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

Jacob C Rudolph,Alexandria G Hounshell,Christopher L Osburn,Hans W Paerl,Carli A Arendt

doi:10.3389/fmars.2020.00018

Abstract

Flooding from extreme weather events (EWE), such as hurricanes, exports large amounts of dissolved organic matter (DOM) to both estuaries and coastal waters globally. Hydrologic connectivity of wetlands to adjacent river channels during flood events is potentially a major control on the DOM exported to coastal waters after EWEs. In this study, a geographic information system based flood model was used to: 1) determine the volume of flooded wetlands in a river corridor following Hurricane Matthew in 2016; 2) compute the resulting volume fluxes of DOM to the Neuse River Estuary-Pamlico Sound (NRE-PS), in eastern North Carolina and 3) use the flood model to quantify the wetland contribution to DOM export. The flood model-derived contributions were validated with a Bayesian Monte Carlo mixing model combining measurements of DOM quality: specific UV Absorbance at 254 nm (SUVA254), spectral slope ratio (SR), and stable isotope ratios of dissolved organic carbon (13C-DOC). Results indicated that 1) hydrologic connectivity of the freshwater riparian wetlands caused the wetlands to become the primary source of organic matter (OM) that was exported into the NRE-PS after Matthew and 2) this source lingered in these coastal waters in the months after the storm. Thus, in consideration of the pulse-shunt concept, EWE such as Hurricane Matthew cause pulses of DOM from wetlands, which were the primary source of the OM shunted from the terrestrial environment to the estuary and sound. Wetlands constituted ca. 48% of the annual loading of DOC into the NRE and 16% of DOC loading into the PS. Results were consistent with prior studies in this system, and other coastal ecosystems, that attributed a high reactivity of DOM as the underlying reason for large CO2 releases following EWE. Adapting pulse-shunt concept to estuaries requires the addition of a “processing” step to account for the DOM to CO2 dynamics, thus a new pulse-shunt process is proposed to incorporate coastal waters. Our results suggest that with increasing frequency and intensity of EWE, strengthening of the lateral transfer of DOM from land to ocean will occur and has the potential to greatly impact coastal carbon cycling.

Highlights

The frequency of extreme weather events (EWE), such as tropical cyclones, has increased in the southeastern United States (US) in recent decades (Paerl et al, 2018) and since the mid-1990s the coast of North Carolina has experienced many such events. Paerl et al (2018) classified EWEs occurring over the past 20 years in eastern NC into three categories and compared each category to non-storm conditions to show how different types of storms would affect the Neuse River Estuary (NRE), the main tributary to Pamlico Sound (PS), 2nd largest estuarine complex in the United States
The Pulse-Shunt concept proposed by Raymond et al (2016), was based on headwater fluvial systems but describes the dynamic we have modeled for coastal rivers and estuaries
Our findings demonstrated that EWE such as Hurricane Matthew, the second 500-year storm to occur in NC within the past 20 years (Paerl et al, 2019), transfer substantial amounts of terrestrial C to coastal waters in just a few weeks

Summary

Introduction

The frequency of extreme weather events (EWE), such as tropical cyclones, has increased in the southeastern United States (US) in recent decades (Paerl et al, 2018) and since the mid-1990s the coast of North Carolina has experienced many such events. Paerl et al (2018) classified EWEs occurring over the past 20 years in eastern NC into three categories (dry and windy, wet and calm, and wet and windy) and compared each category to non-storm conditions to show how different types of storms would affect the Neuse River Estuary (NRE), the main tributary to Pamlico Sound (PS), 2nd largest estuarine complex in the United States. $1.6 billion in damage and 28 deaths in North Carolina alone (Stewart, 2017) In their analysis, Paerl et al (2018) found that “wet and windy” storms, such as Matthew, showed statistical differences in carbon (C) and inorganic nitrogen concentrations, partial pressure of carbon dioxide (pCO2), and pH in the NRE, indicating such EWEs have major impacts on estuarine and coastal water biogeochemistry in the weeks to months after the storms. Depending on the residence time of floodwater in receiving waters such as estuaries and large river plumes, DOM degradation can persist for weeks to months following these events (Osburn et al, 2019a) These episodic large storm events can be significant, yet poorly constrained, influences on coastal C budgets, via terrestrial DOM’s transport to coastal waters (Raymond et al, 2016)

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