Dissolved Organic Matter Composition in a Marsh‐Dominated Estuary: Response to Seasonal Forcing and to the Passage of a Hurricane

Abstract

Dissolved organic matter (DOM) is a large and complex mixture of compounds with source inputs that differ with location, season, and environmental conditions. Here, we investigated drivers of DOM composition changes in a marsh‐dominated estuary off the southeastern United States. Monthly water samples were collected at a riverine and estuarine site from September 2015 to September 2016, 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, 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. At the head of Sapelo Sound, changes in DOM composition were primarily driven by river discharge and possibly by summer algae blooms. Microbial consumption of DOC was larger during periods of high discharge at both sites, potentially due to the higher mobilization and influx of fresh material to the system. The Georgia coast was hit by Hurricane Matthew in October 2016, which resulted in a large input of carbon to the estuary. The DOC concentration was ~2 times higher and DOM composition was more aromatic with a stronger terrigenous signature compared to the seasonal maximum observed earlier in the year during peak river discharge conditions. This suggests that extreme events notably impact DOM quantity and quality in estuarine regions.

Transcriptional activity differentiates families of Marine Group IIEuryarchaeotain the coastal ocean

“…6A), matching clone library data from the SAB [24] and data from the Central California Current and the Black Sea [9,66]. The DOM in productive, turbid SAB coastal waters supports highly active heterotrophic microbial populations [68,69]. Our data suggest large populations of Ca .…”

“…Regression parameters are shown on the plot. -17 -SAB coastal waters supports highly active heterotrophic microbial populations[70,71]. Our data suggest large populations of Ca.…”

Spatio‐temporal changes in dissolved organic matter composition along the salinity gradient of a marsh‐influenced estuarine complex

“…Variation in Altamaha River discharge was shown to be the main driver controlling changes in the quantity and quality of the dissolved organic carbon (DOC) delivered to the estuary (Letourneau and Medeiros 2019). Another important source of organic carbon to estuaries is intertidal salt marshes (Herrmann et al 2015), which are dominated by the salt marsh cordgrass Spartina alterniflora in this region.…”

“…For example, a meta‐analysis of watersheds in the northeastern U.S. showed that up to 86% of annual DOC loads can be exported during large precipitation events (Raymond and Saiers 2010), increasing the contribution of aromatic DOM compared to baseflow conditions (Yang et al 2013; Osburn et al 2019 a ). Severe storm events can mobilize large amounts of terrestrially derived organic matter and quickly transport this material downstream (Raymond et al 2016; Letourneau and Medeiros 2019) and are often accompanied by storm surge and flooding of coastal systems (Cangialosi et al 2018), which can result in the release of organic matter stored in coastal watersheds into adjacent rivers and estuaries (Osburn et al 2019 b ).…”

“…Characterizing changes in the molecular composition of DOM from different parts of the estuarine complex is important to better understand what changes occur in this organic matter pool and how they vary across spatial and temporal gradients. Here, we built on previous studies that investigated DOM changes in estuarine systems (e.g., Sleighter and Hatcher 2008; Hounshell et al 2019; Letourneau and Medeiros 2019) and in other terrestrial/marine gradients (e.g., Ward et al 2013; Medeiros et al 2015 a ) by characterizing DOM composition and variability across the estuary during multiple seasons. We identified and tracked the input of terrigenous DOM from the Altamaha River as it was transported through the estuarine complex and characterized the effects of Hurricane Irma on local changes in DOM quantity, quality, and microbial lability.…”