Impact of land-use change on salt marsh accretion

Abstract

Salt marsh vertical accretion that keeps pace with relative sea-level rise (RSLR) promotes habitat resilience and a continuously growing carbon stock. Vertical accretion, however, hinges on various salt marsh biogeomorphic feedback loops that respond to changing sediment supply, storms events, accelerating RSLR, elevation, and vegetation density. During the latter half of the 20th century, a large proportion of watersheds on the lower coastal plain of North America experienced land cover change. Many salt marshes show slow rates of vertical accretion over that period suggesting degradation and destabilization attributed to reduced upstream sediment loads and changing proportions of mineral sediment to organic matter. Here, we investigated a potential dichotomy observed at many sites in coastal North America between previously published accretion rates of the subtidal bay bottoms and adjacent salt marshes that were exceeding and lagging behind RSLR since 1950 CE, respectively. In 12 small coastal watersheds that formed within tributary and coastal prism incised valleys, we coupled 210Pb-derived accumulation rates of fringing salt marshes and previously published data from adjacent tidal creek bay bottoms with geospatial analysis of watershed land cover change since 1959, inundation duration, and vegetation density. Accumulation rates were higher at marsh sites within tributary incised valleys with lower relief watersheds, smaller tidal ranges, shorter inundation times, higher vegetation densities, and where land cover changes were dominated by increased cleared forest area. Additionally, mass accumulation rates (MAR) accelerated at 8 of the 12 marsh sites after a Major Land Cover Change (MLCC). Despite this acceleration in MAR, only 2 marsh sites recorded average sediment accumulation rates (SAR) greater than RSLR after a MLCC, which suggests that most of the marsh sites are drowning or being converted to subtidal habitat. This may be misleading, however, because modern marsh platform elevations at all but one marsh site were above local mean sea level, so the marshes could be accommodation limited. This work supports the disconnect found in previous research suggesting that estuaries are accumulating sediment at sustainable rates while their fringing salt marshes are not, which illuminates the complexities associated with sedimentation regimes in small local watersheds and their fringing salt marshes.