Dissolved and particulate organic matter source-age characterization in the upper and lower Chesapeake Bay: A combined isotope and biochemical approach

Abstract

In order to characterize the sources and ages of organic matter contributing to river and estuarine outflow waters, the present study investigated D 14 C and d 13 C signatures of the major operationally defined biochemical classes of ultrafiltered dissolved organic matter (UDOM) in conjunction with lipid biomarker and elemental compositions of UDOM and suspended particulate organic matter (POM) in the Chesapeake Bay. Freshwater (Susquehanna River) UDOM was dominated by a molecularly uncharacterized (MUC) fraction, followed by total carbohydrate (TCHO), total hydrolysable amino acid (THAA) and total lipid (TLE) components. In contrast, UDOM at the bay mouth (salinity ,22-24) was comprised mainly of TCHO, followed by MUC, THAA, and TLE. The D 14 C and d 13 C signatures of both UDOM and its major biochemical classes indicate that Susquehanna DOM is derived in part from old allochthonous terrestrial sources, whereas young marine sources dominate at the bay mouth. In contrast to the other biochemical classes, lipophilic DOM at both sites was very old (,5,000-7,000 years B.P.). In addition, factor analysis of lipid biomarker compounds revealed unique signatures for the UDOM and POM pools that imply disparate source and/or recycling properties as well as potential influences due to physical partitioning. Lipid biomarker compounds showed that although autochthonous riverine/estuarine sources dominated both the UDOM and POM pools, terrigenous lipids were elevated in the Susquehanna during high flow conditions. The presence of lipid biomarkers diagnostic of ''fresh'' algal material in UDOM further suggested its greater reactivity than POM. The observed biochemical and lipid biomarker compositions and isotopic signatures of UDOM and POM are consistent with previous findings suggesting that these two major organic matter pools have dissimilar reactivities and cycling times, and they derive from comparatively unique source-age materials in rivers and estuaries.Global riverine discharge of dissolved (DOM) and particulate organic matter (POM) is on the order of ,0.4 3 10 15 g C yr -1 (Meybeck 1982;Hedges 1992), making it a potentially major source of organic carbon (C) to the world's oceans. However, much of this carbon is transported through, and modified within, riverine and estuarine environments before entering the coastal ocean. Organic matter in these systems is comprised of variable amounts of identifiable biomolecules such as carbohydrates, proteins, and lipids (Mannino and Harvey 2000;Minor et al. 2001), as well as operationally defined (e.g., humic and fulvic substances in the dissolved phase) and other noncharacterizable, long-lived geomacromolecules. Radiocarbon ( 14 C) studies indicate that DOM and POM in rivers and estuaries range from modern to thousands of years in age (Hedges et al. 1986;Masiello and Druffel 2001;Raymond and Bauer 2001). Previous studies have also shown, however, that young subcomponents of estuarine DOM can be reactive and cycle on timescales of days to weeks (Guo and Santschi 1997;Mc...

Trophic transfer and habitat use of oyster Crassostrea virginica reefs in southwest Florida, identified by stable isotope analysis

“…Water samples for suspended POM were collected at each site in clean (acid-washed) polycarbonate bottles and then transported on ice back to the laboratory. There, water samples were vacuum-filtered on pre-com busted (500°C, 4 h) 4.7 cm Whatman GF/F glass microfiber filters, dried at 60 to 65°C in an oven for at least 24 h, and stored in precombusted (500°C, 4 h) 20 ml glass scintillation vials at −20°C until further processing (Riera & Richard 1996, Loh et al 2006, Decottignies et al 2007.…”

“…During processing, samples on filters were dried at 60 to 65°C in an oven for at least 24 h, then underwent acid fumigation in a desiccator containing a small, pre-combusted (500°C, 4 h) jar of 12 N HCl (trace metal grade) for at least 24 h. Filters were dried again at 60 to 65°C in an oven and then stored in precombusted (500°C, 4 h) 20 ml glass scintillation vials in a desiccator (Loh et al 2006).…”

Modeling the seasonal autochthonous sources of dissolved organic carbon and nitrogen in the upper Chesapeake Bay

“…In many mid-Atlantic estuaries, the biochemical composition and isotopic signature of DOM changes across the salinity gradient with biomarkers and carbon isotopes indicating that a significant amount of DOM production and modification occurs within the estuary (Loh et al, 2006;Mannino and Harvey, 2000;Raymond and Bauer, 2001). In the largest of these estuaries, Chesapeake Bay, DOC exhibits a non-conservative distribution at certain times of the year, suggesting that seasonal autochthonous DOM production can be significant (Fisher et al, 1998;Rochelle-Newall and Fisher, 2002).…”

Compositions and constituents of freshwater dissolved organic matter isolated by reverse osmosis

“…Carbohydrates and lipids accounted for 25.0-41.5% and 30.2-46.3% of the identifiable DOM samples, followed by proteins (18.2-19.8%) and lignin (7.17-12.8%). The molecular-level analyses revealed that carbohydrates, lipids, proteins, and lignin accounted for 23-35%, 0.2-0.5%, 13-17%, and <1% of freshwater DOC, and a substantial fraction (48-64%) remained unidentified (Loh et al, 2006). The discrepancies suggest that the use of 13 C NMR spectra to evaluate carbohydrates and proteins could be feasible.…”