Swart and Price Salinity Variations in Florida Bay 1

Swart and Price Salinity Variations in Florida Bay Origin of Salinity Variations in Florida Bay Swart, P.K. and Price, R. Stable Isotope Laboratory, Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami Fl 33149. Abstract This paper presents a method of distinguishing the source of freshwater which causes reductions in salinity in the coastal environment of South Florida. This technique, which uses the δ18O and δD of the waters, allows for differentiation of the freshwater derived from precipitation as opposed to runoff, because surface waters in the Everglades have been highly evaporated and therefore have elevated δ18O and δD values relative to precipitation. A time series of monthly δ18O and δD values of surface waters, collected from stations in Florida Bay between 1993 and 1999, has shown that during this time the major source of fresh waters causing depressions in the salinity in the western portion of Florida Bay were derived from precipitation rather than from the runoff of water from the Everglades. In the eastern portion of Florida Bay, close to the boundary between peninsular Florida and the Bay, the proportion of freshwater derived from precipitation drops steadily reaching less than 10%. This method not only allows differentiation between the sources of freshwater, but can in a temporal sense ascertain the effectiveness of water management practices upon the salinity of the estuarine ecosystems of South Florida. 1 Swart and Price Salinity Variations in Florida Bay Introduction Florida Bay is a large triangular body of water located between the Florida Keys and peninsular Florida to the north (Figure 1). The water in the Bay is composed of a mixture of freshwater derived from the Everglades, through Taylor and Shark Sloughs, and directly from precipitation, and seawater which enters from the Gulf of Mexico and the Florida reef tract. This mixture allows salinities to fluctuate between essentially zero, close to the Everglades- Florida Bay interface, to marine values closer to the western and southern margin of Florida Bay. In addition, as a result of the isolation of some of the interior portions of the bay, salinities can attain values as high as 50-70 as a result of evaporation. The amount of groundwater input into Florida Bay is still unknown, but wells which have been drilled indicate that these subsurface fluids are all saline (Bohlke, et al., 1999) and have enriched δ18O and δD values (δ18O= +1.5 to +2.7‰; δD = +13 to +20‰). In fact the saltwater-freshwater interface is located approximately 10 km inland from the present coast line (Price, 2001; Fitterman et al., 1999). Hence groundwater is not considered to be at present a significant source of freshwater input into Florida Bay, Over the past 15 years there has been growing concern regarding the detrimental role salinity plays in controlling the survivorship of seagrasses and other organisms within Florida Bay (McIvor et al., 1994). This concern reached its peak between 1989 and 1991, when salinities within Florida Bay reached values in excess of 60 in some of its interior portions (Boesch et al. 1993). As a result of this concern, extensive monitoring programs were instigated which involved the collection of water samples from a network of stations throughout Florida Bay (Boyer et al., 1999). Initial responsibility for the origin of the high salinity in Florida Bay was believed to be a result of reduced water delivery to Florida Bay caused by anthropogenic management of water flow from the water conservation areas into the Everglades National Park and ultimately into Taylor Slough (Boesch et al., 1993). These conclusions resulted in the initiation of a large engineering project which was designed to divert water into Taylor Slough. Several studies were initiated at this time which attempted to address the historical record of salinity in Florida Bay using proxy indicators contained within the skeletons of calcareous 2 Swart and Price Salinity Variations in Florida Bay material such as corals (Swart et al., 1996) and shell material (Halley and Roulier, 1999). The results of the study of Swart et al. (1999) indicated that Florida Bay had experienced a long history of salinity variation, but that (i) the highest salinities over that past 150 years had indeed occurred since 1960, and (ii) a major influence on the salinity was the construction of the railway between Miami and Key West. These conclusions were based on an analysis of the δ18O of the skeleton of a massive scleractinian coral growing in Lignumvitae Basin near the Peterson Keys (Site 20; Figure 1) and a correlation with salinity records between 1955 and 1986 (Swart et al., 1996; 1999). In order to better understand the relationship between salinity and the δ18O of the water, δ18O and δD measurements were made on samples collected by Florida International University from a series of stations within Florida Bay (Figure 1) starting in 1993. These data were used to demonstrate different relationships between salinity and δ18O for the purposes of the reconstruction of past salinity records from the δ18O of the skeletons of calcareous organisms (Swart et al., 2001). In this paper we report the results of associations between salinity and the δ18O on samples from Florida Bay collected between 1993 and 1999 and discuss the implications that these associations have on the origin of salinity reductions in Florida Bay. In addition to the data collected on samples from Florida Bay we have utilized data measured on samples from the Taylor and Shark Sloughs (Price, 2001; Swart et al, 2001) in the Everglades as well as precipitation samples from four localities in South Miami (Figure 1). Methods Surface water samples were collected on a approximate monthly basis from a network of stations in Florida Bay and the Everglades. The stations in Florida Bay are the same ones analyzed for salinity and other chemical parameters (Boyer et al., 1999). There are 28 stations within Florida Bay. Between November 1994 and August 1996 water samples were only collected from 20 stations. Of the 75 months between October 1993 and January 1999, samples were collected on 56 months. Samples of precipitation were also collected at three locations in 3 Swart and Price Salinity Variations in Florida Bay South Florida (Figure 1) between 1995 and 1999 and analyzed for their δ18O and δD isotopic compositions. Oxygen and hydrogen isotopic measurements were made in the Division of Marine Geology and Geophysics at the University of Miami. Both measurements were made using a water equilibration system (WEST) attached to an Europa GEO (Swart, 2000). In the 18 water equilibration system, the δ O is determined on CO2 which has been injected into serum bottles at slightly above atmospheric pressure containing 1 cm3 of sample. This method is similar to that described by Epstein and Mayeda (1953). The samples are subsequently equilibrated at 35oC for eight hours without shaking. The process is entirely automated with the CO2 being injected and retrieved using an autosampler and the gas being transferred to a dual- inlet mass spectrometer through a cryogenic trap (-70oC) to remove water. The precision of this method for oxygen, determined by measuring 59 samples of our internal standard, is +/ 0.08‰ for δ18O. The hydrogen isotopic composition is determined using the same device as employed for CO2. Equilibration with hydrogen gas takes place in the presence of a platinum catalyst (Hokko Beads) at 40oC (Coplen, et al. 1991). Precision using this method is +/- 1.5‰ . Both oxygen and hydrogen isotopic data are calibrated using V-SMOW and are reported in ‰ according to the conventional notation. Salinity measurements were made by FIU and have been previously reported in various publications (Boyer et al. 1999). Results Florida Bay Salinity: During the period for which δ18O values reported (1993-1999), the mean salinity of Florida Bay studied varied from 18.1 to 48.3. The lowest salinity recorded was 0.10 in Highway Creek site and the maximum 68.4 in Little Madeira Bay (See Figure 1). Little Madeira Bay also exhibited the largest range in salinity, varying from 3.4 in October of 1995 to 68.4 in April 1999. The smallest range in salinity occurred at the Oxfoot Bank site (26.8 to 38.4). A contour map showing the mean salinity values is shown in Figure 1. 4 Swart and Price Salinity Variations in Florida Bay Oxygen: The mean monthly δ18O values for Florida Bay are positively correlated with the mean salinity (r=0.68, n=65) (Figure 2a). In contrast there is no correlation if the mean salinities are compared with the mean δ18O values for the individual sites over the study period (r=0.0007, n=28). The range of mean δ18O values for individual months lies between -1.12 and +3.36‰, with the highest and lowest δ18O values occurring in May 1999 and October 1999 respectively. The range in absolute δ18O values for individual basins ranges from -3.54 to +5.71 ‰. The lowest δ18O value occurred in Joe Bay in October 1999 and the highest δ18O value in August 1994 in Manatee Bay. The largest ranges in δ18O values occurs in Joe Bay (8.37 ‰) while the lowest range at the Sprigger Bank site (2.9‰). Hydrogen: The δD of the monthly means for Florida Bay are positively correlated with salinity (r=0.6, n=65) (Figure 2b). The δ18O and δD values are also correlated (r=0.82). The correlation exhibits a slope of 6 compared to a slope of 8 for the meteoric water line (MWL) (Figure 3). The mean range of δD values in Florida Bay extends from -5.0 to +25.8 ‰.

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