Drought and saltwater incursion synergistically reduce dissolved organic carbon export from coastal freshwater wetlands

The hydrologic transport of dissolved organic carbon (DOC) represents both a primary energetic loss from and a critical energetic link between ecosystems. Coastal freshwater wetlands serve as a primary source of DOC to estuaries; historically the magnitude and timing of DOC transfers has been driven by water movement. Extensive agricultural development throughout the coastal plain of the southeastern US has hydrologically connected much of the landscape via canals to facilitate drainage. The resulting large-scale loss of topographic relief and reduced mean elevation is interacting with increasingly frequent and severe droughts to facilitate the landward movement of seawater through the highly connected artificial drainage networks. The resulting changes in hydrologic regime and salinity are each expected to reduce DOC export from coastal freshwater wetlands, yet their individual and combined impacts are not well understood. Here we show that repeated saltwater incursion during late summer droughts substantially decreased DOC concentrations in surface water (from similar to 40 to similar to 18 mg/L) from a mature and a restored forested wetland in the coastal plain of North Carolina, USA. These declines in DOC concentration reduced annual export of DOC to the estuary by 70 % and dampened storm fluxes by 76 %. We used a long-term experiment with intact soil columns to measure the independent and combined effects of drought, salinity, and sulfate loading as potential drivers of the large changes in DOC concentration. We found that soil drying and salinization each reduced DOC similarly (20 % reduction by drought alone, 29 % by salinization) and their combined effect was additive (49 % reduction in salinization + drought treatments). Our results demonstrate that, well in advance of significant sea-level rise, drought and relatively low levels of saltwater incursion (< 6 ppt) are already significantly altering the timing and magnitude of dissolved organic carbon flux between coastal forested wetlands and downstream estuaries.