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 (d15N and d13C) 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. d15N values varied from 4.8 to 8.1[per thou] in the Chesapeake Bay/MAB area. In the Galveston Bay/Gulf of Mexico region, d15N and d13C values varied from 3.2 to 9.5[per thou] and -26.1 to -20.9[per thou], respectively. Similar distribution patterns of d13C and d15N were observed in both study areas, with values of d15N showing a mid-salinity maximum of about 8 to 10[per thou], whereas d13C continually increased with increasing salinity. The d13C 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 d15N, with d15N 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 d15N values. Vertical profiles of HMW DOM d13C in open-ocean stations generally decrease from surface water to deep waters, whereas the opposite was found for d15N. HMW DOM components with heavier d13C and lighter d15N 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 d15N with previously published D14C values for the same samples support these conclusions about possible ¹⁵N degradation pathways.

KEY WORDS: Dissolved organic matter · Stable isotopes · Colloids · Estuary · Ultrafiltration · Seawater

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Published in MEPS Vol. 252 (2003) on April 30
Print ISSN: 0171-8630; Online ISSN: 1616-1599. Copyright © Inter-Research, Oldendorf/Luhe, 2003