Dissolved organic matter (DOM) is a large and complex mixture of compounds with source inputs that differ with location, season, and environmental conditions. We investigated how DOM composition changes both spatially and temporally along a marsh-dominated estuary off the southeastern U.S., and how it influences microbial processing. Surface samples were collected throughout the estuary from September 2015 to January 2018, and bulk, optical, and molecular analyses were conducted on samples before and after dark incubations. Results showed that river discharge was the primary driver changing the DOM composition at the mouth of the Altamaha River. For discharge higher than ~ 150 m3 s-1, dissolved organic carbon (DOC) concentrations and the terrigenous character of the DOM increased approximately linearly with river flow. For low discharge conditions, a clear signature of salt marsh-derived compounds was observed in the river. The Georgia coast was hit by Hurricane Matthew in October 2016 and Hurricane Irma in September 2017, which resulted in a large input of carbon to the estuary. The DOC concentration was ~ 4 times higher than average values and DOM composition was more aromatic with a stronger terrigenous signature compared to the seasonal maximum. Interesting, microbial degradation of DOC was strongly influenced by DOM composition, with increased degradation rates for DOM with a larger terrigenous character. This suggests that extreme events substantially impact DOM quantity and quality in estuarine regions. Since storm events are becoming more common in the area, the results of this study have implications for composition and processing of DOM prior to export to the Atlantic Ocean.
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