh appears to be the most rapidly retreating: from 2006 to 2014, the seaward bank advanced two erosional stages and lost 3 m horizontally. This headland is dominated by low-marsh S. alterniflora, with mid-marsh grasses Distichlis spicata and Spartina patens also present on the seaward edge. By comparison, the nearby seaward edge of Wequetequock Point has only S. alterniflora and bare patches with no mid-marsh species. Wequetequock Point also appears more stable, with about one quarter of the seaward bank on a normal slope and abundant mussels (mean 4,500 m^-2; max 20,000 m^-2).

Repeat surveys since 2006 show mussel vacancy rate is related to the rate of erosion. Open holes appear in normal slope banks due to wave erosion of rocks and other material embedded in the exposed peat. Banks that remain in the same erosion stage for multiple years show increased mussel occupation of these holes. In contrast, rapidly eroding banks at Barn Island Marsh have very few mussels (<100 m^-2) and are typically fringed by grasses other than S. alterniflora. Much of the Barn Island Marsh bank is eroding too rapidly for mussel settlement and growth and normal marsh grass succession. In addition to documenting the recent rates and mechanisms of marsh loss, using a model that combines multiple indicators of marsh edge stability can help us assess the vulnerability of salt marshes to sea level rise and storms.