Nitrogen and carbon isotopic composition of high-molecular-weight dissolved organic matter in marine environments
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abstract
High-molecular-weight (HMW) dissolved organic matter (DOM) was isolated using cross-flow ultrafiltration from seawater across a salinity gradient in 2 estuarine/coastal marine environments - Chesapeake Bay/Middle Atlantic Bight (MAB) and Galveston Bay/Gulf of Mexico. Nitrogen and carbon isotope ratios (15N and 13C) were measured on the isolated HMW DOM samples (defined here as the size fraction between 1 and 200 nm), which made up 50 to 60% of the total DOM in the estuarine regions and decreased to 35% of the DOM at the MAB and Gulf of Mexico stations. 15N values varied from 4.8 to 8.1 in the Chesapeake Bay/MAB area. In the Galveston Bay/Gulf of Mexico region, 15N and 13C values varied from 3.2 to 9.5 and -26.1 to -20.9, respectively. Similar distribution patterns of 13C and 15N were observed in both study areas, with values of 15N showing a mid-salinity maximum of about 8 to 10, whereas 13C continually increased with increasing salinity. The 13C values clearly demonstrated a shift of HMW organic carbon sources from largely terrestrial inputs in the upper-estuarine areas to marine-dominated organic carbon sources in lower-estuarine and coastal regions. The more complicated distribution patterns of 15N, with 15N values first increasing with salinity in estuarine regions then decreasing towards the seawater endmember, suggest more dynamic N cycling. Thus, in addition to organic matter sources, biogeochemical and isotopic fractionation processes are important factors governing marine HMW DOM 15N values. Vertical profiles of HMW DOM 13C in open-ocean stations generally decrease from surface water to deep waters, whereas the opposite was found for 15N. HMW DOM components with heavier 13C and lighter 15N values seem to be preferentially degraded during their transport from surface to deep waters. However, other processes could also have contributed to this distribution trend. While the carbon isotopic signature can be used as an indicator of DOM sources, nitrogen isotopic composition, on the other hand, appears to be related to both source functions and subsequent recycling in marine environments. Comparisons of 15N with previously published 14C values for the same samples support these conclusions about possible 15N degradation pathways.
