Extreme flooding mobilized dissolved organic matter from coastal forested wetlands

Abstract

Two intense rainfalls [Hurricane Joaquin (2015) and Hurricane Matthew (2016)], one year apart, provided a unique opportunity to examine changes in dissolved organic matter (DOM) dynamics in coastal blackwater rivers under extreme flooding conditions in the southeastern United States. Two sites along Waccamaw River (a coastal blackwater river) and the outflow of 18 sub-basins of Yadkin-Pee Dee Basin were sampled during and after the flooding events. The peaks of dissolved organic carbon (DOC) and nitrogen (DON) concentrations were observed 18 and 23 days after peak discharge in 2015 and 2016, respectively. Moreover, DOM aromaticity and abundance of humic substances significantly increased during the same period. Separation of discharge hydrograph into surface runoff and subsurface flow suggested that temporal changes were mainly due to contributions from subsurface flow flushing organic matter from wetlands and organic-rich riparian zones. The spatial analysis highlighted the key role of the forested wetlands as the only land use that significantly correlated with both DOM quantity (DOC and DON load) and DOM composition (i.e., aromaticity). The Yadkin-Pee Dee River basin alone exported more than 474 million kg DOC into the ocean during high-flow conditions from the 2016 event, indicating that such extreme short-term events mobilized enormous amounts of organic carbon and nitrogen to the ocean. Considering the predicted increase in frequency and intensity of extreme rainfall events in the eastern U.S., the results of this study can shed light on changes in DOM dynamics that may occur under such conditions in the near future

Lingering Carbon Cycle Effects of Hurricane Matthew in North Carolina's Coastal Waters

“…Thus, we posit that as the river became hydrologically connected to adjacent wetlands, mobilization of wetland‐derived DOC into the main channel clearly augmented, if not dominated, any upland sources of DOC. In the Yadkin‐Pee Dee coastal watershed in South Carolina, flow‐separation analysis indicated the drainage of wetlands via subsurface pathways after storm runoff from Matthew had declined and thus extended the shunt of terrigenous, wetland‐derived DOC to coastal waters, similar to what our observations imply (Majidzadeh et al, ). These results, coupled with the long residence time of enclosed coastal waters such as sounds, explain the surprisingly long retention of terrigenous material flushed from the watershed into the NRE‐PS, which persisted in this coastal system well into November and early December roughly 2 months following passage of Hurricane Matthew over the region (Table ).…”

“…Second, the influence of wetlands on terrigenous DOC in the estuary and sound following Matthew was exemplified by the range of wetland δ 13 C and SUVA 254 values (−29.4 ± 0.7‰ and 4.9 ± 0.5 L·mg C −1 ·m −1 , respectively) in contrast with values for the Neuse River above head of tides at Ft. Barnwell during the same period (−25.9 ± 0.6‰ and 3.6 ± 0.5 L·mg C −1 ·m −1 ). The lower δ 13 C values and higher SUVA 254 values of the wetlands bracketed the majority of NRE-PS values, which strongly suggested the importance of wetland DOC to these coastal waters, similar to many upland watersheds with forested wetlands (Lambert et al, 2015;Majidzadeh et al, 2017).…”

Use of Geospatial, Hydrologic, and Geochemical Modeling to Determine the Influence of Wetland-Derived Organic Matter in Coastal Waters in Response to Extreme Weather Events

“…Our results indicate that due to the hydrologic connectivity of the wetlands upstream of the estuary, around 70-77% of the 32% annual load increase in the NRE can be attributed to the wetlands. Similarly, Majidzadeh et al (2017) indicated that in the Yadkin-Pee Dee Hurricane Matthew exported a large amount of wetland DOC.…”

The Role of the Upper Tidal Estuary in Wetland Blue Carbon Storage and Flux

“…These results should be interpreted with caution because C E/I was not measured directly, but they are within the range of component rates measured directly from along the Waccamaw River during two separate and overlapping study years. Majidzadeh et al (2017) found that dissolved organic C (DOC) fluxes, which would represent a large portion of C E/I , ranged from 125 g C · m À2 · year À1 in a dry year to 421 g C · m À2 · year À1 in a wet year, or 28%-95% of our average C E/I estimation in a given year. Studies of hydrologic total DOC fluxes through lateral export from coastal estuaries of the eastern United States have documented an average of 46 g C · m À2 · year À1 from the Gulf of Maine estuaries to 168 g C · m À2 · year À1 from the South Atlantic 10.1029/2018GB005897…”