BULLETIN OF MARINE SCIENCE, 44(1): 274-282, 1989

FRESHWATER FLOW FROM THE TO BAY: A HISTORICAL RECONSTRUCTION BASED ON FLUORESCENT BANDING IN THE CORAL SOLENASTREA BOURNONI

Thomas J. Smith, III, J. Harold Hudson, Michael B. Robblee, George v: N. Powell and Peter J. Isdale

ABSTRACT Fluorescent banding was found in a core taken from a I-m high colony of the coral Solenastrea bournoni which was growing in the Petersen Key Basin region of . Fluorescent banding in massive, hermatypic corals from the Great Barrier Reef, Australia, is known to result from the input of fulvic and humic compounds of terrestrial origin into the nearshore environment via river runoff. Relationships between the fluorescent banding pattern in the Solenastrea skeleton and flow in Slough (SRS) and (TS), the two major outlets of freshwater from the Everglades, were investigated. These relationships were then used to hindcast flow for the period 1881-1939. In hindcasting flow in SRS, 57.2% of the variance in annual flow could be recovered from the fluorescent record, based on the period 1961-1986. When the model was tested on a validation sample (known SRS flow for 1940-1960) approximately 45% of the interannual variation was explained. The fluorescence record shows a sustained, marked, decline which began about 1912 and ended around 1931. Fluorescence is significantly higher (P < 0.001) early in the record (pre 1932) than late in the record (1932 and later). Based on the significant relationship between fluorescence and SRS flow, this decrease is interpreted as recording decreased freshwater flow from the Everglades into Florida Bay and adjacent waters, perhaps by as much as 59%. The onset of decreased flow corresponds with the timing of construction of the extensive network of drainage canals to the east and south of . These canals diverted water into the Atlantic Ocean which would normally have flowed into the Everglades from Lake Okeechobee.

The phenomenon of natural fluorescence in river water, caused by dissolved fulvic and humic acids, has been known for some time (Kalle, 1949). This natural fluorescence has been used as a tracer of freshwater inputs into nearshore envi- ronments (Zimmerman and Rommerts, 1974; Willey and Atkinson, 1982; Willey, 1984). Rivers originating from different sources, such as coastal plain versus piedmont, can be distinguished by their differing fluorescence signatures (Willey and Atkinson, 1982). Recently it has been shown that massive, hermatypic corals (particularly the genus Porites)from nearshore environments of the Great Barrier Reef, Australia, possess fluorescent bands within their skeletons (Isdale, 1984). The frequency and intensity of this fluorescent banding is highly correlated with terrestrial runoff into the nearshore environment (Isdale, 1984). Low relative- molecular weight fulvic acids are responsible for this fluorescence (Boto and Isdale, 1985). Although the exact mechanisms by which the fluorescing compounds be- come incorporated within the coral's skeleton are poorly understood, fluorescent banding in coral skeletons is almost certainly a record of periodic discharges from coastal rivers into nearshore estuarine and marine waters. In October 1986 a core from a I-m high colony of the coral Solenastrea bournoni was collected. The coral was growing in the Petersen Key Basin region of Florida Bay (Fig. I). Fluorescent banding was clearly visible when the core was viewed under long wavelength uv light. In this paper we attempt to quantify the rela- tionships between the frequency and intensity of fluorescent banding and fresh- 274 SMITH ET AL.: HISTORICAL EVERGLADES RUNOFF AND CORAL FLUORESCENCE 275 water flow from and Taylor Slough, the two major outlets of the Florida Everglades. We then used these relationships to present a reconstruc- tion of historical flow patterns from the Everglades into Florida Bay and adjacent waters. Regional Surface Hydrology of Southern Florida. -Shark River Slough (SRS) is the southernmost portion ofthe 23,300 km2 -Lake Okeechobee- Everglades basin (Stephens, 1942; Schrontz, 1942; Jarosewich and Wagner 1985; Fig. 1). Historically, surface waters could flow from Lake Tohopekaliga in the north through the Kissimmee River into Lake Okeechobee and thence southward through the Everglades and into SRS, a distance of 340 km. Surface waters were prevented from passing eastward into the Atlantic Ocean by the Atlantic coastal ridge, although some water was probably lost via groundwater flow and spillover at low points in the ridge. During high stages in Lake Okeechobee water could drain westward into the Gulf of Mexico via the , although at normal lake heights this was thought to be minimal (Schrontz, 1942). The predominant flow from Lake Okeechobee was south to the Everglades. Taylor Slough (TS), a second, much smaller drainage basin, lies immediately to the north of Florida Bay. This slough drains the southeastern Everglades and empties directly into the northeast portion of Florida Bay (Rose et aI., 1981). The headwater region for TS is adjacent to the southern edge of SRS. It is likely that water from SRS could pass into the TS basin as groundwater (Tabb, 1967) or as sheet flow during periods of high water in SRS (Craighead and Holden, 1965). Currently, flow in SRS is approximately 10 times greater than in TS. Peak flows are recorded in September/October for both systems. Beginning in 1910, an extensive network of canals was constructed which run east and southeast from Lake Okeechobee to the Atlantic Ocean (Wallis, 1942). Four major canals had been opened by 1915 (Wagner and Rosendahl, 1988). Two additional canals were completed in 1924. A dirt levee had been constructed around Lake Okeechobee by 1926. In 1928 the highway, which crosses the central Everglades, was completed. Although supplied with numerous culverts, the highway was an impediment to the southward flow of water through the Everglades, especially under high water conditions (Wagner and Rosendahl, 1988). With the construction ofthe water conservation areas between Lake Okee- chobee and SRS and extension of the canal system into southern Dade County during the 1960's, discharge to SRS was almost totally regulated. Flow into both SRS and TS is via regulated discharges from the canals and via rainfall directly into their respective basins (Rose et aI., 1981; Wagner and Rosendahl, 1982).

METHODS AND RESULTS Coral Fluorescence. -In October 1986, cores were collected from two colonies of the coral S. bournoni. The colonies were growing in the Petersen Key Basin of Florida Bay (24°55.0'N, 80045.9'W), approx- imately 6 km west-northwest of Lignumvitae Key (Fig. I). This site is described in detail elsewhere (Hudson et aI., 1988). Cores were collected using previously described drilling techniques (Hudson et aI., 1976; Hudson 1981). It was subsequently discovered that only one core had been successfully drilled down the complete axis of colony growth to give the entire history of the coral. It was this coral core which was used in our analyses. The core was determined to be 107 years old; however, due to asynchronous banding in the first 2 years of growth, only the last 105 years of record could be used. The core was sectioned and then analyzed for fluorescence by stepping a core slice past a Leitz Ploem Opak Illuminator attached to a monochromator in 0.5 mm increments. The fluorescence emission intensity at 460 nm was recorded. This wavelength was used because a preliminary emission intensity scan indicated that it gave the best enhancement of the terrestrial over marine fluorescence signals (Isdale and Kotwicki, 1987). These data were converted into a monthly time series of fluo- rescence values by comparison with the temporal record of annual skeletal density bands visible in x-radiographs of the coral core (Isdale and Kotwicki, 1987). Maximum fluorescence was usually 276 BULLETIN OF MARINE SCIENCE, VOL. 44, NO.1, 1989

Figure I, Relationships between some important geographic features of the study region referred to in the text (modified from Jarosewich and Wagner [1985]), Kissimmee River watershed boundary (solid line), Atlantic Coastal Ridge (- - -), Big Cypress-Everglades divide (. , .. '), historical Everglades (shaded) with Shark River Slough (SRS) and Taylor Slough (TS). Petersen Key Basin, location of coral FB5 as shown.

measured in December, however; both intra- and interannual variations in fluorescence were marked (Figs, 2, 3), Visual observation of the annual fluorescence record (Fig, 3) indicated two changes over its length: I) a decrease in the average annual fluorescence, and 2) a regular periodicity early in the record which disappeared late in the record, The record was divided approximately in half, based on the year with the minimum measured fluorescence, 1931. Student's (-test was used to examine dif- ferences in mean annual fluorescence between the two periods. Nineteen thirty-one was included in the early half of the record to provide a conservative test. Fluorescence in the early half(1931 and SMITH ET AL.: HISTORICAL EVERGLADES RUNOFF AND CORAL FLUORESCENCE 277

200 Fluorescence(relative units)

l~O

ICO

80

MJJASONDJFMA

Figure 2 (Left). Annual cycle in fluorescence intensity for the coral (ii. ± I SD) over the 105-year record. Figure 3 (Right). Average annual mean fluorescence and total annual discharges from Shark River Slough and Taylor Slough. before) of the record was found to be significantly higher than in the latter portion (1932 and later) of the record (t = 10.55, P < 0.001). Power spectra for the pre and post 1931 periods were calculated (Fig. 4). For the 1931 and earlier period, 35% of the variance lies within the broad peak at periods of 4-6 years (P < 0.05). This peak is absent from the spectrum for 1932 and later. The latter half of the record has a peak at the Nyquist frequency (0.5 cycles/yr) which is not present in the pre-1932 spectrum.

Flow in Shark River Slough and Taylor Slough. - Measurements of discharge from SRS and TS into the nearshore zone are not available. Data used in this study consist of measurements of flow in each drainage system which were taken some distance upstream from their respective terminuses. Records of daily flow in SRS were collected along the Tamiami Trail some 64 km upstream from its mouth. The values represent the aggregate flow me~sured in all culverts along the Tamiami Trail from canal L-30 in the east, to 40 Mile Bend (40MB) in the west. Data are available beginning in November 1939 (USGS 1940-1985). TS flow has been measured since September 1960 by the USGS. Measurements are taken where the park entrance (SR 9336) road crosses the slough some 14 km upstream from its mouth. Daily values were summed into monthly totals for both series of flow (Fig. 5). The average value for each month was then calculated and subtracted from the appropriate monthly observation to convert the flow records into a series of monthly flow anomalies. Data concerning groundwater levels in south Florida are also available. Both Tabb (1967) and Sculley (1986) have shown large, inverse correlations between groundwater stage (measured as water table height) and salinity in the waters of northeast Florida Bay. The implication is that substantial freshwater inputs to the northeastern section of Florida Bay are via groundwater. The data used here are groundwater stage records, beginning in October 1957, from well S-196A near Homestead. Groundwater was measured as maximum daily stage, reported every fifth day, for 1957-1973, and daily thereafter. Average stage was calculated for each month and the series converted to anomalies as above. Flow versus Fluorescence. - Relationships between fluorescence, flow, and ground- water stage were investigated using a linear regression approach to time series analysis (Fuller, 1976). Although the biology and physics of the fluorescence/flow interaction dictates that fluorescence would be dependent on flow, because the 278 BULLETIN OF MARINE SCIENCE, VOL. 44, NO.1, 1989

15

10

15 Percentvariance at each cycle

ID <1 g, 100 ~ o

50 o o 0.1 02 0.3 04 05 Cycles per year

Figure 4 (Left). Power spectra of annual fluorescence for pre- and post-1931 periods. Shown is the percent of the total variance at each cycle. Figure 5 (Right). Annual cycle of flow for Taylor Slough (a) and Shark River Slough (b). it ± 1 SE, N = 26 for Taylor Slough, N = 47 for Shark River Slough.

purpose of this work was to hindcast flow, fluorescence intensity was considered to be the independent variable. The dependent variables (considered individually) were SRS flow, TS flow and groundwater stage (GS). Initial regression analyses indicated that at monthly and seasonal timesteps fluorescence recovered only a small, albeit significant, percentage of the variance of the dependent variables. Therefore, the remainder of the analyses considered all variables at the annual level (on a calendar year basis). The largest amount of variance explained among the dependent variables was> 57% for SRS (Table 1). Variance explained by fluorescence decreased to 24% for TS and to < 10% for groundwater stage (Table I). Shark River Slough flow was hindcast using the following strategy. A regression model was developed using known flow and fluorescence for the period 1961- 1986. The model was then tested using the remainder of the measured data (1940- 1960). Once verified, the model was used to hindcast a chronology of annual flow in SRS for the period of measured fluorescence, but unknown flow, 1881-1939. When the model was tested over the validation period, the ,2 was 44%, indicating that the model performs very well (Fig. 6). Average annual flow into SRS, as measured along the Tamiami Trail, for the 1940-1986 period was 471,610 ± 62,829 acre-feet (581.50 ± 77.47 G-liters, x ± 1 SE). The hindcast values indicate an annual flow into SRS for 1881-1939 of 1,145,777 ± 96,700 acre-feet (1412.74 ± 119.23 G-liters). These values indicate a decrease of 59 ± 47% in the present flow compared with historical (hindcast) flows. SMITH ET AL.: HISTORICAL EVERGLADES RUNOFF AND CORAL FLUORESCENCE 279

Table I. Results of regression analyses using coral fluorescence to hindcast flow as indicated (B = regression co-efficient, F = F statistic, P = significance of F, r' = variance explained, Lag = length of lag, in years, for fluorescence, MS = mean square, df = degrees of freedom)

Flow: Taylor Slough Variable B F P Lag I 0.497 5.95 0.05

Overall df MS F P r" Regression I 5.59 5.95 0.05 0.239 Residual 19 0.94

Flow: Shark River Slough Variable B P Lag a 1,854 3.14 0.05 Lag 2 1,211 1.98 0.10 Lag 3 -2,256 -4.00 0.01

Overall df MS F P r" Regression 3 1,075 10.34 0.01 0.572 Residual 18 104

Groundwater Stage: Well S·196A

Variable B F P Lag I 0.027 5.22 0.05

Overall df MS F P r" Regression I 360 5.22 0.05 0.099 Residual 21 70

DISCUSSION Intensity offtuorescence in the Solenastrea bournoni coral skeleton from Florida Bay was most significantly correlated with flow in Shark River Slough, and, secondarily, flow in Taylor Slough. These results are not surprising given the magnitude of the difference between flow in SRS and TS (Fig. 5). Significant correlations between groundwater stages at well S-196A and salinity in northeast Florida Bay have been cited as evidence that groundwater provides inputs of freshwater to the bay (Tabb, 1967; Sculley, 1986). We found that fluorescence and groundwater stages were poorly correlated. However, we do not know if groundwater contains dissolved fulvic and humic compounds which would be incorporated into the coral's skeleton. The lack of a relationship may simply indicate that it does not. On the Great Barrier Reef, Australia, it has been shown that coral fluorescence can delineate large flow events which are separated in time by as little as 1-2 months (Isdale, 1984). We were unable to achieve that level of intra-annual resolution with our fluorescent record. The long lag times between fluorescence and flow in this study are not surprising considering differences in flow regime between south Florida and the Burdekin in Australia studied by Isdale (1984). The Burdekin is characterized by open channel flow. When in flood, river currents are substantial with the plume from the Burdekin passing directly over the coral which Isdale (1984) studied (Wolanski and van Senden, 1983). Shark River Slough 280 BULLETIN OF MARINE SCIENCE, VOL. 44, NO. I, 1989

SRS discharge ( G-Iitres )

2000 ObseNed

Predicted

1000

o 1900 1920 1940 1960 1980 Figure 6. Observed (heavy line) and hindcast (light line) values of flow into Shark River Slough. Values for 1940-1960 show the fit of predictions from the regression model with observed SRS flows. has a poorly defined channel which may be anywhere between 20-30 km wide, depending on the flow regime at the time. Consequently, flow regimes in SRS are dominated by slow moving sheet flow (Parker, 1974; Rosendahl and Rose, 1981). Under high flow conditions the rate of sheet flow may approach 80.5 km per year I (50 mile·year- ), whereas in low flow conditions rates often drop to zero (Leach et al., 1972; Rosendahl and Rose, 1981). On average the rate of sheet flow appears to be 32 km/year (Schomer and Drew, 1982). The picture that emerges of Shark River Slough flow patterns is one of sheetflow pulses, each from a different runoff year, which are connected to one another to form a continuous but oscillating gradient of flow (Schomer and Drew, 1982). Given this, our finding of lags of 2- 3 years do not seem unreasonable. Significant relationships between fluorescence and flow are clearly apparent at the annual level. Although our reconstruction should be interpreted with caution because our data do not strictly meet the assumptions required for time series analysis, several features of the fluorescent record are irrefutable. A highly statis- tically significant difference in mean fluorescence occurs between the early, pre- canal period and later, post-canal segments of the record. Hindcast values ofSRS flow indicate an approximate 59% decrease in flow between the pre- and post- canal eras. Additionally, there is strong evidence for a regular 4-6 year periodicity in the historical annual flow regimes, as years of low flow alternated with years of high flow. This is no longer apparent in the SRS flow record. This feature of the fluorescent record is corroborated by the analyses of south Florida rainfall patterns provided by Thomas (1974). He found a significant 5-year periodicity in rainfall for the southeastern coastal section (Miami region), the Florida Keys, and inland over the central Everglades during the period 1914-1968. Because runoff is driven by rainfall (Parker et al., 1955; Parker, 1974) we can hypothesize that the periodicity observed in the pre-1932 fluorescence record was a result of this periodicity in rainfall and was lost when flows were diverted by canal con- struction. The occurrence of greater flows through Tamiami Trail into Shark River Slough SMITH ET AL.: HISTORICAL EVERGLADES RUNOFF AND CORAL FLUORESCENCE 281

and ultimately into Florida Bay has major significance with respect to current efforts to restore the Everglades towards historic conditions and to arrest apparent declines in habitat quality of Florida Bay. These efforts are based on the earliest available flow data (post 1940). Our fluorescence record reveals that these flows had already been substantially reduced below historic levels. The fluorescence record further reveals that natural periodicities in historic flow regimes have also been lost in modem times. Attempting to revitalize the Everglades and the Florida Bay estuary are unlikey to succeed unless Shark River Slough flows approximate pre-drainage estimates and periodicities.

ACKNOWLEDGMENTS

The senior author thanks the United States National Park Service for support to attend the Sym- posium on Florida Bay. A. Smith (Institute of Food and Agricultural Sciences, University of Florida) and D. Sikkema () spent many hours collating the hydrological data used in this study. J. Lough (AIMS) wrote the algorithm for converting fluorescence vs. distance into fluo- rescence vs. time. T. Daniel and A. Griffith assisted with laboratory work at AIMS. W. Jaap confirmed our identification of the coral. K. Boto and J. Lough provided critical comments on earlier versions of the manuscript. J. Wagner (National Park Service) provided many useful comments, especially concerning early studies of historical flow regimes in the Everglades. Contribution No. 404 from the Australian Institute of Marine Science.

LITERATURE CITED

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Schomer, N. S. and R. D. Drew. 1982. An ecological characterisation of the lower Everglades, Florida Bay and the Florida Keys. U.S. Fish & Wildlife Service, Office of Biological Services, Washington, D.C. FWS/OBS-82/58.1. 246 pp. Schrontz, C. C. 1942. The principal characteristics of the Kissimmee-Everglades watershed. A. Lake Okeechobee. Proc. Soil Sci. Soc. Fla. IV-A: 14-23. Sculley, S. P. 1986. Florida Bay salinity concentration and groundwater stage correlation and regres- sion. Unpublished internal report, 17 pp. mimeo. South Florida Water Management District, West Palm Beach, Florida. Stephens, J. C. 1942. The principal characteristics of the Kissimmee-Everglades watershed: B. The Everglades. Proc. Soil Sci. Soc. Fla. IV-A: 24-28. Tabb, D. C. 1967. Prediction of estuarine salinities in Everglades National Park, Florida, by use of groundwater reocrds. Ph.D. Dissertation, University of Miami, Coral Gables. Florida. 107 pp. Thomas, T. M. 1974. A detailed analysis of climatological and hydrologica] records of south Florida with reference to man's influence upon ecosystem evolution. Pages 82-122 in P. J. Gleason, ed. Environments of south Florida: present and past. Memoir 2: Miami Geological Society. Miami, Florida. Wagner, J. I. and P. C. Rosendahl. ]982. Structure S-12 water distribution to Everglades National Park. South Florida Research Center Report T-650. 42 pp. --- and ---. 1988. An analysis of water delivery schedules for Everglades National Park through 1982. South Florida Research Center Report SFRC-88/XX. 37 pp. Wallis, W. T. 1942. The history of Everglades drainage and its present status. Proc. Soil Sci. Soc. Fla. IV-A: 29-33. Willey, J. D. 1984. The effect of seawater magnesium on natural fluorescence during estuarine mixing, and implications for tracer applications. Mar. Chern. IS: 19-45. --- and L. P. Atkinson. 1982. Natural fluorescence as a tracer for distinguishing between piedmont and coastal plain river water in the nearshore waters of Georgia and North Carolina. Est. Coast. & Shelf Sci. 14: 49-59. Wolanski, E. and D. van Senden. 1983. Mixing of Burdekin River flood waters in the Great Barrier Reef. Aust. J. Mar. Freshw. Res. 34: 49-63. Zimmerman, J. T. F. and J. W. Rommerts. 1974. Natural fluorescence as a tracer in the Dutch Wadden Sea and adjacent North Sea. Neth. J. Sea Res. 8: 117-125.

DATEACCEPTED: March 2,1988.

ADDRESSES:(T.J.S. and P.J.I.) Australian Institute of Marine Science, PMB No.3, Townsville, M.e., Qlds. 4810, Australia; (J.H.H.) U.S. Geological Survey, Fisher Island Station. Miami Beach. Florida 33139; (G. VN.P.) Research Department. National Audubon Society, 115 Indian Mound Trail, Ta- vernier, Florida 33070; (M.B.R.) South Florida Research Center, Everglades National Park, P.O. Box 279, Homestead. Florida 33030; PRESENTADDRESS:(T.J.S.) Florida Cooperative Fish and Wildlife Research Unit. /17 Newins-Ziegler Hal!, University of Florida, Gainesville, Florida 326/1.