Florida Bay
Estuary: Using Corals to Detect and Reconstruct Change
Topical Area: Paleoecology
Richard Dodge and Daniel Anderegg, Nova Southeastern University Oceanographic Center, Dania, FL; Peter Swart, University of Miami/RSMAS, Miami, FL
INTRODUCTION:
The goal of the research is to assess historical changes in water quality
within the Florida Bay estuary utilizing trace elements in skeletons of
resident stony corals. Nutrient and temperature analog trace element
composition (Ba, Sr, Mg) of dated coral samples of skeletons are being analyzed
and compared to available coral stable carbon and oxygen isotopic composition,
growth rate/density, and fluorescence data, as well as to environmental data.
We thank Sea Grant and SFWMD for project support. The project ends in 1998.
METHODS:
Coral samples: Cores were
obtained of the skeletons of two large (>100 yrs) Florida Bay (Lignumvitae
basin) Solenastrea
bournoni
corals (FB-5 and FB-6), located only a few hundred yards away from each other.
Originally cored in 1986, both FB-5 and FB-6 were cored again in 1994 to obtain
a more recent growth record. A Solenastrea from Manatee Bay was collected
whole in 1996. Another Solenastrea coral was cored from Long Key
in 1996. These coral specimens demonstrate annual density banding which
facilitates time stamped chemical sampling and have the ability to grow for a
century or more.
Preparation: Coral specimen cores were cut using a large diamond bit masonry saw
into slabs of approximately .5 cm thickness. Multiple slabs, sectioned normal
to growth band surfaces, were cut from each coral and X-rayed to reveal annual
density bands. To make sampling templates, clear plastic overlays were placed
on X-radiographic positives, and density band positions were traced and transferred
to the slabs. Skeletal subsamples (intra-annual; approximately 4 to 12 per
year) were then cut using a fine dental saw.
Chemical Analysis: Coral samples were cleaned and prepared in the laboratory for Ba
analysis by ICP/MS (inductively coupled plasma mass spectrometer) following
procedures given in Shen and Boyle (1988) and Lea et al. (1989). Mg and Sr
results were obtained using an ICP/AES (inductively coupled plasma atomic
emission spectrometer, see Mitsuguchi et al. 1996). C and O isotopic composition were measured with a mass
spectrometer by Swart et al. (1996). Density of specimens was
measured by scanning densitometry Dodge and Kohler (1985).
Environmental data: This study utilizes a historical dataset
compiled by Mike Roblee from ENP (Everglades National Park) which includes
non-continuous salinity and temperature measurements from various scientific
sources. The date ranges of the data are from 1956 to 1991 (non-continuous).
Data Analysis: Trace element chronologies were
interpolated to a common time series to allow correlations to be calculated
among chronologies of interest. Correlations were calculated among non-smoothed
and smoothed (by a one year moving average) chronologies. Spectral analyses
were performed on all non-smoothed interpolated chronologies to identify
cyclicity (annual peaks) in the data. A linear trendline was fitted to selected
segments of smoothed barium and carbon isotope time series in an effort to
visualize potential historical patterns.
RESULTS:
Trace metal results include:
Important relationships between one year moving average smoothed data sets were significantly negative correlations between barium (FB-6 and FB-5) and salinity (-.61, and -.51 respectively), negative correlations between carbon isotopes and barium (FB-6 and FB-5) (-.44 and -.57, respectively), and negative correlations between barium (FB-6 and FB-5) and oxygen isotopes (-.55 and -.29, respectively).
Spectral
analyses (FFT’s) of the longest (approximately
100 yrs) barium time series for both S. bournoni specimens FB-5 and FB-6
identified a significant annual cycle. FFTs also showed a strong seasonal cycle
for the shorter (approximately 10 years) Ba time series from the recent
re-cores of corals FB5, FB6, and the Manatee Bay sample. FFTs of the existing
carbon and oxygen isotopic time series for coral FB-6 over the same time period
did not reveal any annual cyclicity.
Magnesium and strontium concentration
levels were found to be consistent with those found by other coral researchers
(4.3 to 5.5 mMol/ Mol Ca for Mg and 9.1 to 9.6 mMol/Mol Ca for Sr). Skeletal Mg
in specimen FB-5 and Sr levels in specimen FB-6 revealed no annual cyclicity
(using spectral analysis) and did not correlate significantly with any of the
available environmental data sets and chemical time series.
Overall trends: generally low d13C values for S. bournoni
specimen FB-6 may reflect the d13C of
surrounding waters which are marked by a depletion in 13C of their
dissolved inorganic carbon content produced by decay of organic matter (Swart et al.
1996). Thus, possible gradual infilling of the Florida Keys reducing the
exchange between the ocean and the Bay might lead to decreased d13C and higher Ba
values if Everglades water or precipitation runoff is barium rich. This
potential inverse relationship may be reflected in skeletons of resident corals
by the established negative correlations (r= -.47 for FB-6 and r= -.57 for
FB-5) between the barium and d13C chronologies
one year moving averages. Additionally, and possibly of greater influence,
hurricanes may serve as powerful flushing agents, “cleaning out” Bay waters of
their organic content, shifting their d13C towards more
positive values, hence reducing their barium content.
Overall
trends in FB-5 and FB-6 skeletal barium concentrations since 1910 were examined
in the same manner as carbon isotopes for FB-6 by Swart et al. (1996). The smoothed
record (one year moving average) was broken into four major intervals that
represent times of varying hurricane frequency. A linear trendline was plotted
for each segment of the Ba time series, as was previously done for the carbon
isotopic data. The barium chronology of FB-5 was compared to the isotopic time
series produced for coral FB-6. Work is in progress to verify chronology
reliability through Pb-210 dating in conjunction with Dr. Charles Holmes of the
USGS.
The relationship between coral specimen FB-6’s carbon isotopic and barium results over the chosen time segments was always inverse, however, it was not always the case for specimen FB-5. From present to past, the trends of these three time series (FB-5 and FB-6 Ba, FB-6 carbon isotopes) can be summarized as follows:
· (1994-1966) Ba increase for both FB-5 and FB-6,
d13C decrease.
Inverse relationship holds. Less exchange with Atlantic Ocean waters.
· (1959-1966): Three hurricanes in seven years,
possibly more flushing by marine waters from the Atlantic Ocean. FB-6
Ba decrease, FB-6 d13C increase.
FB-5 barium does not follow the inverse relationship.
·
(1959-1948): No hurricanes. FB-6 Ba increase and
FB-6 d13C decrease.
Less marine waters, Bay more isolated due to fewer hurricanes. Again, FB-5 does
not follow the inverse relationship.
·
(1948– 1910): Long term FB-6 and FB-5 Ba decrease
and d13C increase.
Inverse relationship holds. Increased marine water input due to frequent
hurricanes.
SUMMARY: Barium concentrations in Florida Bay corals offer a potentially powerful tool for understanding past spatial and historical salinity conditions. Higher resolution (finer sampling) work performed on re-cores of FB5, FB6, and Manatee Bay specimens demonstrate a clear seasonal cycle for Ba, with higher values apparently occurring in late summer/early fall. Mg and Sr work continues with existing and newly acquired corals to further understand water salinity/trace element relationships. These may be complicated in Florida Bay because recent water trace metal analysis revealed a positive correlation between salinity and barium in brackish environments and a negative one in more open ocean waters.