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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 envi- ronments —Chesapeake Bay/Middle Atlantic Bight (MAB) and Galveston Bay/Gulf of Mexico. Nitrogen and carbon isotope ratios (δ 15 N and δ 13 C) were measured on the isolated HMW DOM sam- ples (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. δ 15 N values varied from 4.8 to 8.1 ‰ in the Chesapeake Bay/MAB area. In the Galveston Bay/Gulf of Mexico region, δ 15 N and δ 13 C values varied from 3.2 to 9.5‰ and -26.1 to -20.9‰, respectively. Similar distribution patterns of δ 13 C and δ 15 N were observed in both study areas, with values of δ 15 N showing a mid-salinity maximum of about 8 to 10‰, whereas δ 13 C continually increased with increasing salinity. The δ 13 C values clearly demonstrated a shift of HMW organic car- bon 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 δ 15 N, with δ 15 N 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 δ 15 N values. Vertical profiles of HMW DOM δ 13 C in open-ocean stations generally decrease from surface water to deep waters, whereas the opposite was found for δ 15 N. HMW DOM compo- nents with heavier δ 13 C and lighter δ 15 N values seem to be preferentially degraded during their trans- port from surface to deep waters. However, other processes could also have contributed to this distri- bution 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 δ 15 N with previously published ∆ 14 C values for the same samples support these conclusions about possible 15 N degradation pathways.

Nitrogen and carbon isotopic composition of high-molecular-weight dissolved organic matter in marine environments