GCE IV - Key Finding in 2021

What Drives DOM Composition in Estuaries?

Dissolved organic matter is an important component of the carbon pool in estuaries, but it is difficult to characterize because it is not homogenous. DOM is comprised of a heterogenous mixture of aromatic and aliphatic compounds that can originate from a variety of terrestrial and in situ sources. Moreover, DOM is subject to microbial degradation as it transits through the system, and different compounds have differing availability. In recent years, GCE researcher P. Medeiros has conducted a series of studies aimed at teasing apart the sources and transformation patterns in DOM in order to understand what drives variability in estuarine DOM composition. These studies used a combination of approaches including measurements of bulk DOC concentration, optical (CDOM) and molecular (FT-ICR) analyses, as well as bacterial incubations and metatranscriptomics. Letourneau and Medeiros (2019) evaluated the DOM signatures of river water entering the coast via the Altamaha River. They found increased biodegradation of DOC when the discharge was high and the DOM composition was more terrigenous in character. This paper, in JGR Biogeosciences, was featured as an EOS research spotlight. Within the GCE domain, Letourneau et al. (2021) showed that the gradient from terrigenous to marine inputs was the dominant driver of DOM composition along the salinity gradient of the entire estuarine complex (Fig. 1). Although bacterial degradation rates were elevated in DOM with a higher terrigenous character, it was the less aromatic DOM that was preferentially degraded. They were also able to demonstrate large increases in both DOC concentration and biodegradation associated with the passage of Hurricane Irma. Finally, Martineac et al. (2021) showed that the dominant pattern of variability in DOM composition occurs at seasonal scales, which is likely associated with the seasonality of river discharge. Microbial data revealed a similar pattern, with variability in gene expression also occurring primarily at the seasonal scale (Fig. 2). Taken together, these analyses suggest that future changes in river discharge have the potential to significantly impact DOM composition and cycling in coastal systems.