Molluscan Shells as Recorders of Environmental Change in South Florida
G.L. Wingard, R. Stamm, J.B. Murray
U.S. Geological Survey, Reston, VA.
The most critical issue in the Comprehensive Everglades Restoration Plan
(CERP) is to restore more natural patterns of freshwater flow through the
terrestrial ecosystem and into the estuaries and coastal areas. In order to recreate
natural freshwater flow patterns, it is essential to understand the natural patterns
of variation prior to significant human alteration of the system and the natural
sources of water. Seasonality of water delivery is especially critical because the
reproductive cycles of many of the organisms within the environment are tied to
these seasonal patterns of flow. A number of studies have documented the utility
of mollusks as indicators of environmental change (Dodd and Crisp, 1982;
Dettman, et al., 1999; Rodriguez, et al. 2001; Surge, et al., 2001) and the term
sclerochronology has been applied to this type of research (Schöne, et al., 2002).
Current U.S. Geological Survey research is applying this methodology to
determining historical salinity patterns and sources of freshwater influx in Florida
Bay.
Mollusks grow incrementally, secreting calcium carbonate layers that preserve
information about the water at the time of secretion. Analyses of the individual
growth bands of mollusks allow detailed comparisons of seasonal change over
time. Shells analyzed from radiometrically-dated sediment cores taken
throughout the region allow reconstruction of seasonal salinity variations and
freshwater sources in the past, prior to significant human alteration of the system.
These data will provide the resource managers (SFWMD, ACOE and ENP) with
restoration targets and performance measures that will allow them to “get the
water right.”
Significant progress has been made on developing sclerochronology for
application to south Florida ecosystem restoration by means of three discrete
types of experiments: 1) growth studies on selected molluscan organisms; 2)
salinity tolerance experiments and calibration of shell chemistry to water
chemistry (both in the lab and in the field); and 3) testing of instrumentation for
conducting analyses. Determining an accurate growth rate is important because
the timing of seasonal changes in the estuaries is critical to ecosystem health. In
order to use mollusk shells for determining these seasonal changes, it is essential
to understand average growth rate and the timing of growth. For example, if
growth only occurs during certain seasons, then the mollusk will not capture the
full spectrum of seasonal salinity changes. Previously, determination of growth
rates was only assumed or measured. Currently, we have focused our
experiments and analyses on Chione cancellata. Chione was selected as the
initial test organism for several reasons: 1) the thickness of the shell makes
sectioning and analyses of individual layers easier; 2) early salinity tolerance
experiments have demonstrated that this clam will survive in a wide range of
salinities (10-68 ppt); and 3) field evidence suggests it is one of the longer lived
(3-7 years) molluscs present in Florida Bay.
Growth experiments, salinity tolerances, and calibration studies are conducted
both in the field and in the lab. Tanks are set up in the lab and maintained at
specific salinities (15, 25, 35, and 45 ppt). Animals are removed periodically and
digitally photographed to determine growth, water is tested daily, and water
samples are removed weekly for analysis and calibration to the shell chemistry.
Mortality is checked daily, and shells are removed and frozen immediately to be
preserved for biochemical analyses. Additional experiments on salinity extremes
and survivability have been conducted by increasing the salinity to 65 ppt. In the
field, habitats are located close to water monitoring stations so data on water
chemistry can be obtained. Animals are collected, tagged, digitally photographed,
and placed in the habitat. Habitats are checked approximately every 90 days, and
dead specimens are removed, saved for analysis, and replaced with a newly
tagged and photographed specimen; live specimens are photographed and
returned to the habitat.
Four instruments have been tested for analyzing the individual growth layers of
mollusks: 1) SHRIMP – Sensitive High Resolution Ion Microprobe; 2) Cameca
4F SIMS – Secondary Ionization Mass Spectrometer; 3) LA-ICP-MS – Laser
Ablation Inductively Coupled Plasma Mass Spectrometry; and 4) JEOL Electron
Microprobe with multi-wavelength dispersive spectrometers (WDS). The same
specimens of Chione have been analyzed on these instruments allowing for
comparison. A specimen from the mouth of Little Madeira Bay (FB8-1) has been
analyzed on the LA-ICP-MS, the JEOL, and the SHRIMP for Mg/Ca ratios. The
similar results from all three instruments indicate that the techniques are
comparable. A specimen from Bob Allen mudbank in central Florida Bay also
was analyzed for Mg/Ca and for Sr/Ca on the LA-ICP-MS and the JEOL. The
trends on the two machines are nearly identical as noted for specimen FB8-1. In
addition, the Mg/Ca and Sr/Ca trends are parallel. Sr/Ca is primarily dependent
upon salinity. Mg/Ca, however, depends on temperature and salinity, but it has
been argued that in sub-tropical to tropical environments Mg/Ca primarily reflects
salinity. Analyzing both ratios provides reassurance that salinity – the critical
variable for this project – is the variable being measured.
After completing these analyses, it has been determined that there is no single best
instrument. Each instrument is capable of detecting differences in Mg/Ca ratios
in different areas of the Chione shells, and parallel trends in Mg/Ca and Sr/Ca
indicate salinity is the primary controlling variable on Mg/Ca. Each instrument
tested potentially has application to specific aspects of this study. The SHRIMP,
JEOL, and LA-ICP-MS all produce comparable results for Mg/Ca ratios. The
SHRIMP is the most expensive and least accessible, but the most accurate. The
JEOL is the least expensive and the most accessible, but the least accurate. The
Cameca is currently the only choice for isotopic analyses, except for
micromilling, which we have not tested. The Cameca may be suitable for Mg/Ca
analyses as well, but we have not adequately tested this capability. The next step
is to complete testing of the Cameca using Woods Hole Oceanographic Institute
instruments. Woods Hole is preferred because 1) they have machines that can do
both isotopic and elemental analyses and 2) they work with biogenic carbonate
materials and thus have the expertise to contribute to the interpretation of the
resulting data.
Assuming the results on the Woods Hole machine are comparable to the other
tested instruments, the plan is to begin detailed analyses of Chione from the
experimental tanks and habitats early in Spring 2003, in order to calibrate shell
chemistry to water chemistry. These tests will be followed up with analyses of
Chione shells from cores collected throughout Florida Bay to determine historical
seasonal patterns of salinity change. Salinity estimates derived from molluscan
sclerochronology will be compiled with standard paleoecologic faunal and floral
analyses of the cores and with Mg/Ca analyses of ostracode shells (see Cronin, et
al. abstract, this volume). Combined, these biochemical and paleoecologic
analyses of cores will provide an accurate picture of the patterns of salinity
change in Florida Bay over the last 100-200 years.
References cited:
Dettman, D.L., et al., 1999, Geochimica et Cosmochimica Acta, v. 63, p. 10491057.
Dodd, J.R. and Crisp, E.L., 1982, Palaeo., Palaeo. Palaeo., v. 38, p. 45-56.
Rodriguez, C.A., et al., 2001, Conservation Biology, v. 15, p. 249-258.
Schöne, B.R., et al., 2002, The Veliger, v. 45, p. 45-54.
Surge, D., et al., 2001, Palaeo., Palaeo. Palaeo, v. 172, p. 283-296.
Wingard, G. Lynn, U.S. Geological Survey, MS 926A National Center, Reston,
VA 20192. Phone: 703-648-5352, Fax: 703-648-6953, lwingard@usgs.gov,
Question 1.