Subsidence, accretion, and elevation trends in estuarine wetlands and relationships to salinity and sediment stratigraphy

Abstract

Coastal wetlands are important ecosystems that provide many benefits and services including storm buffering, nutrient removal from coastal waters, carbon sequestration, and habitat for migratory birds and economically important fishes. Sea-level rise poses a threat to coastal wetlands through increased flooding and saltwater intrusion, which may stress vegetation communities and increase organic matter decomposition. Coastal wetlands must have soil surface elevations exceeding mean high water levels in order to avoid drowning. Soil elevation is an important response variable mediating many linked biological and physical processes in coastal wetlands, and it is a key variable in forecasting future wetland losses to sea-level rise. Chesapeake Bay marshes, subjected to sea-level rise that is double the eustatic (background) rate, may be vulnerable to loss by drowning. To determine whether Chesapeake Bay wetlands are subsiding, and to understand soil elevation dynamics, three related studies were undertaken. A study was conducted in a tributary of Chesapeake Bay, the Nanticoke River, to quantify elevation and accretion dynamics along an estuarine salinity gradient. Oligohaline wetlands were found to be subsiding at higher rates compared to mesohaline and tidal freshwater wetlands. To further understand subsidence trends, a second study was done to examine stratigraphic and geomorphic differences among the sites where subsidence was measured. Few clear relationships between subsidence and stratigraphy emerged, though subsidence followed a curvilinear pattern, peaking at intermediate values for organic matter, depth of site, and salinity, unique nature of mid-estuarine sites. Finally, an in situ field experiment was conducted examining the effects of saltwater intrusion on elevation and vegetation dynamics in a tidal freshwater wetland, given that tidal freshwater wetlands may be more susceptible than saline wetlands to loss because of saltwater intrusion associated with sea-level rise. Four years of salt addition treatments did not have significant effects on either elevation or vegetation, indicating resilience of tidal freshwater wetlands to saltwater intrusion. These studies as a whole shed light on the complicated sets of interacting factors affecting surface elevation and the necessity for localized assessments of coastal wetlands to determine vulnerability to sea-level rise.