Salinity History of Florida Bay: An Evaluation of Methods, Trends, and Causes
T. M. Cronin, L. Wingard, J. H. Murray
USGS Reston, VA
G. Dwyer Duke University, Durham NC
M. Robblee, USGS Miami FL
Concern exists over hypersalinity – salinity exceeding marine salinity of ~35 ppt
– in Florida Bay and its negative impact on the ecosystem. High salinity is one of
several factors that may cause changes in seagrass distribution, abundance, and
density (Fourqurean and Robblee 1999). To determine the causes of historical
changes in seagrass communities, as well as other ecosystem disturbances, and to
improve model ability to simulate ecosystem response to future restoration, the
patterns and causes of salinity variability in Florida Bay must be determined.
Distinguishing natural salinity variability due to climate variability from changes
caused by water diversion in the Everglades during the 20th century requires
information on Florida Bay salinity history. Variability in salinity in Florida Bay
reflects the bay’s geometry, which features the Gulf of Mexico to the southwest,
the Florida Keys to the southeast, and the Everglades to the North, and its shallow
bathymetry and circulation resulting from the complex of mudbanks in the bay.
Florida Bay salinity varies over seasonal, interannual, and decadal timescales but
the instrumental record prior to the 1990s is insufficient to attribute cause to
decadal trends. This study addresses the application of quantitative “retrodictive”
estimates of past salinity patterns derived from geochemical studies of ostracodes
from sediment cores from the central part of Florida Bay. Parallel studies of
molluscan shell chemistry and seasonal salinity variability using the same
sediment cores will be given in a separate presentation at the meeting.
The ratio of magnesium to calcium (Mg/Ca) ions in the calcium carbonate shells
of the crustacean group ostracodes is strongly influenced by the salinity and
temperature in which the organism secretes its shell (see Dwyer et al. 2002). The
variability in Mg/Ca ratios of Florida Bay water is “captured” in the Mg/Ca ratios
of the ostracode shell when it secretes its adult shell. It is conventional to express
the relationship between shell and water chemistry as follows:
(Mg/Ca) ostracode calcite = (K D-me) (Mg/Cawater)
where Mg/Ca represents the atomic ratio of Mg to Ca and K D-me is the partition
coefficient for magnesium (Dwyer and Cronin 2001). Mg/Ca ratios in fossil
ostracodes from sediment cores were used to construct a paleosalinity curve for
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central Florida Bay. The results showed that 20th century decadal oscillations in
salinity were related mainly to regional rainfall variability, which in turn was
influenced by climate processes associated with interannual El Nino-Southern
Oscillation and decadal Pacific North American patterns (Cronin et al. 2002).
In this study, the accuracy of the Mg/Ca salinity curve was “groundtruthed”
against instrumental records of salinity from Florida Bay (Nuttle et al. 2000).
Figure 1 shows the Mg/Ca paleosalinity curve plotted against the instrumental
record of salinity for the last 50 years. The instrumental record was constructed
from mean monthly salinity values for Whipray, Rankin, and Bob Allen Basins
pooled to yield a grand mean for the central region. Table 1 summarizes the
paleosalinity-instrumental salinity comparison for Russell Bank.
Salinity
Maxima
Mg/Ca
Salinity
Instrumental
Salinity
Salinity Max.
Difference
Salinity Minima
Mg/Ca
Salinity
Instrumental
Salinity
Salinity Min.
Difference
1990-1993
late 1970s
mid 1960s
1950s
40.63
45.03
51.37
57.02
41.70
44.08
48.41
47.32
-1.08
0.95
2.96
9.70
1993-1995
1980s
late 60s-early 70s
~1960
36.15
24.91
28.35
27.05
36.45
28.53
30.22
27.89
-0.30
-3.61
-1.87
-0.84
Table 1. Comparison between paleosalinity and instrumental of salinity minima and maxima
Both records exhibit large decadal swings in salinity in central Florida Bay
ranging from > 50 ppt to the low 20s. For salinity maxima during the early
1990s, late 1970s and mid-1960s the difference between paleo and instrumental
salinity was ~ -1.1., 0.9, and 3 ppt, respectively. The paleosalinity method
overestimated the maxima for the 1950s by 9.7 ppt, however the instrumental
record stops in 1957 and it is unlikely the two records had a similar period of
record for this high salinity event. The differences for four periods of Florida Bay
salinity mimima ~ 1993-1995, the 1980s, the late 1960s-early 1970s and ~ 1960
were 0.3, 3.6, 1.9 and 0.8 ppt, respectively. Given the temporal gaps in the
instrumental record, the error bar on the sediment core age estimates, and the
spatial and temporal averaging used for both records, these comparisons provide a
remarkable confirmation that the Mg/Ca-based shell chemistry method yields
accurate estimates of past salinity to within < 1 to 4 parts per thousand.
Figure 1 also compares paleosalinity with hydrological instrumental records from
south Florida: mean monthly water height at USGS well 196a, Homestead,
Florida, annual discharge at Taylor Slough since 1961, and NOAA monthly
regional rainfall anomalies. The results reveal decadal patterns in rainfall and
water well height since the 1930s and an inverse correlation between salinity and
well height, rainfall and discharge. For example, wet periods (i.e., ~1960, late
1960s, early 1980s) were characterized by positive precipitation and well height
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anomalies and relatively low Florida Bay salinity. Periods of low salinity recorded
in the Mg/Ca paleosalinity curve during the early 1950s and the early 1940s are
also evident in the precipitation and well height curves. Decadal salinity
oscillations over the past century are also evident from Mg/Ca analyses from Park
and Bob Allen Keys, and Little Madeira Bay (Dwyer and Cronin 2001).
Tay lor Sloug h Dis ch. (cfs )
0
20000
40000
60000
2000
2000
Taylo r Disch
1990
1990
mea sured
1980
1970
1970
1960
1960
1950
1950
year
1980
1940
1940
Mg/Ca
pal eosa lini ty
Homes tead
Wel l he ight
1930
1930
1920
1920
60
50
40
30
20
Florida Bay s alinity (ppt)
10
-3
-2
-1
0
1
Homes tead well hgt. anom.
2
-2
-1
0
1
2
3
4
Regional precip. anomaly
Figure 1. Comparison between paleosalinity and measured salinity (left), Homestead well level and Taylor
discharge (center), and NOAA regional rainfall (right).
These results demonstrate that the Mg/Ca method reconstructs salinity patterns
that faithfully record salinity extremes to within a few parts per thousand and that
the Mg/Ca method could be applied to new core sites to build a network of longterm salinity records in Florida Bay and adjacent bays.
References
Cronin, T. M., et al. 2002. Climate Research 19: 233-245.
Dwyer, G. S. and Cronin, T. M. 2001. Bull. of Amer. Paleont. 361: 249-276.
Dwyer, G. S., et al. 2002. AGU Monograph 131: 205-225.
Fourqurean, J. W. and Robblee, M. B. 1999. Estuaries 22: 345-357.
Nuttle, W. K. et al. 2000. Water Resources Research 36: 1805-1822.
Robblee, M. et al. 1991. Marine Ecology Progress Ser. 71: 297-299.
Cronin, Thomas M. 926A US Geological Survey, Reston, V 20192
Phone: 703-648-6363, Fax: 703-648-6953, tcronin@usgs.gov
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