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workshops > water quality > lignin phenols from sediments of florida bay as indicators of seagrass dieoff

Lignin Phenols From Sediments Of Florida Bay As Indicators Of Seagrass History

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

Scientific Question
Is recent seagrass dieoff in Florida Bay caused by anthropogenic change to the ecosystem (decreased freshwater flow from the Everglades and increased nutrient load), or part of a long-term natural cycle in seagrass abundance? photo of scientists piston coring in seagrass beds
Piston Coring in Seagrass Beds, Florida Bay [larger version]

Hypothesis
photo of seagrass beds and mangroves
Seagrass Beds and Mangrove-Covered Keys, Florida Bay [larger version]
High densities of seagrass create conditions that promote dieoff when stressors occur. Stressors could be both natural and anthropogenic. Thus, seagrass dieoff followed by re-colonization likely occurred in the past as a result of natural stressors (biological or physical). Recent seagrass dieoff may result from anthropogenic stressors on a high density seagrass population susceptible to any anthropogenic or natural stressors.

Approach

Examine dated cores (210Pb) for historical changes in seagrass abundance using lignin phenols as a proxy. Also examine historical changes in nutrient (N and P) and organic carbon loads to the sediments in these cores.

map of USGS coring sites in Florida Bay
USGS Sediment Coring Sites in Florida Bay [larger version]

photo of scientists piston coring in seagrass beds
Piston Coring in Seagrass Beds, Florida Bay [larger version]

Summary

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.
structure of part of the lignin biopolymer
Part of the Lignin Biopolymer [larger version]

Lignin Phenols from CuO Oxidation
(click on the table to view a larger version)
table of lignin phenol structures


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
(click on the profiles to view a larger version)

depth profiles of organic carbon in sediments


Florida Bay - Nutrient Concentrations (N and P) in Sediments
(click on the profiles to view a larger version)

depth profiles of N concentrations in sediments

depth profiles of P concentrations 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
Total Lignin Phenols in Sediments
(click on the profiles to view a larger version)

depth profiles of total lignin phenols in sediments at Pass Key


Florida Bay - Bob Allen Keys
Total Lignin Phenols in Sediments
(click on the profiles to view a larger version)

depth profiles of total lignin phenols in sediments at 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 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.

Florida Bay - Pass Key
Lignin Phenol Ratios in Sediments
(click on the profiles to view a larger version)

depth profiles of lignin phenol ratios in sediments at Pass Key


Florida Bay - Bob Allen Keys
Lignin Phenol Ratios in Sediments
(click on the profiles to view a larger version)

depth profiles of lignin phenol ratios in sediments at 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.

Rationale

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
(click on the plot to view a larger version)
plot of lignin phenols in Florida Bay


Florida Bay - Pass Key and Bob Allen Keys
Lignin Phenol Ratios in Vascular Plant Source Organic Matter

Source S/V C/V P/V
Mangrove Leaf 0.609 2.53 1.52
Mangrove Twig 2.55 0.267 0.159
Seagrass 0.019 0.446 3.99
Seagrass 0.023 0.434 6.30




Related information:

SOFIA Project: Geochemistry of Wetland Sediments from South Florida



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Last updated: 04 September, 2013 @ 02:09 PM (KP)