Terrestrially derived dissolved organic matter in the Chesapeake Bay and the Middle Atlantic Blight
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Concentrations of lignin-phenols were analyzed in high molecular weight dissolved organic matter (0.2 m > HMW DOM > 1 kDa) isolated from surface waters of the Chesapeake Bay (C. Bay), and surface and bottom waters of the Middle Atlantic Bight (MAB). The abundance of lignin-phenols in HMW DOM was higher in the C. Bay (0.128 0.06 g L-1) compared to MAB surface waters (0.016 0.004 g L-1) and MAB bottom waters (0.005 0.003 g L-1). On an organic carbon-normalized basis, lignin-phenol abundances in the HMW DOM (i.e., 6), were significantly higher (p < 0.05) in bottom waters compared to sediments at some stations in the MAB. Ratios of syringyl to vanillyl phenols (S/V) in HMW DOM, indicative of angiosperm-derived lignin, ranged from 0.165 to 0.422 in C. Bay, 0.100 to 0.314 in MAB surface waters, and 0.076 to 0.357 in MAB bottom waters. Ratios of vanillic acid to vanillin (Ad/Al)(v) in HMW DOM, indicative of lignin decay, ranged from 0.611 to 1.37 in C. Bay, 0.534 to 2.62 in MAB surface waters, and 0.435 to 1.96 in MAB bottom water. Ratios of S/V and (Ad/Al)(v) shows no significant differences between each environment, providing no evidence of any compositionally distinct input of terrestrial organic matter into each environment. When considering depth profiles of suspended particulate matter in the MAB, and C:N ratios, and bulk radiocarbon ages and stable carbon isotopic values in HMW DOM isolated from these areas, two scenarios present themselves regarding the sources and transport of terrestrially derived HMW DOM in the MAB. Scenario 1 assumes that a low amount of refractory terrestrial organic matter and old DOC are uniformly distributed in the oceans, both in surface and bottom waters, and that primary production in surface waters increases DOC with low lignin and younger DOC which degrades easily. In this case, many of the trends in age and biomarker composition likely reflect general patterns of Atlantic Ocean surface and bottom water circulation in the area of the MAB. Scenario 2 assumes terrestrial organic matter in bottom waters of the MAB may have originated from weathered shelf and slope sediments in nearshore areas via a combination of mechanisms (e.g., diffusion, recent resuspension events, and/or desorption of DOM from riverine POM buried deep in these sites) and entered bottom waters offshore in the MAB by diffusion along isopycnal surfaces. These results complement recent work which proposes that transport of DOM across continental shelves may be a significant source of 'old' organic matter to the deep ocean. Copyright (C) 2000 Elsevier Science Ltd.
