MONITORING SALT MARSH ELEVATION AT CAPE COD NATIONAL SEASHORE: UNDERSTANDING THE RESPONSE TO SEA LEVEL RISE

Tidal marshes are critical coastal resources at Cape Cod National Seashore and throughout the northeast. Salt marsh integrity and the ability of marshes to build vertically are impacted by human activities such as dikes or other tidal restrictions, which alter natural sediment transport patterns and contribute to the loss of salt marsh habitat. Salt marsh surface elevation must keep pace with sea level rise. When relative sea level rise is greater than marsh surface elevation increase, marshes may convert to intertidal mudflats or subtidal shallow open water habitat. To understand the response of Seashore salt marshes to sea level rise, as well as to the impacts of tidal restriction, Cape Cod National Seashore has been employing the Surface Elevation Table-Marker Horizon (SET-MH) method for over a decade at three systems (Hatches Harbor, Herring River and Nauset Marsh). At Nauset Marsh, a back-barrier system, marsh surface elevation is increasing at a rate that is comparable to the regional sea level rise rate. At Herring River and Hatches Harbor, the monitoring design includes sampling within tide-unrestricted and tide-restricted or tide-restored portions of these systems. Trends in marsh surface elevation in relation to sea-level rise are variable at these sites, related to bioturbation, altered hydroperiod, and sediment supply, among other factors. The concept of elevation capital is offered as a method for coastal managers to apply SET data toward forecasting the long-term status of salt marshes under a regime of rising sea levels.