MEPS 252:51-60 (2003)  -  doi:10.3354/meps252051

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 (δ¹⁵N and δ¹³C) 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. δ¹⁵N values varied from 4.8 to 8.1” in the Chesapeake Bay/MAB area. In the Galveston Bay/Gulf of Mexico region, δ¹⁵N and δ¹³C values varied from 3.2 to 9.5” and -26.1 to -20.9”, respectively. Similar distribution patterns of δ¹³C and δ¹⁵N were observed in both study areas, with values of δ¹⁵N showing a mid-salinity maximum of about 8 to 10”, whereas δ¹³C continually increased with increasing salinity. The δ¹³C 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 δ¹⁵N, with δ¹⁵N 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 δ¹⁵N values. Vertical profiles of HMW DOM δ¹³C in open-ocean stations generally decrease from surface water to deep waters, whereas the opposite was found for δ¹⁵N. HMW DOM components with heavier δ¹³C and lighter δ¹⁵N 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 δ¹⁵N with previously published Δ¹⁴C values for the same samples support these conclusions about possible ¹⁵N degradation pathways.

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