Tidal wetlands adapt to accelerating sea level rise rates by building soil elevation via a simple ecogeomorphic feedback in which vegetation traps mineral sediment and adds organic matter. Rapid organic matter accumulation is a globally significant sink for carbon (C), and this ‘blue carbon’ is a primary contributor to vertical accretion and the persistence of tidal wetlands. While the fate of wetlands is typically evaluated in the context of sea level rise alone, anthropogenic activities that alter the delivery of sediment, nutrients, and freshwater to the coast are expected to alter the strength of these ecogeomorphic feedbacks via complex changes in plant physiology and microbial activity. Here, we explore how changes in freshwater and nutrient delivery regulate marsh accretion and C accumulation using point-based numerical models of tidal wetland evolution that explicitly incorporate physiological and biogeochemical responses to these anthropogenic disturbances. We then examine how spatial variation alters the strength and direction of these feedbacks at the landscape scale.
We show that reduced freshwater delivery reduces accretion and C accumulation, but that both enhanced freshwater and nutrient delivery accelerate elevation gain and C accumulation in the majority of tidal wetlands because they increase aboveground productivity, enhancing mineral sediment deposition, and increasing C accumulation by increasing total belowground productivity. However, nutrients are detrimental to C accumulation for wetlands low in the intertidal zone where aboveground productivity is not nutrient limited and belowground biomass declines in response to nutrients. Applied across the landscape, we show that these biological responses are intimately linked with hydrology, geomorphology, and suspended sediment availability in complex ways not apparent in point-based models. These spatial feedbacks tend to enhance wetland persistence in the face of sea level rise and therefore enhance landscape scale C pools. For instance, in the Altamaha River estuary (Georgia), low freshwater discharge allows sediment-rich brackish water penetrate further upstream, enhancing mineral accretion and counterbalancing short-term reductions organic accretion resulting from decreased productivity and accelerated mineralization. In the long term, increased estuarine salinity favors community shifts toward species with higher rates of C burial and accretion. While these disturbances may have positive consequences for the accumulation and persistence of blue carbon in many coastal settings, our analysis shows that they are deleterious when they occur in wetlands low in the tidal frame or in concert with reduced sediment supply.