by 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 - A biopolymer produced only by vascular plants and used as a structural component in wood and other plant tissues. Lignin constitutes up to 30% of the organic matter in wood, and lesser amounts in other plant tissues. Lignin is typically present combined with cellulose as the lignocellulose biopolymer. The lignin structure is a three dimensional network constructed from phenyl propane derivatives. Lignin is relatively resistant to biodegradation in sediments, especially under anoxic conditions.

Part of the Lignin Biopolymer [larger version]

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.

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.

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 60’s and early 70’s. At Bob Allen Keys, seagrass was dominant from about 1900 to the mid 1940’s. A decline in seagrass occurred from the min 1940’s to the early 1970’s but seagrass rebounded again to dominance after the early 1970’s.

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.

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.

Related information:

SOFIA Project: Geochemistry of Wetland Sediments from South Florida