Fertilizer legacies and saltwater incursion constrain recovery of water quality benefits in a restored coastal wetland

Abstract:

Coastal wetland restoration is being used as a climate change mitigation tool and to decrease excess nutrient loading to sensitive estuarine systems. In coastal areas with a history of agricultural practices, the interactions between fertilizer legacies, changes in storm magnitude, and increasing frequency and duration of saltwater incursion events could limit the recovery of water quality benefits from wetland restoration. Here, we use twelve years of weekly surface water samples from a coastal restored wetland, a mature forested wetland, and an agricultural field to examine patterns in dissolved organic carbon, nitrogen, and phosphorus. Our study period included six drought events (Palmer Drought Severity Index less than -2) and five major hurricanes (higher than category 3). We found that drought-induced saltwater incursion increased the release of legacy nitrogen, but not phosphorus from the restored wetland. Both droughts and storms led to increases in the release of nitrogen and phosphorus from all three land-uses, but the magnitude of storm-driven pulses in the restored wetland has declined since the initial reflooding. Our results suggest that elevated nutrient release after reflooding of coastal wetlands is an important, but temporary problem, which is exacerbated by saltwater release of legacy fertilizer. An improved understanding of the vulnerability of fertilizer legacies in coastal areas to increasing exposure to salinity is necessary to inform large-scale coastal wetland restoration efforts.