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

Abstract

The interconnected estuarine complex of the Altamaha River and adjacent sounds located in Georgia (USA) functions as a hotspot for organic matter transformation as it is transported to the Atlantic Ocean. Here, we investigated how dissolved organic matter (DOM) composition changes both spatially and seasonally along the estuary and how it influences bacterial processing. Surface samples were collected during high tide at fifteen stations throughout the estuary in April, July, October 2017, and January 2018. Bulk, optical, and molecular analyses were conducted on samples before and after dark incubations to assess DOM sources and transformation patterns in the system. The dominant driver of change in DOM composition was found to be the terrigenous‐marine gradient in organic matter sources. Six distinct clusters were identified based on the terrigenous signature of the DOM pool, explaining 45% of the variance in DOM composition in the system. Bacterial consumption of dissolved organic carbon (DOC) was strongly influenced by DOM composition, with increased degradation rates for DOM with a larger terrigenous character. However, changes in optical properties suggested that less aromatic DOM that co‐varied with the terrigenous material was preferentially degraded. The passage of Hurricane Irma in September 2017 resulted in a 27% ± 7% increase in DOC content, likely due to inundation associated with storm surge and increased local precipitation, and DOC biodegradation was 17% ± 8% higher than during summer. These effects lasted for at least one month after the storm, revealing that hurricanes can have a large impact on DOM composition and cycling in coastal systems.

Assessing the Contribution of Seasonality, Tides, and Microbial Processing to Dissolved Organic Matter Composition Variability in a Southeastern U.S. Estuary

“…Residence time at the Altamaha River averages only a few days (Sheldon and Alber, 2002) but can reach several weeks in many parts of the estuary (Wang et al, 2017). For example, DOM composition was significantly enriched in terrigenous molecules 30 days after the passage of Hurricane Irma in 2017, indicating a residence time of at least 1 month for organic material introduced during the passage of the storm (Letourneau et al, 2021) and sufficient time for microbial transformation before export to the coastal ocean.…”

“…Doboy Sound is characterized by marine influence augmented with terrestrial inputs from the Altamaha River (Medeiros et al, 2015a), the dominant source of freshwater to the area (Schaefer and Alber, 2007). This study location was selected due to extensive prior research on the gene expression from ambient microbial communities (e.g., Poretsky et al, 2005Poretsky et al, , 2010Gifford et al, 2011Gifford et al, , 2013Hollibaugh et al, 2011Hollibaugh et al, , 2013 coupled with characterization of the DOM pool (Medeiros et al, 2015a(Medeiros et al, , 2017bLetourneau et al, 2021). Samples from July and October 2014 analyzed here have been previously described in Vorobev et al (2018).…”

“…Tides also have an impact on the DOM composition in estuaries as sources of DOM shift on short-time scales (Tzortziou et al, 2008;Seidel et al, 2014). Substantial transformations of DOM composition due to bacterial consumption have also been observed in Southeastern U.S. coastal waters Hodson, 1989, 1994;Miller and Moran, 1997;Moran et al, 1999;Medeiros et al, 2015aMedeiros et al, , 2017bVorobev et al, 2018;Letourneau et al, 2021). These environmental complexities make quantifying the influence of sources and biogeochemical processes affecting DOM composition in estuaries difficult.…”

Distribution, Sources, and Biogeochemistry of Carbon Pools (DIC, DOC, and POC) in the Mangrove-Fringed Zhangjiang Estuary, China

“…For example, with the largest runoff of 80,000-250,000 m 3 /s in the world, the Amazon Estuary mouth and the offshore displayed a CO 2 sink by the dilution of seawater and high chlorophyll contents (Lefèvre et al, 2017). However, in the Altamaha Estuary, the amount of dissolved organic matter (DOM) decreases from the upstream area to the estuary, which is mainly driven by seasonal variations in the river runoff (Letourneau et al, 2021). In addition, anthropogenic activities have a strong influence on the spatiotemporal distribution of estuarine biogenic matter.…”