Abstract
Riverine dissolved organic matter (DOM) is a major source of reduced carbon exported from land to marine environments, and the inflow of riverine organic matter greatly affects biogeochemical cycling in estuaries and bays. Thus, any change in DOM composition, such as changes caused by flood waters as a result of storms and hurricanes, can subsequently affect estuarine environments. To investigate the impact of high flow events on riverine DOM, multidimensional molecular level information of DOM from four south Texas Rivers (Aransas, Lavaca, Mission, and Nueces Rivers) was acquired using high-resolution Ion Mobility Quadrupole Time of Flight Liquid Chromatography Mass Spectrometry (IM Q-TOF LCMS). Base-flow samples were collected in May, July and October of 2016, June of 2017, and March of 2018, while high-flow samples were collected in September of 2017, and June and September of 2018. Based on the molecular formula assigned from IM Q-TOF LCMS, H/C ratio decreased during high-flow event (1.52 to 1.51 in ESI+; 1.19 to 1.07 in ESI-), while O/C ratio increased (0.31 to 0.33 in ESI-). Furthermore, DOM shifted from a protein-like and lipid-like dominated community at base flow condition, to a lignin, tannin and condensed aromatic structure dominated one during high flow event based on MS and tandem MS data. These changes in high-flow riverine DOM indicate an increase of terrestrial signal, which likely is a result of mobilization of terrestrial organic matter from the watersheds by flooding. These mobilized DOM, though refractory at high-flow condition in rivers, could be reactive in coastal regions when condition changes, and thus potentially fuel microbial activities downstream. In addition, about 3.76 – 21.8% of DOM molecules contain structural isomers among different flow conditions. This low number of isomer percentages suggests that as the products of various enzymatic biochemical reactions, the number of isomers in DOM is constrained. Taken together, our study provides insights into structural changes of riverine DOM in response to extreme climate events in subtropical regions and have implications in understanding biogeochemical changes in estuaries under a changing climate.
Highlights
Dissolved organic matter (DOM) represents a major pool of reduced carbon (C) and nitrogen (N) in aquatic systems
During an extreme flooding event caused by Hurricane Matthew in 2016, the Yadkin-Pee Dee River basin alone could export over 474 million kg of dissolved organic carbon (DOC) (Majidzadeh et al, 2017); and a single two-to-three-day storm event could account for ca. 31% of the DOC flux in an Ontario headwater stream in Canada (Hinton et al, 1997)
Our results showed that high-flow events driven by storm or hurricane, as well as the subsequent biotic and abiotic process, changed the chemical composition of riverine dissolved organic matter (DOM)
Summary
Dissolved organic matter (DOM) represents a major pool of reduced carbon (C) and nitrogen (N) in aquatic systems. Acknowledged is that the increase in stream water DOM concentration during high-flow events is generally concurrent with a change in the DOM composition (Gremm and Kaplan, 1998; Hood et al, 2006; Zhang et al, 2007; Spencer et al, 2008). This shift is a result of mobilization of a different terrestrial DOM pool, which is not available under base flow scenarios (Vidon et al, 2008; Fellman et al, 2009). A quick search on Web of Science with the key words of (“dissolved organic matter” OR “DOM”) AND (“composition”) AND (“flood” OR “storm”) returns a total number of 77 publications, suggesting that data are still scarce in evaluating how the chemical composition of riverine DOM changes during high-flow events
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