Carbon source and weather in coastal North Carolina

Abstract

The increasing frequency of extreme weather events, including tropical cyclones, is altering the delivery and quality of organic matter (OM) exported from landscapes to estuaries. However, the mechanisms regulating these shifts, especially during multi‐year climatic oscillations, remain poorly understood. We used a new fluorescence–based mixing model to estimate dissolved OM (DOM) sources to the Neuse River Estuary, during a dry period (2010–2012) and a wet period (2015–2019), both punctuated by extreme precipitation events. Wet periods were characterized by positive anomalies in dissolved organic carbon (DOC) and a 350 —a proxy for colored DOM (CDOM)—relative to dry period median values. During the wet period, extreme rainfall events collapsed the longitudinal gradient of wetland‐derived DOC between the tidal freshwater reach and the estuary mouth, resulting in spatial homogenization of source contributions. These events coincided with DOC and a 350 values that significantly exceeded dry‐year medians, indicating hydrologically driven expansion of land–ocean connectivity and downstream replacement of background river–ocean mixtures of DOM with wetland‐derived carbon. We propose the Carbon Source and Weather (C‐SAW) paradigm, in which wetter conditions poise coastal watersheds to release pulses of DOM stored in the coastal landscape following extreme rainfall. These events overwhelm long‐residence time estuaries with distinctive DOM from wetland soils, sediments, and water, and potentially lead to dramatic changes to the estuarine carbon cycle. C‐SAW integrates elements of the Flood Pulse and Pulse‐Shunt Concepts, providing a unifying framework linking watershed carbon storage, hydrologic connectivity, and estuarine carbon cycling.

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