An Evaluation of the Scientific Basis
for "Restoring" Florida Bay
by Increasing Freshwater Runoff from the Everglades

 

Larry E. Brand
Rosenstiel School of Marine and Atmospheric Science
University of Miami
4600 Rickenbacker Cswy.
Miami, FL 33149

 

Introduction

    Florida Bay and the Florida Keys are at the downstream end of the Kissimmee River-Lake Okeechobee-Everglades watershed (Fig. 1). Their ecological health depends on what happens upstream. Within the past 20 years, a number of ecological changes have occurred in South Florida coastal waters. In Florida Bay, large algal blooms have developed and persisted, large areas of seagrasses and sponges have died off, and major changes have occurred in fish populations (Zieman et al, 1994; Robblee et al., 1991; Boesch et al., 1993; Durako, 1994; Thayer et al., 1994; Butler et al., 1995; McPherson and Halley, 1997). In the Florida Keys, macroalgae have overgrown many coral reefs, coral diseases appear to be spreading, and many corals have died (Dustan and Halas, 1987; Porter and Meier, 1992; Ogden et al.,1994; Kuta and Richardson, 1996; Richardson, et al., 1996; Richardson, 1997). Many of these changes are classical indicators of nutrient eutrophication.

    The dominant hypothesis for explaining many of these changes, however, is that reduced water flow into Florida Bay from the Everglades led to hypersaline conditions, which then led to massive seagrass dieoff (Robblee et al., 1991; Durako, 1994; Carlson et al., 1994; Zieman et al., 1994). This hypothesis further proposes that the seagrass dieoff and subsequent organic decomposition and sediment resuspension released nutrients which then generated the algal blooms. This hypothesis has been used as a rationale for pumping more freshwater into Florida Bay as part of a large scale alteration of water management in South Florida (United States Army Corps of Engineers and South Florida Water Management District, 1999). The hypothesis is a reasonable one to begin with, but an examination of the data available leads to serious doubts about the validity of the hypothesis and the predicted ecological consequences of pumping more freshwater into Florida Bay.

Salinity and Seagrass Dieoff

    While hypersaline water may generate physiological stress on seagrasses, there is very little temporal or spatial correlation between high salinity and the seagrass dieoff in 1987 in Florida Bay. McIvor et al. (1994) have documented that salinities up to 70 psu have occurred in Florida Bay in the past 50 years. The highest salinities observed in various studies conducted in Florida Bay, summarized by McIver et al. (1994), are presented in Table 1. The highest salinity observed during the time of the seagrass dieoff in 1987 was 46.6 psu, while no massive seagrass dieoffs were observed at previous times when salinities up to 70.0 psu were measured.

    Although the spatial distribution of salinity in 1989-1990 (Fig. 2) has been used to explain the seagrass dieoff, freshwater flow through Taylor Slough and the South Dade Conveyance System (SDCS) into Florida Bay was greatly reduced in 1989 and 1990 due to drought, compared to 1987 (a non-drought year) when the seagrass dieoff occurred (Fig. 3). Rainfall data (Fig. 4) also show the major drought was 2 to 3 years after the seagrass dieoff and that freshwater input to Florida Bay in 1987 was similar to that observed in 1984 to 1986. Apparently there are no baywide salinity data for 1987, but it was likely not as saline in 1987 as in 1989 and 1990 when the drought occurred and probably not much different from 1984 to 1986. There is no evidence that the seagrass died off in 1987 as a result of increasing salinity.

    While some seagrass dieoff in 1987 did occur in areas of high salinity, much of the seagrass dieoff reported by Robblee et al (1991) occurred in areas (Fig. 5) that had near normal marine salinity even in the drought years of 1989 and 1990 (Fig. 2). Using satellite imagery, Stumpf et al. (1999) have shown that even larger areas of seagrass to the west of Florida Bay (Fig. 5) died off as well, as local fishers and other boaters had reported at the time (DeMaria, 1996). The fact that this area of seagrass dieoff has normal marine salinity and occasionally lower salinity because it is downstream of the Shark River outfall suggests that high salinity was not the dominant cause of the seagrass dieoff. The lack of significant spatial or temporal correlations between high salinity and seagrass dieoff suggests that simply pumping more freshwater into Florida Bay will not solve the ecological problem. Indeed, the data (Fig. 3) show that South Florida Water Management District began pumping more freshwater into Florida Bay from the Everglades well before the seagrass dieoff.

Nutrients and Algal Blooms

    A second part of the dominant hypothesis is that the seagrass dieoff and subsequent organic decomposition and sediment resuspension then released nutrients that generated the algal blooms that now persist in Florida Bay (Durako, 1994; Carlson et al., 1994). Again, this is a reasonable hypothesis, but the facts do not support the hypothesis that it is the major source of nutrients generating the algal bloom.

    Unfortunately, there appear to be no quantitative data on the early development of the algal bloom, so we have to rely on the visual observations of persons who were on the bay for long periods of time. Many fishers and other boaters who are frequently in Florida Bay have been quoted by DeMaria (1996) as observing algal blooms and water "discoloration" beginning in 1981 and increasing thereafter, well before the 1987 seagrass dieoff. They stated "In 1981, the water got dirtier, the blooms grew, and the seagrass started dying."; "From 1981 to 1986, the decline [of the bay] was gradual. In 1987, the bay started to collapse quickly."; and "The commercial fishermen saw the water [in western Florida Bay] changing - first by becoming dirty, then algae blooms started, and then seagrass died off.". These observations suggest that there was another source of nutrients, well before the seagrass dieoff.

    It has now been 13 years since the seagrass dieoff, yet the algal blooms in Florida Bay persist, and have even increased in the 1990s. Residence time of water in the bay has been estimated on the order of a month (Jackson and Burd, 1999; Top et al., 2000). Given the repeated observations of the algal bloom in north central Florida Bay being driven to the south every winter by cold fronts plus the flushing of the bay by Hurricane Georges in 1998, it is hard to believe that the nutrients from decomposed seagrasses 13 years ago have not been flushed out and are instead continuing to fuel the bloom. This is particularly true in the deep channel south of Cape Sable where tidal currents flow in and out daily and there was no significant seagrass dieoff; and yet there is persistently high phosphorus and algal blooms.

    Seagrass dieoff and the resulting biomass decomposition and sediment resuspension also cannot explain the spatial distribution of the algal blooms or nutrients in Florida Bay. It is well established that the general flow of water is from the northwest (around Cape Sable) to the southeast through the passes between the Florida Keys (Smith, 1994; Wang et al, 1994; Lee and Williams, 1999). If seagrass dieoff was the major source of nutrients, the nutrients and algal blooms should be at the same place or downstream of the seagrass dieoff locations. A comparison of the distribution of average chlorophyll concentrations from 1996 to 2000 (Fig. 6) with the distribution of seagrass dieoff in 1987 (Fig. 5) indicates that much of the algal bloom area is upstream, not downstream of the seagrass dieoff areas. Furthermore, a comparison of the inorganic nitrogen (N) and total phosphorus (P) spatial distributions (Figs. 7 and 8, respectively) with seagrass dieoff areas reveals that the highest concentrations of nutrients are neither downstream nor at the same location as much of the seagrass dieoff. The data of Fourqurean et al. (1993); Frankovich and Fourqurean (1997), and Boyer et al. (1997) show the same pattern. While there is some overlap of high N and high P areas in northcentral Florida Bay where some of the seagrass died, seagrass decomposition could not generate a nutrient pattern with P upstream to the northwest and N to the northeast of the seagrass dieoff area, with currents flowing to the southeast. While seagrass dieoff most likely did result in the release of nutrients, it cannot explain the spatial pattern of nutrients or algal blooms in Florida Bay today.

An Alternative Hypothesis

    Proposed here is an alternative hypothesis for the source of the phosphorus (P) and nitrogen (N) that generate the algal blooms in Florida Bay. Ratios of total N : total P and inorganic N : inorganic P are well above the Redfield ratio of 16 (ratio of N and P required for algal growth) throughout Florida Bay. This has led many researchers to assume that P is the primary limiting nutrient and that inputs of N to Florida Bay are not a cause of the algal blooms. It is well known however that many organic N molecules are not readily available to phytoplankton while many organic P molecules are, due to the activity of phosphatase enzymes (Vitousek and Howarth, 1991). This reflects the fact that organic N is bound by direct carbon bonds while organic P is bound by more easily broken ester bonds. Examination of the ratio of inorganic N : total P (Fig. 9) indicates ratios greater than the Redfield ratio in eastern Florida Bay and ratios less than the Redfield ratio in western Florida Bay. This suggests the potential for P limitation in the east and N limitation in the west. The results of around 1000 nutrient bioassays conducted over a year and a half indeed show mostly P limitation in the east and N limitation in the west (Fig. 10), with a spatial distribution similar to the inorganic N : total P ratios (Fig. 9). Lavrentyev et al. (1998) and Tomas et al. (1999) have also observed P limitation in eastern Florida Bay and N limitation in western Florida Bay. The observation of N limitation in western Florida Bay suggests that indeed much of the organic N from the west is not available to the phytoplankton. The largest algal blooms are in northcentral Florida Bay (Fig. 6) where high P from the west (Fig. 8) meets high N from the east (Fig. 7), and the inorganic N : total P ratio is close to the Redfield ratio (Fig. 9).

The source of P

    The most likely source of the high P in the west is thought to be phosphorite deposited during an ancient upwelling event during the Miocene (Riggs 1979, 1984; Mallinson et al., 1994; Compton, 1997) and subsequently transported south into South Florida by erosion as a result of the uplifting of the Appalachian Mountains. Two different transport mechanisms are thought to account for the high P observed in west Florida Bay.

1. Phosphorus from erosion of phosphorite deposits in central Florida, enhanced by phosphate mining, and transported down the Peace River and along the southwest coast of Florida could be a source. It is well established that the coastal currents along the southwest coast of Florida generally flow from the Peace River area toward Florida Bay (Lee and Williams, 1999). Nutrient bioassay data (Fig. 11) confirm that nitrogen limitation is quite strong along the entire coast from the Peace River to Florida Bay, suggesting high P concentrations.

2. Brand (1996, 2000) has hypothesized that phosphorite deposits mixed with quartz sand (Fig. 12) may have groundwater moving up through them, transporting P up into Florida Bay. The distribution of water column P (Fig. 13) correlates well with the phosphorite deposits (Fig. 12). Top et al. (2000) have confirmed, using 4He, 3He, and 222Rn tracer data, that significant amounts of groundwater are entering Florida Bay.

    It is hypothesized that the phosphorite deposits are a persistent source of P that has not changed significantly over the past few thousand years. Actually, phosphate mining over the past century may have increased the input of P into coastal waters, but there is no evidence that it has increased substantially in the past two decades. Perhaps it is the N source that has changed and led to the ecological changes observed in Florida Bay. This is plausible because the nutrient bioassays indicate that N is limiting in parts of Florida Bay. The question becomes: Where is the N coming from and has it increased in the past two decades?

The source of N

    The spatial distribution of N (Fig. 7) and its correlation with low salinity (Fig. 14) suggests that freshwater runoff from the Everglades through Taylor Slough and the South Dade Conveyance System (SDCS) is the major source of N to Florida Bay. There are no significant algal blooms at the mouth of Taylor Slough because of the lack of P. The natural Everglades ecosystem was an oligotrophic wetland, in which the natural source of nutrients was primarily in rainfall, and the vegetation sequestered most of the nutrients before they reached the coastal waters (Lodge, 1994). During the 20th century, much of the Everglades were drained and converted to agricultural land (DeGrove, 1984; Light and Dineen, 1994). Acreage of sugar cane farms in the Everglades Agricultural Area (EAA) south of Lake Okeechobee (Fig. 15) increased dramatically after the Cuban Revolution of 1959 (Fig. 16; Snyder and Davidson, 1994). Initially, most of the nutrient-rich runoff from the EAA was pumped back up into Lake Okeechobee through pump stations S2 and S3 (Fig. 17; Light and Dineen, 1994). This eventually helped lead to the eutrophication of Lake Okeechobee (Havens et al., 1996). As a result, it was decided to instead pump the nutrient-rich EAA runoff south through the extensive canal system (Fig. 18) of the Central and South Florida Project for Flood Control and Other Purposes (C & SF Project), operated by the South Florida Water Management District, starting around 1979 (Lodge, 1994). It is that diversion of nutrient-rich water to the south into the Everglades canal system that is thought to have led to the ecological changes observed in the Everglades in the past two decades (Lodge, 1994; Davis, 1994). From the 1960s to the 1990s, N input into Water Conservation Area 2A south of the EAA increased by a factor of 12.4 (Davis, 1994). Davis (1994) estimates that P input into Everglades National Park increased 3-fold as a result of the diversion of agricultural runoff.

    At about the same time, the South Dade Conveyance System (Fig. 19), which was designed to enhance the flow of water from the Everglades canal system into Florida Bay, was completed (Light and Dineen, 1994; South Florida Water Management District, 1992). The system was designed to transport more freshwater into Everglades National Park and Florida Bay and lower groundwater levels to the east. This allowed farmers to better drain their agricultural fields in south Dade County and shift from seasonal to year round farming (Ley, 1995) and for developers to greatly expand suburbs to the west of Miami, closer to the Everglades. This engineering was successful and more water was injected into Florida Bay, beginning around 1980 (South Florida Water Management District, 1992). Fig. 3 shows the increase in water flowing through the South Dade Conveyance System just north of Florida Bay. It is hypothesized that this led to the injection of not only more freshwater into Florida Bay, but also more N. Figure 20 shows that N and P are scavenged from the water as it moves south from the Everglades Agricultural Area to Florida Bay, but approximately 60 mM N remains, as it is not scavenged by the limestone and vegetation as efficiently as P. The C111 Interim Plan implemented in the early 1990s increased flow into northeast Florida Bay even more (Ley, 1995). This is associated with a 42% increase in nitrate and 229% increase in ammonia in Florida Bay from 1989-1990 to 1991-1994, and a 42% increase in chlorophyll (Ley, 1995). The C111 canal was altered so that more water would flow to Taylor Slough further to the west and less water would flow to the east. This had the effect of injecting N-rich water closer to the area of high P in western Florida Bay.

    Unfortunately, there do not appear to be any good N concentration data in Florida Bay before the beginning of the large regional monitoring program in 1989. Upstream, however, Walker (1991) found nitrate and phosphate inputs increased between 1983 and 1989 in Taylor Slough and between 1977 and 1989 in the Shark River. Walker (1991) observed a 20%/year increase in nitrate in water flowing through the C111 canal into Florida Bay between 1983 and 1989 and a 11%/year increase at the S12A station where water flows into the Shark River system.

    A comparison of chlorophyll concentrations over time at 4 stations in northcentral Florida Bay where the major bloom occurs (Fig. 21), with the flow of the N-rich water into Florida Bay through the Taylor Slough system shows a general correlation (Fig. 22), with highest chlorophyll concentrations in 1994 and 1995, when water flow was highest. The same correlation appears when the data are examined on a monthly basis (Fig. 23). There appears to be a 1 to 2 month lag between when the water is pumped into the bay and when the algal bloom appears to the west of the input site. This is to be expected, as the N-rich, P-poor water from Taylor Slough entering Florida Bay in the east will not generate an algal bloom until it mixes with the P-rich water to the west. A comparison of the spatial extent of the bloom during the dry season when there is little input of N-rich water from Taylor Slough (Fig. 24) and during the rainy season when there is a large input of N-rich water from Taylor Slough (Fig. 25) also demonstrates how input of the N-rich water from the agricultural areas through Taylor Slough helps generate the algal blooms.

    It is worth noting that fishers and other boaters began to observe a systematic decline in water quality around 1981 (DeMaria, 1996), right after nutrient-rich agricultural runoff from the EAA was diverted south into the Everglades canal system and Shark River, and the South Dade Conveyance System was enlarged to enhance the flow of Everglades canal water into eastern Florida Bay. While we do not have quantitative data on N concentrations in Florida Bay before 1989, the temporal correlation between the diversion of nutrient-rich agricultural runoff into the Shark River, Taylor Slough and Florida Bay and the observed ecological changes is rather remarkable.

    The spatial and temporal correlations between the freshwater runoff from agricultural lands in the Everglades and the algal blooms in Florida Bay suggest that the increased freshwater runoff is the cause of the algal blooms in Florida Bay. The U.S. Army Corps of Engineers and the South Florida Water Management District have proposed to increase freshwater runoff even more into Florida Bay (United States Army Corps of Engineers and South Florida Water Management District, 1999). The dominant hypothesis that hypersalinity is the fundamental problem in Florida Bay predicts that more freshwater runoff will improve the environmental situation in Florida Bay. The data indicate that increased freshwater runoff increases the algal blooms.

Downstream effects

    It is generally agreed that ecological changes are occurring in the Florida Keys coastal ecosystem (Environmental Protection Agency, 1992; Ogden et al., 1994). One of the suspected causes of some of these changes, increased nutrients, is well known to alter ecosystems. For example, increased nutrients can cause macroalgae to overgrow corals (Smith et al., 1981; Maragos et al., 1985; Bell, 1992) and for epiphytes to overgrow seagrasses (Cambridge and McComb, 1986; Silberstein et al., 1986; Tomasko and Lapointe, 1991; Lapointe et al, 1994). This has been observed in many areas throughout the world. It is clear that nutrient rich Florida Bay water makes its way to the northern bay side of the lower Keys, and to the southern ocean side of the Keys into Hawk Channel and over the coral reefs primarily throught the large passes between the middle Keys (Smith, 1994; Lee and Williams, 1999).

    Plumes of turbid, low salinity, nutrient-rich, high chlorophyll water have been observed and documented being transported from Florida Bay all the way out to the coral reefs. Quite often, one observes plumes out into Hawk Channel but they do not make it out to the reefs and the water over the reefs appears quite clear. Sometimes, however one can observe plumes of turbid water all the way out to the reefs and beyond. Figure 26 shows highly turbid water moving south from Florida Bay through the passes around Long Key. Figure 27 shows chlorophyll concentrations along a transect in Hawk Channel (Fig. 28) along the Florida Keys from Key Biscayne to Vaca Key collected 2 days after the satellite image. Especially high chlorophyll concentrations are observed at stations 14 and 15 around Long Key and continuing at somewhat elevated concentrations to the south as the coastal countercurrent transports the plume south. The data of Smith (1994) and Wang et al. (1994) confirm that this is the general flow pattern. A comparison of Figs. 24 and 25 shows elevated average chlorophyll concentrations on the reef side of Long Key during the rainy season. Sometimes one can observe plumes of turbid water with a rather different trajectory. For example, on March 23, 1996 a plume of very turbid, low salinity, high chlorophyll water was observed out over the reefs at Looe Key, which could be seen in a satellite image of reflectance (Fig. 29). Along a transect across this plume from Big Pine Key out to the reefs and beyond was observed lower salinity (Fig. 30), higher turbidity (Fig. 31) and higher chlorophyll (Fig. 32) right over the coral reefs. Declining turbidity and chlorophyll was not observed until well beyond the reefs. It appears that this water may have originated in western Florida Bay or in the area west of Key West, perhaps somewhere around the Marquesas. It appears this water was pushed by the northerly winds into the Gulf Stream, which picked it up and carried it east out over the reefs. This plume appears to be squeezed between the Gulf Stream and the coastal countercurrent as a narrow front.

    The data of Lee and Williams (1999) show that drifters released near the outflow of Shark River usually end up flowing southeast through the passes around Long Key or Vaca Key out over the reefs or southwest along the north coast of the lower Keys and then south into the Gulf Stream (Fig. 33). These data indicate that not only Florida Bay water makes it to the reefs, but also the outflow from the Shark River. Figure 34 shows a negative correlation between salinity and chlorophyll at 5 stations south of the Shark River outfall (Fig. 35), suggesting that N from the Shark River is stimulating algal blooms downstream. Dawes et al. (1999) have documented that there has been a decline in macroalgal species diversity and an increase in species that proliferate under eutrophic conditions in the Content Keys. This is of interest because the Content Keys are a long distance from any human habitation or sewage source, but are downstream of the Shark River outflow. Figure 36 shows a strong positive correlation between annual N flux through the Shark River and average annual chlorophyll concentrations measured by Brian Lapointe (Lapointe et al., 2000) in the area around Looe Key in the lower Florida Keys.

    Taken together, these data suggest that nutrients from Everglades-agriculture runoff are being transported to not just Florida Bay, but also the Florida Keys and coral reefs, and contributing to their eutrophication. Because the net flow of water is from the northwest to the southeast (from Florida Bay to the coral reefs), the proposal to open up more passages along the Florida Keys between Florida Bay and the coral reefs to the south for greater water exchange (United States Army Corps of Engineers and South Florida Water Management, 1999) will lead to increased nutrient loading and eutrophication of the coral reefs.

Conclusions

    The dominant hypothesis for explaining many of the ecological changes that have occurred in Florida Bay in the past 2 decades is that reduced water flow into Florida Bay from the Everglades led to hypersaline conditions, which then led to massive seagrass dieoff. This hypothesis further proposes that the seagrass dieoff and subsequent organic decomposition and sediment resuspension released nutrients which then generated the algal blooms. The data, however, show that hypersaline conditions cannot explain either the spatial or temporal distribution of seagrass dieoff in Florida Bay. Furthermore, seagrass dieoff cannot explain the spatial or temporal distribution of nutrients and algal blooms in Florida Bay.

    It is hypothesized that the large algal blooms in Florida Bay are the result of N-rich waters in eastern Florida Bay meeting P-rich waters in western Florida Bay. Nutrient bioassays confirm that P is the limiting nutrient in the east and N is the limiting nutrient in the west, as predicted by the spatial distribution of N:P ratios.

    It is hypothesized that much of the P comes from Miocene phosphorite deposits by way of Peace River erosion and subsequent coastal current transport along the southwest coast, and by way of groundwater through phosphorite-rich quartz sand deposits underneath certain areas of the coastal waters. It is argued that this P source has not changed significantly over the past few decades. It appears that much of the N comes from freshwater runoff from agricultural lands through the Everglades. It is argued that changes in water management practices in the past two decades have led to an increase in N inputs to eastern Florida Bay. Mixing of this water from the east with the P-rich water from the west has led to the large algal blooms that have developed in northcentral Florida Bay, altering the entire ecosystem. Some of this enriched water is transported to the middle and lower Florida Keys, where it may be adversely affecting the coral reefs and other oligotrophic ecosystems there.

    In conclusion, it is hypothesized that if more freshwater from the Everglades-agricultural system is pumped into Florida Bay, as proposed (United States Army Corps of Engineers and South Florida Water Management District, 1999), the algal blooms will increase and the ecological problems of Florida Bay will get worse, not better. It is also hypothesized that if more passages along the Florida Keys between Florida Bay and the coral reefs are opened up, as proposed, the coral reefs will experience lower water quality.

 

Acknowledgements

    Funding for this research has been provided by the South Florida Ecosystem Restoration Prediction and Modeling Program of NOAA's Coastal Ocean Program, EPA's Florida Keys National Marine Sanctuary Water Quality Protection Program, and The Morris Family Foundation. I have been helped with much of the field sampling, sample processing, and data processing by Amie Gimon and Maiko Suzuki. Fred Tooker has taken us throughout Florida Bay in his boat many times for sampling and has provided excellent insights into the bay and its history. Numerous others have helped with field sampling on various occasions, particularly Dave Forcucci. Discussions with Brian Lapointe over the years have provided many insights into South Florida ecosystems. The website maintained by Joe Boyer showing the data collected in the FIU water quality monitoring program has been extremely useful and stimulated some of the ideas presented here. The nutrient data and water flow data were taken from South Florida Water Management District hydrometeorological and water quality data bases and we thank Angela Chong for helping us obtain data from those files.

 

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Table 1. Highest salinities observed in various studies in Florida Bay (from McIvor et al., 1994).

Years

1955-1957
1956-1958
1964-1965
1965-1966
1967-1968
1973-1976
1984-1985
1984-1986
1986-1987
1989-1990

Highest salinity observed (psu)

70.0
57.8
65.3
67.7
44.8
66.6
44.0
50.0
46.6
59.0

 

Figure Legends

Figure 1. Map of South Florida. Dashed line shows boundaries of the Kissimmee-Lake Okeechobee-Everglades watershed.

Figure 2. Salinities in Florida Bay measured from June 1989 to August 1990. (redrawn from Fourqurean et al., 1992)

Figure 3. Estimated annual water flow into Florida Bay through the South Dade Conveyance System. Data are calculated as flow through S332, S175 and S18C structures minus S197. S332 and S175 pump water into the Taylor Slough system. S18C pumps water into northeast Florida Bay and Barnes Sound. S197 flow into Barnes Sound is subtracted to leave only water entering Florida Bay directly. (calculated from SFWMD data files)

Figure 4. Rainfall data taken from Climatological Data Florida, NOAA.

Figure 5. Location of the major seagrass dieoff in 1987. (redrawn from Robblee et al., 1991 and Stumpf et al., 1999)

Figure 6. Chlorophyll concentrations measured between February, 1996 and January, 2000. The data (approximately 2850 measurements with 3 replicates each) were plotted using fixed radius surface interpolation. For chlorophyll measurements, three 100 ml replicate water samples were filtered (after adding 1 mg of MgCO3) through GF/F glass fiber filters and the filters frozen until extracted (within a few days). These filters were then extracted for 60 minutes with 10 ml of dimethyl sulfoxide and then with an added 15 ml of 90% acetone at 5oC overnight and measured fluorometrically before and after acidification for the measurement of chlorophyll and phaeopigment concentrations (Burnison, 1980; Parsons et al., 1984). Fluorescence measurements were made with a Turner Designs 10-000R or 10-AU fluorometer equipped with an infrared-sensitive photomultiplier and calibrated using pure chlorophyll a.

Figure 7. Average concentrations of dissolved inorganic N measured in monthly sampling from 1991 to 1999. Data were plotted using nearest neighbor surface interpolation. (calculated from SFWMD data files)

Figure 8. Average concentrations of total P measured in monthly sampling from 1991 to 1999. Data were plotted using nearest neighbor surface interpolation. (calculated from SFWMD data files)

Figure 9. Average ratios of dissolved inorganic N to total P measured from 1997 to 1998. (calculated from SFWMD data files)

Figure 10. Results of approximately 1000 nutrient bioassay experiments conducted between March 1998 and November 1999. Nutrient bioassays were conducted by adding either no nutrients (as a control); 250 mM NO3; 25 mM PO4; or 250 mM NO3 and 25 mM PO4 (as a control) to 30 ml water samples and monitoring algal biomass daily fluorometrically for 1 to 2 months. The highest in vivo chlorophyll fluorescence measurements achieved in each enrichment were compared to determine if the initial water samples were originally N or P limited. An index of nutrient limitation was calculated using the equation:

FLN - FLP
FLN + FLP

where FLN is the highest chlorophyll fluorescence observed in the incubation with 250 mM NO3 added and FLP is the highest chlorophyll fluorescence observed in the incubation with 25 mM PO4 added. Data are plotted using fixed radius surface interpolation. Positive numbers indicate N limitation and negative numbers indicate P limitation.

Figure 11. Bioassay data collected along the southwest coast of Florida in November, 1999 and combined with the Florida Bay bioassay data shown in Figure 10. All data are calculated as in Fig. 10.

Figure 12. Location and thickness of phosphorite-rich quartz sand deposits. (from Cunningham et al., 1998)

Figure 13. Average total P concentrations in South Florida measured in monthly sampling from 1996 to 1998. Data were plotted using nearest neighbor surface interpolation. (calculated from SFWMD data files)

Figure 14. Average salinity measured with a YSI meter between February, 1996 and January, 2000. The data (approximately 2540 measurements) were plotted using fixed radius surface interpolation.

Figure 15. Map of Everglades Agricultural Area (EAA), Water Conservation Areas (WCA), and South Florida Water Management District (SFWMD) canal system. (redrawn from Light and Dineen, 1994)

Figure 16. Changes in sugar cane farm area in EAA over time. (data from Snyder and Davidson, 1994).

Figure 17. Annual water flow through S2 pump station. Negative values indicate backpumping north from the Everglades Agricultural Area into Lake Okeechobee. Positive values indicate pumping south from the Everglades Agricultural Area into the Everglades. (calculated from SFWMD data files)

Figure 18. Map of South Florida Water Management District canal system. (redrawn from Light and Dineen, 1994)

Figure 19. Map of the South Dade Conveyance System. (from Light and Dineen, 1994)

Figure 20. Average concentrations of total N and total P in 1998 along the canal system from the Everglades Agricultural Area to Florida Bay. (calculated from SFWMD data files)

Figure 21. Location of 4 SFWMD stations superimposed on a map of chlorophyll measured as in Fig. 6.

Figure 22. Annual water flow through the Taylor Slough system as measured at structures S332 and S175 and annual chlorophyll concentrations measured at the 4 stations shown in Fig. 21. (calculated from SFWMD data files)

Figure 23. Monthly water flow through the Taylor Slough system as measured at structures S332 and S175 and annual chlorophyll concentrations measured at the 4 stations shown in Fig. 21. Each year is calculated from March to the following February to avoid splitting water flow during the high runoff period. (calculated from SFWMD data files)

Figure 24. Average chlorophyll concentrations (measured as in Fig. 6) during the dry season (February - April) 1996 to 1999 plotted using fixed radius surface interpolation .

Figure 25. Average chlorophyll concentrations (measured as in Fig. 6) during the rainy season (September - November) 1996 to 1999 plotted using fixed radius surface interpolation .

Figure 26. Satellite imagery of reflectance taken November 2, 1997 and processed by Richard Stumpf. (from website http://coastal.er.usgs.gov/flbay/html/199711/19971102_13ref.tif)

Figure 27. Chlorophyll concentrations (measured as in Fig. 6) at stations shown in Fig. 28 on November 4, 1997.

Figure 28. Station locations where samples were taken November 4, 1997.

Figure 29. Satellite imagery of reflectance taken March 23, 1996 and processed by Richard Stumpf. (from website http://coastal.er.usgs.gov/flbay/html/199603/19960323_14ref.tif)

Figure 30. Salinity measured with a refractometer on a transect from Big Pine Key to the Gulf Stream on March 23, 1996.

Figure 31. Turbidity measured with a Lamotte 2008 turbidity meter on a transect from Big Pine Key to the Gulf Stream on March 23, 1996.

Figure 32. Chlorophyll concentrations measured as in Fig. 6 on a transect from Big Pine Key to the Gulf Stream on March 23, 1996.

Figure 33. Trajectories of drifters released near the mouth of Shark River. (from Lee and Williams, 1999 and website: http://mpo.rsmas.miami.edu/flabay/latest.html#table)

Figure 34. Average annual chlorophyll concentrations and salinity at stations shown in Fig. 35. (calculated from SFWMD data files)

Figure 35. Location of 5 stations sampled 6 times a year from 1996 to 1999. (from SFWMD data files)

Figure 36. Annual nitrogen flux through the Shark River as measured at the S12A, S12B, S12C, and S12D structures (SFWMD data files) and average chlorophyll concentrations measured by Brian Lapointe near Looe Key in the lower Florida Keys (Lapointe et al., 2000). N flux is calculated monthly by multiplying N concentrations times water flow rates.