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projects > florida cooperative mapping > abstract
Patterns and Possible Causes of Temperature and Salinity Variability in Central Florida Bay 1880-1998T. M. Cronin, G. S. Dwyer, T. Kamiya and S. Schwede This project investigated patterns and causes of temperature (T) and salinity (S) variability in Florida Bay using isotopically-dated sediment cores from Russell Bank, Park Key, Bob Allen Key, and Whipray Key. The methods used include morphological (Loxoconcha shell length), geochemical (Mg/Ca ratios in ostracodes), and faunal (ecological) data designed to provide quantitative estimates of decadal-scale changes in T and S for the past 150 years. Temperature and salinity reconstructions were based mainly on the ecology and the shell chemistry of the ostracode Loxoconcha matagordensis, a common species in modern Florida Bay and as a fossil in sediment cores. Temperature reconstructions are based on the carapace length of fossil shells of L. matagordensis. Carapace size in some epiphytal species of Loxoconcha is inversely proportional to the water temperature in which the adult carapace is secreted during final ecdysis (Kamiya 1988). Such a relationship is evident in L. matagordensis populations obtained from temperate to subtropical climatic zones of eastern North America, and from Zostera collected seasonally in Chesapeake Bay and Thalassia in Florida Bay (Figure 1). Because most adults grow during spring and early summer when seagrass growth isoccurring, adult carapace size is an indicator of warm season water temperature.
Salinity was reconstructed using the Mg/Ca ratios in fossil shells of L. matagordensis. As discussed in Dwyer and Cronin (in press), the Mg/Ca ratios provide estimates of the Mg/Ca ratios of the Florida Bay waters over the period of record. These fossil shell ratios can then be used to estimate past Florida Bay salinity due to the strong positive correlation between salinity and water Mg/Ca. The relationship between shell Mg/Ca and water Mg/Ca can be described as follows: (Mg/Ca)ostracode calcite = (KD-Mg)(Mg/Cawater) where (KD-Mg) is the partition coefficient for magnesium. Using a KD for L. matagordensis of 0.00743 (see Dwyer and Cronin in press), in conjunction with the water Mg/Ca-salinity relationship, we calculate estimates of past warm-season salinity of Florida Bay. Kd-Mg may be partially a function of water temperature, which may contribute to the observed variability in shell Mg/Ca. Results (Figure 2) show that mean carapace length for both male and female adult L. matagordensis varies significantly downcore at Russell Bank in central Florida Bay for the period 1880-1998. These trends have been replicated in a second core from Russell Bank and at other sites in central Florida Bay and provide evidence for decadal variability in central Florida Bay temperatures.
Using the length-temperature relationship for modern adult females shown in Figure 1, and the Mg/Ca-salinity relationship described above, we estimated temperature and salinitytrends at Russell Bank from replicate cores taken in 1996 and in 2000 (Figure 3). This plot reveals several important features to the central Florida Bay T and S history. First, warm season T and S covary for the past 120 years; T and S oscillate between 27°C and 32°C and 20 and 50 ppt, respectively. This pattern would be expected because, other factors being equal, warmer temperatures should lead to greater evaporation and increased summer salinity. Second, there was a trend in both T and S from relatively high values during the late 19th century towards minima about 1940-1950. This period was followed by a steep rise in salinity and, to a lesser degree temperature, towards maxima in the 1960s and 70s. Anthropogenic diversion of fresh water of the 1950s and 60s may have caused the apparently anomalous salinity during this period. It should be mentioned that shell size and Mg/Ca ratios vary in cores from Bob Allen, Park and Whipray Keys suggesting those observed at Russell Bay represent broad patterns characteristic of this part of Florida Bay. The causes of decadal salinity and temperature variability can be examined in light of climatic patterns that influence the south Florida region and Florida Bay. Using the Mg-based salinity curve above, and two faunal indicators of salinity (relative abundance of Malzella floridana and Loxoconcha matagordensis), we found evidence for quasi-cyclic oscillations in the salinity of central Florida Bay that includes a 5.5 year Mg/Ca-based salinity periodicity, and three predominant modes of variability (6-7 year, 8-9 year, and 13- 14 year) in all salinity proxies. Since 1950 an 8-year periodicity has been prominent in the faunal indicators. What are the causes of these patterns? Climatatological observations and modeling studies suggest that climate (especially precipitation) in the southeastern United States is strongly influenced by ‘teleconnections’ to decadal and interannual ocean/atmospheric processes originating in the Pacific Ocean, and perhaps also the Atlantic Ocean. To explore whether these processes might influence salinity in Florida Bay, we compared the Russell Bank paleosalinity curve to records of south Florida winter rainfall and to five climate indices: the Southern Oscillation Index (SOI), winter North Atlantic Oscillation (NAO), winter Pacific North American (PNA) index, and a surrogate for winter PNA, the Central North Pacific (CNP) index (Cayan and Peterson 1989). Patterns in the SOI, PNA, and CNP appear to correlate with south Florida winter precipitation. Spectral analyses of SOI and winter rainfall for the period 1910-1998 suggest ~5 year, 6-7 year, and 13-14 year cycles. The 6-7 year and the 13-14 year frequencies are similar to those observed in the faunal and geochemical time series from Russell Bank. Spectral analyses of post-1950 winter rainfall exhibits a 5 year cycle, whereas the PNA index shows an 8 year cycle for this period, similar to that observed for the paleosalinity indicators. The main periods of the CNP index are 5-6 and 13-15 years, similar to those observed in Florida Bay paleosalinity. In summary it appears that decadal salinity trends in the Florida Bay reflect regional rainfall variability associated with climate processes, except possibly during the 1950s and 60s when human factors were important. These studies give us the tools to further evaluate proposed management actions on the Everglades and adjacent coastal ecosystems. References
(This abstract was taken from "Programs and Abstracts - 2001 Florida Bay Science Conference". (PDF, 6.8 MB))
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