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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan 2 Potential Effect of the Comprehensive Everglades Restoration Plan (CERP) on Florida Bay An important assumption often made by scientists and managers associated with the Comprehensive Everglades Restoration Plan (CERP), and by the public, is that the increased flows of water deemed necessary to restore Everglades habitats also will contribute to the restoration and enhancement of Florida Bay (e.g., SFWMD, 1998). This assumption appears to rest on three pillars: (1) that there has been an undesirable (but poorly quantified) long-term increase in salinity in the Bay as a result of water management practices beginning around the 1920s (McIvor et al., 1994) and possibly also as a result of changes in circulation because of railroad construction in the Keys during the early 1900s (Halley and Roulier, 1999); (2) that the increase in salinity resulted in increasing frequency, severity, and duration of hypersaline conditions in parts of the Bay, and a corresponding decrease in the spatial and temporal extent of oligohaline-mesohaline conditions; and (3) that the restoration plan will ameliorate this problem by delivering more freshwater to the Bay than it currently receives. Hypersalinity is undesirable, at least in part, because it is thought by some researchers to have been a major factor leading to a dramatic die-off of Thalassia around 1987. While the Thalassia die-off was first attributed to a direct salinity effect (Robblee et al., 1991), various alternative hypotheses have been advanced more recently. These include hypoxia and sulfide toxicity (Carlson et al., 1994), loss of the estuarine nature of the Bay, overdevelopment of seagrass beds, abnormally warm late summer and fall water temperatures, sedimentation due to a lack of severe storms (Zieman et al., 1988), eutrophication (Lapointe and Clark, 1992), pathogens (Durako and Kuss, 1994), and the decline of herbivory due to the decline of green turtle and manatee populations (Jackson et al., 2001). Of these hypotheses, hypersalinity and eutrophication are mostly likely to be influenced by the CERP. The view that increasing freshwater flow to the Everglades and to Florida Bay is a “win-win situation” may not be entirely correct. Brand (2002) concluded that “if more freshwater from the Everglades agricultural system is pumped into Florida Bay, as proposed, the algal blooms will increase and the ecological problems of Florida Bay will get worse, not better.” Brand’s concern was not with the freshwater itself, but with the nutrients, especially nitrogen, that it would contain. Another review of recent changes in the ecology of Florida Bay and the coral reef communities of the Florida Keys also concluded that excess nitrogen inputs were responsible for many undesirable changes in the marine system, and that the CERP had not addressed this issue adequately (Lapointe et al., 2002). Furthermore, Prager and Halley (1999) and Halley (2002) cautioned that estuarine conditions, which might result from restoring freshwater flows to the Bay to their historic levels could lead to increased turbidity from suspended sediment and loss of the “gin clear” bay remembered to have occurred in the 1960s and 1970s. Indeed, anecdotal evidence exists for generally turbid conditions in Florida Bay 100 years ago. Gregg (1902) stated that “Florida Bay…is a shallow bay, and the water is usually roily and whitish from the disintegration of the coral rock,” and “Now we will have quite a sail through milky water, until we near
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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan Tavernier Creek.” Thus, Halley (2002) described the potential increase in turbidity as “simply a natural response to creating a bay more like the one that existed at the turn of the century.” This vision, however, is different from the view that associates pristine conditions in the Bay with clear water. This chapter first examines anticipated changes in patterns of freshwater inflows to the Bay under the CERP, and then considers whether these changes, which are designed to lessen the likelihood of hypersaline conditions developing in the Bay, may be undermined by the accompanying influx of nutrients. Freshwater Inflows to the Bay Simulating past, present, and future freshwater inflows to Florida Bay is challenging due to the large number and tidal nature of the creeks that drain the coastal mangrove zone, and to poor linkages between Everglades and Florida Bay models (see Chapter 3). Some flow measurements are now being made (e.g., Hittle et al., 2001), but flows are more commonly modeled. To date, the Natural System Model (NSM) has been the primary tool for simulating the hydrologic behavior of the pre-drainage Everglades. The South Florida Water Management Model (SFWMM) has been used to simulate the system infrastructure and operations as they are currently (“1995 Base”), as they will be in 2050 without any CERP projects in place (“2050 Base”), and as they could be in 2050 with CERP projects completed (“D13R”). The models are discussed in Chapter 3, and the model runs are described in more detail in Appendix B. Taylor Slough and Craighead Basin Direct fresh surface water flow into Florida Bay occurs primarily through Taylor Slough and, to a lesser extent, Craighead Basin. These modeled flows are shown in Figure 3. Slightly to the east, southward flow through the C-111 canal to the Eastern Panhandle area of Everglades National Park (Figure 3, right-most set of bar graphs) also occurs. Most of this flow, however, does not end up in Florida Bay. Rather, it is discharged east of U.S. Hwy 1, eventually passing into Barnes Sound and thence northeastward through Card Sound to Biscayne Bay. Very minor quantities of Eastern Panhandle water discharge to northeastern Florida Bay through the degraded C-111 embankment (Richard Punnett, USACE, Personal commun., July 2002). From Figure 3 it can be seen that more water flows into the Eastern Panhandle under the 1995 Base or “current condition” relative to the NSM simulation. This is reduced under alternative D13R4 (a scenario based on D13R that would capture additional water “lost” to tide). The surplus (119,000-50,000=69,000 acre-ft per year (8.5x107 cubic meters per year) would be redirected upstream into Shark River Slough and downstream to a very small degree into Craighead Basin and Taylor Slough, in an effort to replicate NSM flows (although CERP flow targets may differ from NSM estimates). Hence, simulated fresh surface water flows to Florida Bay through Craighead Basin and Taylor Slough increase only minutely from 32,000+94,000=126,000 acre-ft per year (15.5x107 cubic meters per year) under the 1995 Base scenario to 45,000+82,000=127,000 acre-ft per year (15.6 cubic meters per year) under the D13R4 scenario. The Taylor Slough flow under the D13R4 scenario would be close to the NSM-estimated flow shown in Figure 3.
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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan FIGURE 3 Average annual overland flows toward Florida Bay across Craighead Basin, Taylor Slough and Eastern Panhandle for the 31-year simulation. Comparison of surface water flows in eastern Everglades National Park (ENP). Flow is generally southward across the Craighead Basin-Taylor Slough-Eastern Panhandle (CB-TS-EP) cross-section (shown in inset map of northeastern ENP; see Figure 1 for larger map). CB, TS, and EP are each an eight-mile long segment of the cross-section. Output is shown for three different simulations: Natural System Model (NSM), 1995 base or “current condition” (95B) and D13R4 (a variation of D13R, which is the year 2050 CERP simulation from USACE, 1999). Short descriptions of these are given in the text; more complete definitions are given in Appendix B. Note: Note: NSM water depths at key ENP gage locations are used as operational targets for most alternatives. NSM flows are NOT targets and are shown for comparative purposes only. Source: USACE, 2002. The total annual flow is, of course, only one aspect of the data. The distribution of flow between the wet and dry seasons (shown in the Figure 3 graphs) may be important as well. In addition, the simulated averages do not reflect annual variability in discharges; this must be accounted for when analyzing the full 31-year output from the SFWMM and its ultimate interface with Florida Bay modeling. Finally, changes to rainfall-based water management practices that occurred between the mid-1980s and 1995 resulted in increasing the amount of freshwater flow into eastern Florida Bay, relative to rainfall, at least since 1993 (Sklar et al., 2002). The 1995 base is a simulation for the present period since those changes occurred; it does not represent conditions of the preceding decades, which are less well known. Overall, however, total fresh surface-water inflows to Florida Bay via Craighead Basin and Taylor Slough are predicted to be about the same with CERP as under current conditions. If these predictions are correct, the salinity of this region of the Bay may not change materially. The lack of an operational hydrodynamic model (Chapter 3) increases the uncertainty of such predictions. Shark River Slough In contrast to these minor proposed changes to flows in Craighead Basin and Taylor Slough, the CERP plans a dramatic increase in flow down Shark River Slough relative to the current condition (Figure 4). This is significant, because recent measurements strongly suggest that some of this flow eventually will reach the inner parts of Florida Bay. A detailed review by Smith and Pitts (2002)
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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan indicated, “a generally west-to-east movement of water through the interior of the Bay that eventually exits through the tidal channels between keys on the southeastern and southern sides of the bay.” These authors concluded their analysis of 15 years of physical observations by emphasizing that “[a]veraging over tidal periods and the longer time scales associated with meteorological forcing…reveals transport pathways that represent a clear coupling between Gulf and Atlantic sides of the Keys. Gulf-to-Atlantic transport can be either around the Keys…or it can involve a more complex route through Florida Bay and the tidal channels…” Numerous drifter studies have shown that Shark River Slough water tends to pass along the western boundary of Florida Bay and must often have access to the central Bay (Lee et al., 2002). Boyer et al. (1999) believed that they could see the effect of “a freshening of the waters of the southwest Florida Shelf from Shark Slough drainage” on salinity declines in western Florida Bay. Additional qualitative information about the linkage of the Bay with Shark River Slough discharge is provided by D’Sa et al. (2002) on the basis of remote sensing of salinity patterns. These researchers concluded that “Gulf waters entering the bay primarily from the northwest (near East Cape) entrain freshwater from the Shark River Slough and other smaller rivers in southwest Florida as indicated by the lower salinities observed in the vicinity of Cape Sable…” (Figure 2). FIGURE 4 Average annual overland flows toward Whitewater Bay and Florida Bay for the 31-year simulation period. Comparison of flows across Shark River Slough (SRS) and Craighead Basin/Taylor Slough/Eastern Panhandle (TS) cross-sections for different modeled scenarios. (Cross-section locations are shown in inset map of northeastern Everglades National Park; see Figure 1 for larger map.) Definitions of NSM45F (Natural System Model), 95BSR (1995 base or “current condition”), 50BSR (2050 base or “without project condition”), and D13R4 are given in Appendix B. The TS data include Eastern Panhandle flows that discharge to water bodies other than Florida Bay (see Figure 3 for details); total flows to Florida Bay for the 1995 base are actually similar to those of the other model runs. Note: NSM water depths at key ENP gage locations are used as operational targets for most alternatives. NSM flows are NOT targets and are shown for comparative purposes only. Source: USACE, 1999. (Also available at http://www.everglades-plan.org/pub/restudy_eis.shtml.)
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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan Groundwater The long-term effect of the CERP on the magnitude and salinity of groundwater fluxes to Florida Bay is uncertain. At present, the freshwater-saltwater interface in the surficial aquifer system is at least six kilometers inland of the Bay (Fitterman and Deszcz-Pan, 2001). The salinity of shallow groundwater along the coast of the mainland is close to that of seawater (Fitterman and Deszcz-Pan, 2001). For this reason, discharge of fresh groundwater from the Everglades actually occurs on the mainland, and subsequently enters the Bay as surface runoff. The discharge volume is estimated to be small relative to that of surface water by CERP modelers (USACE, 1999). However, if the CERP raises overall water levels in the southern Everglades, the freshwater-saltwater contact in the surficial aquifer system may be pushed southward over time toward Florida Bay in certain areas along the coast, and could result in fresh groundwater discharge directly to the Bay. This might result in a small net increase in freshwater from this source because the loss due to evapotranspiration during the transit between inland groundwater discharge and the Bay would be eliminated. Although discharge of fresh groundwater to the Bay appears to be negligible under current conditions, there have been several attempts to estimate the magnitude of saline groundwater discharge. Saline discharge can be the result of buoyant counterflow of saline porewater induced by flow in a freshwater lens (Kaufman, 1994), geothermal convection induced by temperature differences between ocean water and fluids in the interior of a carbonate platform (Sanford et al., 1998), or local recirculation driven by waves and tidal pumping (Li et al., 1999). Recent estimates are based on the chemical tracers 222Rn, CH4, and 4He, and seepage meters (Corbett et al., 1999, 2000; Top et al., 2001). Corbett et al. (1999, 2000) estimated a discharge of 1 to 3 centimeters per day, which implies a flux for the Bay as a whole of 7 to 22x109 cubic meters per year. Top et al. (2001) reported even higher estimates of 6 to 12 centimeters per day, or about 45 to 85x109 cubic meters per year. These values are many times larger than the direct surface freshwater flow into northeast Florida Bay in the very wet year of 1997–1998 (0.24x109 cubic meters per year; Patino and Hittle, 2000). It is difficult to imagine how this modest freshwater input could have resulted in the observed reductions in salinity in north-central and eastern Florida Bay if there were a large net input of saline groundwater. The upper estimates of saline groundwater flow would displace a one meter deep water column in 8 to 16 days, while the average residence time of water in the isolated basins of this region of the Bay is likely on the order of months (George Jackson, Texas A&M University and Ned Smith, Harbor Branch Oceanographic Institution, personal commun., April 2002). All of these estimates seem unreasonably high unless these fluxes are dominated by recirculation of Bay water through shallow aquifers under and adjacent to the bay. A comparative study of groundwater flux estimation techniques in the northeastern Gulf of Mexico (Burnett et al., 2002) noted that estimates from seepage meters and tracer measurements may include flow due to tidal pumping and wave action, which are generally not included in estimates from steady-state groundwater flow and transport models. Such recirculation might be an important mechanism to transfer nutrients from the aquifer to the Bay, but it would not represent a net input of water. Nevertheless, discharge of recirculated saline groundwater to the central and eastern Bay may influence local water budgets, especially during droughts. Nutrient Fluxes to the Bay In the previous section, it was shown that there are significant unknowns with respect to the volumes and spatial patterns of freshwater flows to Florida Bay that will result from the restoration effort. In this section, the possible effects on the Bay of the nitrogen and phosphorus that may accompany those flows is discussed. There appears to be consensus that the Everglades wetlands intercept large quantities of phosphorus and that the phytoplankton of eastern Florida Bay are phosphorus limited. Seagrasses throughout the Bay appear to be phosphorus limited (Fourqurean et al., 1992). There is less agreement
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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan about nutrient issues in the central and western Bay. There, the ratio of dissolved inorganic nitrogen (DIN) to dissolved reactive phosphate (DIP) usually suggests strong phosphorus limitation of phytoplankton growth (Boyer et al. 1999), but many bioassay measurements indicate that nitrogen is the nutrient of chief concern for phytoplankton and macroalgal blooms (Brand, 2002; Tomas et al., 1999). It is difficult to reconcile these two lines of evidence unless a significant fraction of the dissolved organic phosphorus (DOP) in Florida Bay is also accessible to the phytoplankton while much of the DON is not. Brand (2002) has developed this argument and summarized data indicating that phytoplankton blooms develop in the zone where relatively nitrogen-rich but phosphorus-poor water from the eastern Bay mixes with the phosphorus-rich but nitrogen-poor water flowing south along the inner southwest Florida shelf. There also is strong evidence that nitrogen (mostly in dissolved organic form) and phosphorus are exported as a function of water flow from the Everglades (Rudnick et al., 1999). Thus, it seems likely that increasing water flows through Shark River Slough will result in a larger net flux of nitrogen and phosphorus out of the Everglades. It is interesting with regard to the CERP that Rudnick et al. (1999) also found an apparent net increase in phosphorus concentration as water from Shark River Slough passed through the mangrove zone. While the source of this phosphorus remains unknown, it persisted through the three-year period they analyzed. Brand (2002) hypothesized that the relatively elevated phosphorus concentrations of southwest Florida shelf water may be from phosphorite-rich quartz sand deposits that underlie much of the northwestern Bay. The hydraulic potential in the underlying Upper Floridan aquifer is higher than that of the Bay (Bush and Johnston, 1988), which would force groundwater upward through these sand deposits. A major uncertainty with regard to effects of CERP-modified water deliveries to the Bay on nutrient loading/limitation in the Bay concerns the role of dissolved organic nitrogen (DON) in promoting the growth of phytoplankton and macroalgae. DON constitutes by far the largest fraction of the total nitrogen concentration in both the freshwater sources to the Bay and in the Bay water itself (Rudnick et al., 1999, Boyer et al., 1997). Until recently, research on the role of nitrogen in Florida Bay focused primarily on dissolved inorganic nitrogen (DIN). However, globally 70 percent of the dissolved nitrogen transported to the sea in rivers is DON (Maybeck, 1982). In temperate regions, the proportion of the total dissolved nitrogen (TDN) pool consisting of DON rises during warm summer months when nitrate concentrations decrease in freshwater (Pardo et al., 1995) and marine (Carlsson and Graneli, 1998) environments. This suggests that in subtropical regions like south Florida, DON may be an even greater fraction of the TDN. Worldwide, few studies have evaluated the bioavailability of DON, and even fewer have characterized DON in terms of its chemical structure (Stepanauskas et al., 1999). DON is commonly defined operationally as the organic nitrogen that passes through a filter with a nominal pore size of 0.45 µ. Thus defined, DON includes the colloidal fraction (including some bacterial biomass that is rich in nitrogen), as well as molecules that are in a truly dissolved state. Although most of the nitrogen in organisms occurs in amino acids, only a small fraction of DON, perhaps 5–10 percent, can be identified as rapidly cycling free amino acids, amines, and urea (McCarthy et al., 1997). This fraction (i.e., amino acids and urea) may be used by some algae (Antia et al., 1991); higher molecular weight DON must be mineralized for the nitrogen to become available to phytoplankton, macroalgae, or submerged aquatic plants. The majority of DON appears to be “contained in amide functional groups, suggesting that most long-lived DON is hydrolysis-resistant or composed of recalcitrant non-protein amide-containing biochemicals” (McCarthy et al., 1997) raising questions about the bioavailability of high molecular weight DON. The prevailing view that DON exported from terrestrial systems is largely refractory has been challenged by recent studies. Several investigators have shown that heterotrophic bacterioplankton mineralize humic-bound DON (Carlsson and Granéli, 1998; Carlsson et al., 1993; Bushaw et al., 1996; Carlsson et al., 1999), and Seitzinger and Sanders (1997) demonstrated that up to 80% of total DON was metabolized by bacteria in Hudson and Delaware River water. Furthermore, in well-mixed coastal systems like Florida Bay, photochemical breakdown of DON has been shown to enhance DON availability to microbes (Bushaw-Newton and Moran, 1999; Tarr et al., 2001; Wiegner and Seitzinger,
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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan 2001; Koopmans and Bronk, 2002). Recent laboratory work by Seitzinger et al. (2002) using water from Barnegat Bay, a New Jersey estuary, showed complex, non-linear relationships between phytoplankton production and DON addition. DON bioavailability varied seasonally, but overall, urban/suburban storm water runoff had a higher proportion of bioavailable DON (59%±11) than pastures (30%±14) and forests (23%±19). Preliminary chemical analyses of Everglades DON by Rudolf Jaffe at Florida International University showed that it contains a significant (5–10%) fraction of proteins and other compounds that should be readily available to bacteria (Rudolf Jaffe, FIU, personal commun., 2001). The relatively long residence time of water within eastern Florida Bay (on the order of months; see earlier references) in contrast to typical bacterial generation times as high as once per day (Joseph Boyer, FIU, personal commun., April 2002) suggests that residence times of this magnitude provide ample time for bacteria to mineralize a significant fraction of the DON entering this portion of the system. Like DON, little is known about the reactivity of dissolved organic phosphorus (DOP) in coastal systems, although Brand (2002) has suggested that it is likely to be more available than DON. As with nitrogen, DOP makes up the greatest fraction of the total phosphorus entering Florida Bay in creek discharge from the Everglades (Rudnick et al., 1999). However, the contribution of this source of phosphorus to the Bay is likely only a small fraction of the Bay’s phosphorus budget (Rudnick et. al., 1999). In addition to the large source of phosphorus entering Florida Bay from the Gulf of Mexico, internal Bay phosphorus-cycling (e.g., algal and bacterial alkaline phosphatase activity) (Beardall et al., 2001; Wright and Reddy, 2001) and macrophyte mining of sediment phosphorus (Jensen, et. al., 1998) could be significant. A preliminary evaluation by Rudnick et al. (1999) of nitrogen and phosphorus inputs to Florida Bay from various sources deserves comment. Their analysis led the authors to conclude that the flux of nitrogen and phosphorus from the Everglades is so small compared with other sources that it likely would not affect the Bay if it increased modestly. Although the inventories of nutrient inputs that Rudnick et al. (1999) developed for the entire Florida Bay and its eastern portion provide a useful perspective for some issues, it is not clear that they are adequate to evaluate the finer-scale spatial and temporal patterns of the phytoplankton blooms. For example, while nutrient discharges from a sewage treatment plant may be a small part of the total nitrogen or phosphorus input to an estuary, they can have large impacts on the areas where the effluents are discharged. Similarly, the higher loadings of nitrogen and phosphorus that may accompany increasing freshwater fluxes from the Everglades could increase the frequency, intensity, and duration of phytoplankton blooms in certain regions of Florida Bay, even though they would be small relative to nutrient flows through the Bay from the western border or from the atmosphere. Boyer et al. (1999) noted that increased freshwater flows from the Everglades to the eastern Bay could account for some water-quality changes, such as lower salinity in that region. Correlations between water discharge from the Everglades and phytoplankton blooms (chlorophyll a) in the north central Bay (Brand, 2002) also provide some circumstantial evidence that nitrogen enhancement of phytoplankton growth may be a consequence of increased freshwater flow from the Everglades. Once generated, such blooms may be spread over larger areas within the Bay or carried through the Keys to the coral reefs (Smith and Potts, 2002). There is some reason to be concerned that impacts to Florida Bay that would be caused by any increased nitrogen or phosphorus discharges that may occur through Shark River Slough are not fully understood. Smith and Potts (2002) and Lee et al. (2002) show a slow but steady current of water through the central Bay from the west that likely contains some fraction of Shark River Slough water and nutrients. As noted earlier, Boyer et al. (1999) observed declining average salinity in the central and western Bay with increasing fresh water discharge from Shark Slough. If the currents in 2050 are similar to present patterns, the central and western portions of Florida Bay would be exposed to increased nitrogen fluxes from Everglades restoration, even if water flows remain about the same in Taylor Slough and the eastern Bay. In conclusion, the ecological response to increased freshwater discharge to Florida Bay seems less certain than it once appeared. Moreover, it seems likely that increased freshwater flow, if it does occur, will increase nutrient loading to the Bay, whether it comes directly into the Bay or indirectly
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Florida Bay Research Programs and their Relation to the Comprehensive Everglades Restoration Plan through Shark River Slough. Field and laboratory observations provide circumstantial, but strong, evidence that the response of these marine ecosystems, which historically have been very low in nutrients, to increased nutrient loading will be an increase in phytoplankton blooms and a decrease in water clarity. The effects of such a fundamental water column shift on the seagrasses and associated resources of Florida Bay will be important to resource managers in the region, particularly because it is likely that these changes will be viewed by many as undesirable.
Representative terms from entire chapter: