Award: OCE-1902131

Intellectual Merit: This award supported the investigation of changes in dissolved organic matter (DOM) composition and microbial degradation that occur in a marsh-dominated estuary in response to the passage of hurricanes and tropical storms. These extreme events often result in large fluxes of dissolved organic carbon (DOC) to estuaries. Precipitation associated with tropical storms may be increasing in the southeastern U.S. Thus, understanding how extreme events impact DOM dynamics in these complex coastal ecosystems is key to understand their potential impact on carbon cycling and CO2 air-sea exchange. After decades escaping major storms, the Altamaha River and Estuary off Georgia (U.S.A.) was hit by multiple hurricanes in consecutive years (Matthew in 2016, Irma in 2017 and Dorian in 2019), providing an opportunity to investigate their impacts on DOM dynamics in coastal systems. In this project, we investigated the DOM composition at the estuary over multiple years capturing the passage of hurricanes which resulted in significant storm surges, as well as high precipitation events and seasonal variations in river discharge. Our results showed that river discharge is the primary driver of changes in DOM composition in the system at seasonal scales, with the terrigenous character of the DOM increasing approximately linearly with river flow for discharges higher than ~150 m3 s-1. During low discharge conditions, the signature of marsh-derived compounds becomes more important. The passage of hurricanes resulted in large increases in DOC content and biodegradation rates throughout the estuary. Traditional measures of terrigenous DOM, such as those obtained from optical analyses, indicate that the terrigenous content in the estuary is linearly related to freshwater content, and that the signatures are similar after extreme events and during peak river flow. Analyses at the molecular level revealed significant differences, however, with a large increase of highly aromatic compounds after extreme events exceeding what would be expected by freshwater content alone, likely associated with remobilization of DOM stored in adjacent forested wetlands. Although biodegradation rates increase after the passage of hurricanes, the terrigenous material added during those extreme events does not appear to be particularly labile and may thus be exported to the coastal ocean. Additional studies are needed to characterize the components of the added DOM that may be responsible for the increased biomineralization observed after extreme events, since they likely play a key role controlling carbon exchange with the atmosphere. Broader Impacts: This project supported the training of two female graduate students in organic geochemistry. Findings supported by this study were described in five scientific articles and were presented at various international scientific conferences and invited seminars. Observations have been made publicly available at BCO-DMO (https://www.bco-dmo.org/project/472758, BCO-DMO 472758) and at the Georgia Coastal Ecosystems LTER data portal (https://gce‐lter.marsci.uga.edu; ORG-GCET-1904). Last Modified: 03/29/2022 Submitted by: Patricia M Medeiros Observations are also publicly available at https://www.bco-dmo.org/project/876719. Last Modified: 07/11/2022 Submitted by: Patricia M Medeiros