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

Date

2003

Authors

Guo LD
Tanaka N
Schell DM
Santschi PH

Journal Title

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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 (delta(15)N and delta(13)C) were measured on the isolated HMW DOM samples (defined here as the size fraction between 1 and 200 nm), which made up similar to50 to 60 % of the total DOM in the estuarine regions and decreased to similar to35 % of the DOM at the MAB and Gulf of Mexico stations. delta(15)N values varied from 4.8 to 8.1 parts per thousand in the Chesapeake Bay/MAB area. In the Galveston Bay/Gulf of Mexico region, delta(15)N and delta(13)C values varied from 3.2 to 9.5 parts per thousand. and -26.1 to -20.9 parts per thousand, respectively. Similar distribution patterns of delta(13)C and delta(15)N were observed in both study areas, with values of delta(15)N showing a mid-salinity maximum of about 8 to 10parts per thousand, whereas delta(13)C continually increased with increasing salinity. The delta(13)C 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 delta(15)N, with delta(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 delta(15)N values. Vertical profiles of HMW DOM delta(13)C in open-ocean stations generally decrease from surface water to deep waters, whereas the opposite was found for delta(15)N. HMW DOM components with heavier delta(13)C and lighter delta(15)N 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 delta(15)N with previously published Delta(14)C values for the same samples support these conclusions about possible N-15 degradation pathways

Description

51-60

Keywords

dissolved organic matter, stable isotopes, colloids, ESTUARY, ultrafiltration, SEAWATER, CROSS-FLOW ULTRAFILTRATION, STABLE CARBON, NATURAL ABUNDANCE, CHESAPEAKE BAY, water column, OCEAN, PARTICULATE, PARTICLES, RELEASE, PACIFIC

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