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lignin phenols from sediments of
florida bay as indicators of seagrass dieoff
Lignin Phenols From Sediments Of Florida Bay As Indicators Of Seagrass Historyby William H. Orem, Harry E. Lerch, Anne L. Bates, and Margo Corum, U.S. Geological Survey, 956 National Center, Reston, VA 20192 and Charles W. Holmes and Marci Marot, U.S. Geological Survey, 600 4th St. S., St. Petersburg, FL 33701
The purpose of this study is to examine historical trends in seagrass abundance (Thalassia testudinum) in dated sediment cores from Florida Bay using lignin phenols as a proxy. In order to understand the underlying causes of recent seagrass dieoffs in the bay, it is essential to establish the natural variability of seagrass within the estuary. Lignin phenols are a useful proxy for seagrass abundance, and analysis of these compounds in the fine sediment fraction from dated cores provides an historical record of the seagrass beds within a region. Historical trends in organic carbon, total nitrogen, and total phosphorus were also examined to correlate changes in seagrass abundance to changes in nutrient loading to the bay.
Lignin Phenols from CuO Oxidation Results for nutrients (below) indicate that recent increases in nutrient concentrations have occurred at several of the sites studied. Historical peaks in organic carbon and total nitrogen are also observed, especially at the Whipray Basin site. Florida Bay - Organic Carbon in Sediments Florida Bay - Nutrient Concentrations (N and P) in Sediments Total lignin phenol contents of sediments (below) at the Pass Key and Bob Allen Key sites ranged from 0.2 to 6.0 ug/mg organic carbon. Concentrations are higher at the Pass Key sites by a factor of 2-3 due to its greater proximity to terrestrial sources of lignin. Total lignin phenol contents at Pass Key are highly variable downcore, reflecting changes in terrestrial runoff over time. Florida Bay - Pass Key Florida Bay - Bob Allen Keys Lignin phenol ratios (S/V, C/V, P/V) provide information on sources of lignin to the sediments. The syringyl/vanillyl (S/V) values (below) ranged from 0.1 to 0.6. Values closer to 0.1 are indicative of seagrass dominance while values closer to 0.6 indicate greater mangrove dominance. Overall, S/V values were higher at Pass Key, reflecting its proximity to the mangrove zone bordering the bay. At Pass Key, seagrass appears to have been abundant around 1960, but declined in the late 60s and early 70s. At Bob Allen Keys, seagrass was dominant from about 1900 to the mid 1940s. A decline in seagrass occurred from the min 1940s to the early 1970s but seagrass rebounded again to dominance after the early 1970s. The p-hydroxy/Vanillyl (P/V) ratio provides information on algal versus vascular plant contributions to the sedimentary organic matter. Higher P/V values at Bob Allen Keys suggest greater algal contributions here. At both sites, increased algal contribution in more recent times is indicated. Florida Bay - Pass Key Florida Bay - Bob Allen Keys Results show that historical variability in seagrass abundance has occurred during this century. No correspondence between declines in seagrass abundance and nutrient loading has been observed in these cores.
Lignin from seagrass (Thalassia testudinum) contains very low levels of syringyl phenols (S/V @ 0.02). Lignin from Thalassia, therefore, has a lignin composition that is readily distinguishable from that of mangroves (S/V > 0.5), the other major lignin source in Florida Bay. Thus, the composition and amount of lignin phenols in sediments provides an interpretable record of historical changes in seagrass abundance. The total lignin phenol content of sediments normalized to total organic carbon also provides information on historical trends in relative amounts of vascular plant and algal contribution to the sedimentary organic matter. Florida Bay - Lignin Phenols Florida Bay - Pass Key and Bob Allen Keys
Related information: SOFIA Project: Geochemistry of Wetland Sediments from South Florida |
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 22 December, 2004 @ 02:14 PM (KP)