Marsh Equilibrium Theory: A Paleo Perspective

Abstract:

Salt marshes adapt to changes in sea level by means of biogeomorphological feedback. These feedbacks maintain a dynamic equilibrium with sea level, within limits. Reconstructions of sea-level changes derived from salt‑marsh sediment provide a paleo perspective for evaluating these feedbacks and for predicting the ecological effects of future sea-level rise. The Marsh Equilibrium Model (MEM) was modified to accommodate long records of sea level and sediment physical and chemical variables derived from high resolution (decadal and decimeter) reconstruction of sea level spanning the late Holocene using foraminifera preserved in North Carolina salt-marsh sediments. Model outputs from a run of nearly 1100-yr show periods of time when the marsh was predicted to be positioned near the top of the tidal frame (inundation time near zero) and times when the marsh was much deeper in the intertidal zone (inundation time of ca. 0.4). An elevation at mean sea level would correspond to an inundation time of 0.5, which is close to the lower limit of the vegetation and is indicative of a marsh that is just forming or, alternatively, a marsh that is on the verge of collapse. The model also indicates that the standing biomass on the marsh surface and sediment organic matter (SOM) content would have varied in harmony with the inundation time. In times past when the inundation time and the opportunity for mineral sedimentation decreased, the sediment organic matter (SOM) content increased. The low SOM concentration near the marsh surface today is consistent with a marsh that is low in the tidal frame. The SOM depth profile is a function of the relative elevation of the marsh, as well as changes in the input of inorganic sediment to the estuary. To effectively manage and preserve valuable salt-marsh ecosystems it is critical to accurately predict their response to projected sea-level changes.