The presence of extensive seagrass beds is a key biological feature of the Florida Bay ecosystem. Seagrass beds provide habitat and nursery areas for many species of marine animals, including commercial species. Beginning in 1987, seagrass (Thalassia testudinum) began dying over large areas, mostly in western Florida Bay. Less extensive occurrences of Thalassia dieoff were observed in 1975 and 1984 in central Florida Bay (Whipray Basin), and anecdotal observations of seagrass dieoff were reported as early as the 1950’s. The principal causes of the recent seagrass dieoff are unclear, but various hypotheses implicate chemical factors (nutrients, sulfide toxicity), hydrology (reduced freshwater input and resulting hypersalinity), climate (fewer severe storms to flush out the bay), and biological factors (pathogens such as Labyrinthula slime mold).

A major unanswered question is what is the natural variability of seagrass abundance in Florida Bay? Also, what physical or biological factors control seagrass variability? Answers to these questions are critical, as ecosystem managers need to develop target criteria for restoration and to understand the natural variability of biotic assemblages within the ecosystem. The purpose of this study is to examine historical trends in seagrass abundance in dated sediment cores from Florida Bay using lignin phenols as a proxy. We are also determining historical trends in nutrients and organic carbon in the same cores to examine linkages between nutrient load to the bay and seagrass abundance. Collaborators are examining seagrass indicator species (foraminifera, ostracodes, molluscs, and diatoms) in the same cores. This combination of geochemical and paleoecological studies of seagrass abundance provides complimentary information.

Rationale

Lignin phenols are a series of methoxy and dihydroxy phenols derived from the lignin of vascular plants. These compounds are generally absent from algae. Lignin phenols fall into families of compounds (vanillyl, syringyl, p-hydroxy, and cinnamyl phenols), based on the methoxyl and hydroxyl substituents on the aromatic ring. All vascular plants contain vanillyl phenols in their lignin structure. Angiosperms also contain syringyl phenols, while gymnosperms lack syringyl phenols. Lignin from Thalassia contains little or no syringyl phenol content (syringyl/vanillyl ratio 0.02), similar to gymnosperm lignin but distinctly different from angiosperm vegetation such as mangroves (syringyl/vanillyl 0.5). Thus, Thalassia has a lignin composition that readily distinguishes it from the other major lignin source to Florida Bay (mangroves) using the ratio of syringyl/vanillyl phenols (S/V). The composition and amount of lignin in the sediments provides an interpretable record of historical changes in seagrass abundance. Total lignin phenol content normalized to the total 1999 Florida Bay and Adjacent Marine Systems Science Conference organic carbon content of sediments also provides information on historical trends in relative amounts of vascular plant and algal contributions to the sedimentary organic matter.

Methods

Sediment piston cores and samples of living seagrass were collected on mudbanks in June 1996 and 1997 from five sites in eastern and central Florida Bay: Pass Key, Russell Key, Bob Allen Keys, Whipray Basin NE, and Whipray Basin SW. Sediments were wet sieved into fractions (>63 and <63 mm) in order to separate coarse material (seagrass fragments and shells) from the fine-grained carbonate mud, prior to analytical procedures. After sieving, the sediment fractions and living seagrass specimens were lyophilized, weighed, and stored in glass vials.

Lignin phenols were determined using CuO controlled oxidation at 170°C in mini-bombs to release the free phenols from the lignin biopolymer. The free phenols were extracted diethyl ether, reduced to dryness, derivatized, and quantified by gas chromatography using an external standardization method. Individual lignin phenols were identified by gas chromatography/mass spectrometry using computer comparison of mass spectra of peaks in the chromatograms to those of authentic standards. Total carbon, organic carbon (pre-treated to remove carbonates) and total nitrogen were determined on lyophilized/powdered samples using a Leco 932 CHNS analyzer. Total phosphorus was determined by baking preweighed samples at 550°C, extracting the residue with 1M HCl, and determining total phosphorus concentration by colorimetric analysis. Cores were dated using ²¹⁰Pb geochronology for the last 100 years, and ¹⁴C dating for older sediment intervals. The use of tradenames is for descriptive purposes only; no endorsement by the U.S. Geological Survey is implied.

Results

Total lignin phenol contents of sediments at the sampling sites ranged from 0.2 to 6.0 µg/mg organic carbon. Concentrations were highest at Pass Key (the site closest to land); concentrations at the most marine site (Bob Allen Keys) were about 2 to 3 times lower than those at Pass Key. The higher lignin phenol concentrations at Pass Key reflect the influence of mangrove-derived lignin transported from the nearby coastal zone mangrove forests. Downcore variability in total lignin content was relatively high over short time frames (several years) at Pass Key, probably reflecting variability in the flux of mangrove-derived lignin from the coastal zone. At Bob Allen Keys, lignin phenol concentrations were: (1) very high around 1900 (6 µg/mg organic carbon), (2) gradually decreased to concentrations of only about 1.5 µg/100 mg organic carbon in the late 1930’s, and (3) increased to concentrations of 3 to 6 µg/g organic carbon after 1950.

Values of syringyl/vanillyl phenol (S/V) ratios ranged from 0.1 to 0.6, indicating that the sediments at all sites contained a mixture of lignin derived from seagrass and mangroves. Values of S/V closer to 0.1 reflected a greater dominance by seagrass, while values closer to 0.6 indicated dominance by mangroves. Overall, S/V values were highest at Pass Key, reflecting the proximity of this site to coastal sources of mangrove-derived lignin. Downcore trends in S/V at Pass Key showed considerable variability over short time intervals, for example from 0.6 in 1990 (mangrove dominance) to 0.1 in 1993 (seagrass dominance). This may reflect short-term variability in transport of mangrove-derived lignin from the coastal zone and tidal creeks controlled by rainfall. Seagrass appears to have been abundant at this site around 1960, but declined in the late 1960’s and early 1970’s. Seagrass became abundant again at this site in the late 1970’s and has been fairly stable since that time. At Bob Allen Keys long-term trends in seagrass abundance were observed from downcore changes in S/V. From about 1900 to the mid 1940’s, S/V values were fairly constant at about 0.2, indicating a mix of both seagrass and mangrove lignin with a slight dominance by seagrass. The S/V values began to rise to about 0.4 from the mid 1940’s to the early 1970’s, suggesting a decline (moderate dieoff?) in seagrass abundance over this period. After the early 1970’s S/V values gradually decreased to values approaching 0.1, indicating an increase in seagrass abundance over this most recent period.

Results show that historical variability in seagrass abundance has occurred during this century. No correspondence between declines in seagrass abundance and nutrient concentrations has been observed in the cores studied to date. Current work is focused on the analysis of cores from additional sites in Florida Bay.

(This abstract was taken from "Programs and Abstracts - 1999 Florida Bay and Adjacent Marine Systems Science Conference". (PDF, 1 MB))

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