RESPONSES OF COASTAL WETLANDS TO RISING SEA LEVEL

TLDR

Salt marshes depend on macrophytes that build elevation through organic matter and sediment trapping, and their long‑term stability arises from interactions among sea level, land elevation, primary production, and sediment accretion, though elevation adjustments lag sea‑level changes, causing productivity variability. The study develops a theoretical model to predict marsh stability relative to sea‑level changes based on surface elevation relative to optimal primary‑production levels. The model assumes that when surface elevation exceeds the optimal level for primary production, the marsh remains stable to relative sea‑level changes. The model shows that equilibrium elevation rises with Spartina production and falls with relative sea‑level rise, that elevations below the optimal level lead to instability, and that an optimal RSLR rate exists—up to 1.2 cm yr⁻¹ for high‑sediment estuaries, which is 3.5 times the current long‑term rate.

Abstract

Salt marsh ecosystems are maintained by the dominant macrophytes that regulate the elevation of their habitat within a narrow portion of the intertidal zone by accumulating organic matter and trapping inorganic sediment. The long-term stability of these ecosystems is explained by interactions among sea level, land elevation, primary production, and sediment accretion that regulate the elevation of the sediment surface toward an equilibrium with mean sea level. We show here in a salt marsh that this equilibrium is adjusted upward by increased production of the salt marsh macrophyte Spartina alterniflora and downward by an increasing rate of relative sea-level rise (RSLR). Adjustments in marsh surface elevation are slow in comparison to interannual anomalies and long-period cycles of sea level, and this lag in sediment elevation results in significant variation in annual primary productivity. We describe a theoretical model that predicts that the system will be stable against changes in relative mean sea level when surface elevation is greater than what is optimal for primary production. When surface elevation is less than optimal, the system will be unstable. The model predicts that there is an optimal rate of RSLR at which the equilibrium elevation and depth of tidal flooding will be optimal for plant growth. However, the optimal rate of RSLR also represents an upper limit because at higher rates of RSLR the plant community cannot sustain an elevation that is within its range of tolerance. For estuaries with high sediment loading, such as those on the southeast coast of the United States, the limiting rate of RSLR was predicted to be at most 1.2 cm/yr, which is 3.5 times greater than the current, long-term rate of RSLR.