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5 Lake Okeechobee Regulation
Pages 133-158

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From page 133...
... Additionally, storage has implications for habitats and resident biota within the lake, including wading birds, a commercially valuable sport fishery, and the endangered Everglade snail kite. In the next few years, decisions will be made about the provision of storage by Lake Okeechobee as part of an upcoming Lake Okeechobee Regulation Schedule review, scheduled for 2019-2023 (USACE, 2018b)
From page 134...
... The chapter begins with an overview of Lake Okeechobee water quality and an update of the Herbert Hoover Dike rehabilitation project, which have important implications for a new regulation schedule. Next, the committee provides an overview of processes by which lake levels affect lake ecology, including uncertainties and key research needs.
From page 135...
... average annual load of total phosphorus to Lake Okeechobee (531 metric tons/year) greatly exceeds the Total Maximum Daily Load (TMDL)
From page 136...
... , the in-lake concentration of total phosphorus was 150 ppb, still far in excess of the goal of 40 ppb. The long-term increases in total phosphorus concentrations in the lake likely are a combined result of the high concentrations in inflowing waters, mobilization of phosphorus from the sediments, and a decreased ability of lake sediments to remove phosphorus from that water (Havens et al., 2007)
From page 137...
... HERBERT HOOVER DIKE REHABILITATION In 1930, Congress authorized the Herbert Hoover Dike, which now encircles most of Lake Okeechobee with 143 miles of embankment, five inlets/outlets,
From page 138...
... based on a regulation schedule that is a set of seasonally varying rules guiding lake operations. If lake level exceeds an upper boundary set by the regulation schedule, water must be released to reduce the risk of failure of the Herbert Hoover Dike (Figure 5-3)
From page 139...
... A USACE dam safety modification report (USACE, 2016d) identified the final measures needed to reduce intolerable risks in the remaining reaches of the Herbert Hoover Dike, based on the current lake regulation schedule.
From page 140...
... The rehabilitation project has already installed 21.4 miles of cutoff wall in Zone A
From page 141...
... USACE (2016d) noted that any revisions to LORS 2008 would require an updated Herbert Hoover Dike risk assessment, which could be undertaken concurrently with rehabilitation efforts.
From page 142...
... This section provides an overview of the key characteristics of Lake Okeechobee that influence the effects of chang ing lake levels on lake ecology and discusses potential ecological effects of a deeper lake. Conditions That Modulate the Effects of Altered Lake Levels To fully discern how changes in the magnitude, timing, and duration of high and low water affect the ecosystem, it is critical to understand some unique prop erties of Lake Okeechobee that influence water quality, currents, shearing stress of wind and waves, and horizontal transport of nutrients.
From page 143...
... much of the near-shore zone and the central pelagic zone is a transition zone where underwater irradiance and nutrient conditions are optimal for formation of algal blooms. Westward of the near-shore zone is the littoral zone, with water column depths of 1 to 3 feet and emergent, floating, and SAV.
From page 144...
... Lake Sediment Type, Hydrodynamics, and Spatial Distribution of Nutrients Historically most of the area encompassed by the Herbert Hoover Dike had sand sediments, with peat sediments at the southern end of the lake. Because of agricultural activity and the straightening of the Kissimmee River upstream, the central pelagic zone has accumulated nearly 5 million cubic meters of organic mud sediments (Fisher et al., 2001)
From page 145...
... Maintaining a high lake level does not appear to adversely affect the ecology of the pelagic zone. Near-Shore Zone The near-shore zone has two different states -- a clear-water state with abundant SAV and a turbid, phytoplankton-dominated state without SAV (Aumen and Wetzel, 1995)
From page 146...
... , stabilizing lake sediments, preventing resuspension, and reducing water column turbidity. The near-shore zone with its SAV is a transi tional location for a variety of fish species, as they start life in the littoral zone and subsequently move into the near-shore and then the pelagic zone as they mature (Fry et al., 1999)
From page 147...
... The spatial extent of macro-algae is inversely related to lake level. The spatial extent of vascular plants is inversely related to minimal lake level 2 years prior (reflecting the longer growth/response time of those plants)
From page 148...
... Emergent plants including bulrush (Schoenoplectus spp.) occur in bands along the western shoreline of Lake Okeechobee, providing ecosystem services such as serving as a substrate for invertebrates consumed by various species of fish, producing seeds valuable as food for many species of water birds, attenuating wave energy that protects the edge of the littoral zone, and serving as a refuge where SAV can grow on the leeward side of the emergent plants (Aumen and Wetzel, 1995; Coops et al., 1996)
From page 149...
... In the 9-month experiment, the ratio of below-ground to above-ground biomass decreased with increasing water level, as did the ratio of live to dead shoot biomass. The authors concluded that "even moderate changes in water level may be an important factor [affecting]
From page 150...
... . High lake levels and associated advection of phosphorus into littoral zone areas that normally are nutrient poor could have a variety of adverse effects on the structure and function of the ecosystem (Havens, 2002)
From page 151...
... During times of sustained high water levels (near 17 feet) , there has been considerable erosion of the edge of the littoral zone and accumulation of large amounts of organic debris along the littoral-nearshore fringe, particularly during high wind events (Havens et al., 2002)
From page 152...
... 152 Progress Toward Restoring the Everglades FIGURE 5-8  Maps of the vegetation in the southwest region of the littoral zone of Lake Okeechobee showing a large expansion of cattail from 2012 to 2016. Cattail is shown in red, with treated areas of cattail in black.
From page 153...
... However, because wading bird density in Lake Okeechobee is partially a function of the condition in wetlands outside the lake, it remains challenging to fully understand how wading birds respond to variations in water level in Lake Okeechobee. From January 1988 to September 2002, a comprehensive field study was conducted to quantify the foraging habitats of wading birds in the Lake Okeechobee littoral zone (Smith et al.
From page 154...
... MONITORING TO GUIDE OPERATIONAL OPTIMIZATION From the past research, there is a robust understanding of how certain eco logical attributes respond to water level in Lake Okeechobee, yet uncertainty about a number of critical responses remains. There is relatively high certainty that water level reversals negatively affect snail kites and wading birds during the nesting season, and that a receding spring water level into the 12- to 13-foot range supports foraging by birds on small fish.
From page 155...
... Because the model can also predict the transport of suspended solids and phosphorus within the lake and the wave energy on the western shoreline, it could be used to predict how a particular water level regime might influence a variety of conditions, including SAV spatial extent, erosion potential at the littoral fringe, and phosphorus movement into the littoral zone. If an empirical relationship can be derived from historical monitoring data, it may also be possible to use the model to predict cattail expansion.
From page 156...
... The lake regulation schedule will soon be revisited to determine new operational rules. The completion of the Herbert Hoover Dike rehabilitation project could enable higher water levels to be held within Lake Okeechobee, although the feasibility of higher water levels must still be determined through an updated risk assessment.
From page 157...
... Monitoring could also improve the understanding of the potential impacts from inundation to emergent vegetation in the near-shore zone. Further, the Lake Okeechobee Environment Model is a tool to use in concert with regional hydrologic and ecological models to evaluate the implications of alternative regulation schedules and lake operations, particularly as new data become available to refine the model's SAV component.


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