Topical Area: Sediment Chemistry

William H. Orem, Charles W. Holmes, Carol Kendall, Harry E. Lerch, Anne L. Bates, Ann Boylan and Margo Corum, U.S. Geological Survey, Reston, VA

Recent events of seagrass dieoff and massive microalgal blooms in Florida Bay have focused attention on the ecological health of this ecosystem. Changes in nutrient input to and the nutrient dynamics of Florida Bay are hypothesized to be linked to the problems of seagrass dieoff and microalgal blooms, but crucial baseline data is still lacking. Restoration of Florida Bay to a natural nutrient regime will require information on the nutrient history of the Bay and the history of seagrass abundance and past microalgal blooms. The purpose of this study was threefold: (1) examine distributions of organic C, total N, and total P in carbonate sediments from sites of continuous and known sedimentation rate to provide a record of historical changes in nutrient input to the bay, (2) determine nutrient conditions in sediments of seagrass-covered and adjacent barren sites to examine the possible origin of the barren areas, and (3) use lignin phenols as a proxy to examine historical changes in seagrass abundance.

Sediment cores were obtained in June 1996 from five sites in eastern and central Florida Bay: Pass Key (25 08.866N/80 34.471W), Russell Key (25 03.841N/80 37.511W), Bob Allen Keys (25 01.378N/80 39.371W), Whipray Basin NE (25 04.309N/80 44.312W), and Whipray Basin SW (25 03.432N/80 45.325W). Piston cores and samples of living seagrass were collected on mudbanks extending out from the mangrove-covered islands at each location. Coring was conducted to bedrock at the Russell and Whipray Basin SW sites, while coring stopped short of bedrock at the other sites. At the Pass, Russell, and Bob Allen sites, cores were collected in both seagrass-covered (Thalassia testudinum) and adjacent barren areas to examine possible differences in nutrient conditions. A layer of surface water between the piston and surface sediment limited movement of the soupy surface layer during transport and handling. Cores were sectioned into 2, 5, or 10 cm intervals. Duplicate cores from each site except Whipray Basin SW were dated using ²¹⁰Pb and ¹³⁷Cs analysis. Calculation of sedimentation rates at each site used the constant rate of supply (CRS) model. All sediment samples contained coarse material (seagrass fragments and shells), as well as fine grained carbonate mud. Sediments were wet sieved into three fractions: >850 m m, 850-63 m m, and <63 m m in order to separate out much of the coarse sediment fraction prior to analytical procedures. After sieving, each fraction of the sediment and the living seagrass specimens were lyophilized, weighed, and stored in clean glass vials.

Chemical analyses conducted for this study included: total carbon (TC), organic carbon (OC), total nitrogen (TN), total phosphorus (TP), delta ¹³C of OC, delta ¹⁵N of TN, and 15 lignin phenols. TC and TN were determined on lyophilized/powdered samples using a Leco 932 CHNS analyzer. OC was determined on the Leco 932 CHNS analyzer after using a fuming acid procedure to remove carbonates from the samples. TP was determined colorimetrically after baking preweighed samples at 550° C to convert organic P to phosphate, and extracting the phosphate with 1M HCl. Stable isotopes (delta ¹³C and delta ¹⁵N) were determined using a Micromass Optima continuous flow mass spectrometer coupled to a Carlo Erba elemental analyzer. The delta ¹³C analyses were performed on samples pretreated using the fuming acid method to remove carbonates, while the delta ¹⁵N analyses were conducted on untreated samples. Lignin phenols were determined using CuO controlled oxidation at 170° C in mini-bombs to release the free phenols from the lignin biopolymer, followed by extraction of the phenols into diethyl ether. The phenols in the ether phase were reduced to dryness, derivatized, and analyzed by gas chromatography, with quantification by Turbochrom chromatography software using an external standardization method. Final confirmation of peak identities in the gas chromatograms was accomplished by gas chromatography/mass spectrometry by comparison of mass spectra of identified peaks to those of authentic standards. The use of tradenames here is for descriptive purposes only; no endorsement by the U.S. Geological Survey is implied.

Concentrations of OC, TN, and TP were typically highest at the Whipray Basin sites (up to 7% OC, 0.75% TN, and 250 m g/g TP) and lowest at Pass Key (up to 2% OC, 0.27% TN, and 125 m g/g TP). Accumulation rates (AR), however, showed the opposite geographic trend. This reflects the much higher sedimentation rates and sediment AR at Pass Key compared to Whipray Basin. AR in the 0-2 cm intervals at the Pass Key and Whipray Basin NE sites were as follows:

AR of OC, TN and TP typically increased with depth at all sites, reflecting higher sediment AR at earlier times.

Results from this study suggest that recent nutrification has occurred at all five sites beginning in the early to mid 1980’s. This was indicated by sudden and sometimes dramatic shifts in the concentrations of OC, TN, TP, and the atomic C/N and C/P ratios of sediments. The timing of the recent nutrification event observed in the sediment data directly precedes the first observation of massive microalgal blooms and seagrass dieoff in Florida Bay in 1987. The observed nutrification was greater for TP than for TN, and was most pronounced at the most northeasterly site sampled (Pass Key). Stable isotope data (delta ¹⁵N) at the Pass Key site from 1980 to 1996 was characterized by large fluctuations in values (swings of more than 2.5 permil), suggesting that rapidly changing sources of N accompanied the recent nutrification event observed at this site.

The longer sediment records at the Russell Key and Whipray Basin sites revealed earlier events of nutrification or enhanced productivity in eastern and central Florida Bay. At Russell Key, an earlier event of increased TP input to the sediments was observed during the 1950’s. The two cores from Whipray Basin had very similar vertical profiles of OC, TN, and TP, suggesting that the processes controlling historical changes in OC, TN, and TP acted across the entire basin. At the Whipray SW site, a very large, broad peak in OC and TN dated at 1730 to 1800 was observed in the downcore profile. The peak values of OC and TN dated at 1741 were nearly equal to surface sediment concentrations. Another, somewhat smaller peak in OC and TN was dated at 1850-1900, and was observed at both Whipray Basin sites. The decadal scale of these events suggest control by longer scale factors (shifts in water circulation patterns or rainfall) rather than isolated events such as hurricanes. Surprisingly, the historical peaks in OC and TN at the Whipray Basin sites were periods of low or moderate TP concentrations. Thus, at least for Whipray Basin, TP appeared to be decoupled from OC and TN inputs to sediments. A downcore peak in TP at the Whipray Basin sites was dated at 1800-1850, a period of relatively low OC and TN contents.

Results of the study of seagrass-covered and adjacent barren areas at the Pass, Russell and Bob Allen sites showed that in the near surface zone, barren areas had distinctly lower OC, TN, and TP contents, and were generally finer-grained. At depth, however, the chemical composition (OC, TN, and TP) and grain size of seagrass-covered and barren areas were generally similar. It was also observed that barren areas contained abundant seagrass fragments at depth. This suggests that the barren areas represent former seagrass-covered areas that have been buried by relict sediment moved to the site by physical processes within the estuary.

Lignin phenol data may provide useful information on the seagrass history of Florida Bay. The dominant seagrass of the bay (Thalassia testudinum) has a lignin composition that readily distinguishes it from the other major lignin sources (mangroves, sawgrass) to Florida Bay. Lignin in 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 ³ 0.5) which dominate the higher plants contributing lignin to the bay. Lignin phenol contents of sediments were examined from the seagrass-covered areas at Pass and Bob Allen Keys. Total lignin contents were significantly higher at the Pass Key compared to Bob Allen. Syringyl/Vanillyl ratios were also generally higher at Pass Key. These results suggest that Pass Key is receiving substantially more lignin input from terrestrial runoff (i.e. angiosperm lignin), consistent with its greater proximity to the coast and Taylor Slough. At both sites, historical changes in total lignin phenol content content showed a trend of decreasing values in recent times. At Bob Allen, this trend was accompanied by a concomitant general decrease in syringyl/vanillyl ratios toward the surface, suggesting that the recent decrease in total lignin was the result of a decrease in angiosperm lignin (probably mangrove). At Pass Key, the changes in lignin phenol content and composition may reflect changes in freshwater inflow to the bay.

Current work is focused on analysis of duplicate cores collected at these sites to verify the results presented here. The lignin phenol work is being expanded to examine the downcore variations in lignin at the Whipray Basin SW site, where large historical changes in TC and TN were observed. A synopsis report on Phase I of this study will be completed during 1999. Phase II will emphasize expansion of the study to additional sites in Florida Bay, and exploration of new proxies for examining historical trends in seagrass abundance.