Progress Toward Restoring the Everglades The Second Biennial Review - 2008 (2008) / Chapter Skim
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5 Lake Okeechobee and Its Place in the Restoration of the South Florida Ecosystem
5 Lake Okeechobee and Its Place in the Restoration of the South Florida Ecosystem
Pages 143-188
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From page 143... ...
do not have the capacity to treat increased volumes of nutrient-enriched water. Changes in water quantity and degradation of water quality (Havens and Gawlick, 2005; Johnson et al., 2007)
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This chapter explores several facets of the management of Lake Okeechobee and the potential role it might play in the Everglades restoration. The chapter also includes a discussion of the downstream effects of the disturbed lake, including
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Figure 5-2.eps bitmap impacts on the estuaries that receive direct flows from it. Allowing the lake to function as the heart of the Everglades as it used to in the pre-drainage system requires large additional restoration efforts.
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when water levels reach 20 feet above mean sea level. When the lake is at an elevation of 9 feet, a very low level, it contains about 1.75 million acre-feet (MAF)
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estimated that before the first dike was constructed, the lake had a mean stage of 20.5 feet (USACE, 1999) , but today the USACE aims to maintain the water level at about a 12-foot elevation to protect the integrity of the Herbert Hoover Dike (USACE, 2006)
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Lucie River (as regulatory discharges and environmental releases) , 7 percent to the lower east coast (as regulatory discharges)
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When it does so in the watershed, it contributes to the external phosphorus load to the lake; when it does so in the lake, it creates an internal phosphorus load to the lake water. The effects of legacy phosphorus on water quality can last several decades.
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The recent loads represented a decline from the previous 5-year average of 715 mt, largely due to the drought in 2007 when the total phosphorus load to the lake was 203 mt (Figure 5-5)
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Internal Phosphorus Loads Excessive external phosphorus loads to the lake have accumulated in mud sediments in the center of the lake (Figure 5-6) , and they create the current internal phosphorus loads to the lake water column.
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. These internal loads occur through diffusive flux of phosphorus from sediments to overlying water and during resuspension of surface sediments into the water column during wind events.
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Implications of Legacy Phosphorus Once the external phosphorus loads from uplands are curtailed through the implementation of best management practices (BMPs) and other phosphorus management strategies in the drainage basin, the critical question concerns how the lake will respond to reduction of the external phosphorus load (Havens et al., 2007)
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, although the littoral zone before the lake was diked is not as well documented. The size, community composition, and geographic arrangement of these communities have been strongly affected by the Herbert Hoover Dike and by water level fluctuations in the lake.
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The littoral zone provides essential habitat for fish, wading birds, and other animals for nesting and feeding, and functions like the SAV zone as a keystone community to structure lake food webs and to affect water quality through uptake and stabilization or remobilization of P-rich bottom sediments (Havens and Gawlick, 2005; Johnson et al., 2007)
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. The South Florida Water Management District (SFWMD)
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However, reduced use appears to be associated with declines in kite foraging habitat from prolonged periods of both high and low water levels that have affected apple snail (Pomacea paludusa) populations (USFWS, 2007)
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Controlling populations of planktivorous fishes that reduce zooplankton grazing or benthic fishes that cause sediment resuspension -- either by removing them or by introducing larger fish to prey on them -- has been used to assist recovery in some lakes. Drawing down water levels to remove sediments or to promote vascular plant growth also has been used, as is illustrated by a short-term experimental drawdown of the lake in 2000 (Steinman et al., 2002b)
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EFFECTS OF THE LAKE'S CONDITION ON THE SOUTH FLORIDA ECOSYSTEM The construction of levees, canals, and other water control structures has significantly altered Lake Okeechobee and its interactions with the South Florida ecosystem. The Herbert Hoover Dike interrupted the southward flow of water, and the water quality difficulties described above pose considerable constraints to the movement of water from Lake Okeechobee to the southern part of the ecosystem in its current condition.
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. Lake Worth Lagoon was historically a freshwater lake, but the creation of permanent inlets has made it estuarine.
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. These hydrologic changes, particularly as a result of drainage from Lake Okeechobee and the northern Everglades, have resulted in large ecological changes and a reduction in productivity of living resources.
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and C-139 basins have been identified as the major sources impacting the downstream Everglades Protection Area. Source control strategies such as BMPs and STAs have been used to reduce phosphorus loads from these basins to the Everglades Protection Area (Adorisio et al., 2007)
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During the 2006 Florida legislative session, moderating provisions were added to the Everglades Forever Act that effectively extended the water quality compliance deadline until 2016, at the earliest. Special attention was given to water quality in Lake Okeechobee with passage of the Lake Okeechobee Protection Act (LOPA)
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All monitoring sites must: • have a geometric mean of total phosphorus concentrations of less than or equal to 10 ppb in 3 of 5 years, • have annual total phosphorus concentrations of less than or equal to 11 ppb across all stations, • have total phosphorus concentrations less than or equal to 15 ppb annually at all monitoring stations, and • have a five-year geometric mean of total phosphorus concentrations aver aged across the network of less than or equal to 10 ppb.
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rapid spread of exotic and nuisance plants. Improving water quality and the hydrologic regimes in and around Lake Okeechobee is critical to the long-term success of the Everglades restoration and to improving the condition of the northern estuaries.
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However, unless additional source controls are implemented to reduce phosphorus loads to the lake, the lake would progressively return to the original contaminated state, because the surface of aluminum oxy-hydroxides would become fouled and buried with sediments over time.
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That reality may make achieving water objectives downstream of the lake much more difficult. Managing External Phosphorus Loads As noted in Box 5-2, over the past several decades a variety of federal and state agricultural programs have been used to reduce the flow of phosphorus from watersheds that empty into Lake Okeechobee.
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It provides good background on earlier actions to improve water quality and ecological conditions of the Kissimmee River–Lake Okeechobee watershed. It also provides an extensive list and assessment of management measures relative to their state of design, likelihood of implementation, state of information about benefits, and costs.
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All watersheds that flow toward the lake are covered by the plan. LOWCP-II lists and briefly discusses approximately 120 management measures that contribute to storage capacity and/or phosphorus load reduction (the water storage components are discussed in more detail later in this chapter)
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. TABLE 5-2 Preferred Plan Features Management Measure Levela Local Project Features Lake Okeechobee Watershed Phosphorus Source Control Programs SFWMD Phosphorus Control Programs 1&2 FDACS Agricultural BMP Programs Supplemental Nonagricultural BMP Programs Land Management Programs Comprehensive Planning/Land Development Regulations 3 Farm and Ranchland Protection Program Partnership 4 Florida Ranchlands Environmental Services Project 2 Alternative Water Storage Facilities 1, 3, & 4 Local Initiatives 1 Regional Features Storage 4 Stormwater Treatment Areas 1–5 Reservoir Assisted Stormwater Treatment Areas 4 Aquifer Storage and Recovery (contingent on findings from test well)
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, application of soil amendments and/or soil removal can improve the overall longevity of STAs to maintain water quality. The first STA became operational in 1994 when just 3,800 acres of cells 1–4 of STA-1W were brought online (Figures 5-9 and 5-10)
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172 Progress Toward Restoring the Everglades FIGURE 5-9 Location of stormwater treatment areas.5-9.eps Figure bitmap SOURCE: Pietro et al.
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Although general processes by which STAs remove phosphorus were well known at the design stage, their actual performance could be determined only through a rigorous monitoring program. Several studies have been undertaken to investigate why certain parts of the STAs were performing below expectations (Pietro et al., 2008)
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. 200 180 STA 1 W Effluent Concentrations of Total P, ppb STA 2 160 STA 3/4 STA 5 140 STA 6 120 100 80 60 40 20 0 2000 2006 2004 2005 2003 2002 2007 2001 1996 1998 1999 1997 FIGURE 5-12 Effluent concentrations of total phosphorus of five stormwater treatment areas.
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This level of contamination was a substantial improvement from the survey conducted in September 1995 in which 57.8 ± 7.8 percent of the area exceeded total phosphorus concentration of 10 µg/L. While trends TABLE 5-3 Geometric Mean of Total Phosphorus Concentrations for the Everglades Protection Area for Water Years 2005–2007 Everglades Areas Inflow Interior Refuge 65.9 11.1 WCA-2A 26.2 14.8 WCA-3A 24.0 9.4 Park 9.8 5.8 NOTE: Concentrations are µg/L.
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Given the phosphorus loading to the STAs and their long-term removal efficiency, it seems unlikely that the current configuration will allow for the 10 µg/L geometric mean criterion to be achieved. To decrease the loading of phosphorus into the Everglades and ultimately to achieve the total phosphorus criterion, the STA area needs to be expanded north of the Everglades Protection Area to allow for greater capacity for phosphorus removal, and improvements are needed in watershed management practices to decrease the inputs of phosphorus to the STAs.
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Despite the generous detail provided for various components of the system (e.g., lake water phosphorus concentrations and loads, STA outflow concentrations) , the lack of integration of all the reports is a significant barrier to an evaluation of overall progress toward understanding and managing phosphorus loads to the Everglades Protection Area.
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Increasing Water Storage A fundamental premise of the CERP is that significantly increased water s torage is needed to improve the condition of the South Florida ecosystem, includ ing Lake Okeechobee, the estuaries, and the remnant Everglades ecosystem. As discussed previously in this chapter, modifications to the system (e.g., levees, canals, lake operations)
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13) includes a water storage reservoir (48,000 acre-feet)
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STAs are not included for the C 43 reservoir as they are for the Indian River Lagoon-South reservoirs. This deep water reservoir is not expected to reduce phosphorus or nitrogen concentrations greatly because of its lack of macrovegetation, frequently limited residence time, and susceptibility to sediment resuspension; thus, nutrient loading to the Caloosahatchee Estuary may exceed that needed to achieve water quality objec tives.
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Given the constraints on lake water discharges, there is a limited range of options for modifying the operating policy. Large releases to the Everglades ecosystem will be possible only with the appropriate conveyance and seepage management structures in place and reduced phosphorus concentrations, either from additional STAs or improvements to lake water quality.
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Optimal lake levels, however, are also determined by the desire to enhance conditions for lake biota and protect the lake's littoral zone. Nevertheless, rehabilitation of the Herbert Hoover Dike may offer synergistic opportunities for creating additional CERP storage and managing water levels for the benefit of the littoral zone, and the costs, benefits, and hydrologic and ecological viability of these options should be considered in any analysis of CERP storage alternatives.
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After all, the littoral zone did not exist at its current location historically; rather, it developed based on the management of the lake at lower water levels. An expanded dike configuration, if politically and societally feasible, could allow the lake to function at higher water levels once the dike has been rehabilitated.
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Flows discharged out of such a spillway might logically enter a flow way, although it is unclear as to the degree to which such flows could be integrated into CERP storage and conveyance needs, given the current water quality issues in Lake Okeechobee. CERP agencies continue to
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Increases water storage capacity of the Reduces water quantity through additional ET natural system. from standing water in flow way.
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However, the lake presently is plagued by both high and, more recently, very low water levels as well as poor water quality, especially phosphorus, that have affected its structure and functioning. The challenges of water quantity and quality in the lake have important ramifications for the entire ecosystem because the lake supports important elements of the region's biota, and it also has the potential to serve as a major source of water storage and water supply for downstream ecosystems.
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Given questions concerning the long-term effectiveness of STAs in phosphorus removal, the current phosphorus loadings to the STAs suggest that their current configuration will be insufficient to achieve the 10 μg/L phosphorus criterion in the Everglades Protection Area. Meanwhile, failure to achieve the water quality goals in Lake Okeechobee will affect the condition of the lake and the northern estuaries, and it will reduce the amount of additional water that can be delivered to the Everglades ecosystem.
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Short-term and long-term trade-offs will be needed in the rehabilitation of Lake Okeechobee and northern estuaries. Moving appropriate volumes of water south into the Everglades and managing flows into the northern estuaries may pose conflicts with sustaining adequate water levels for the lake biota and other in-lake goals, and until the Herbert Hoover Dike is rehabilitated, the risk of its failure at high lake levels will constrain options.
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