The Role of Flow in the Everglades Landscape
The Comprehensive Everglades Restoration Plan (CERP) aims to improve water quantity, timing, distribution, and quality of the Greater Everglades System (Figure 1) over the next 35 years (SFERTF 1998). Not only is it the most expensive (10.9 billion dollars, Sheikh and Carter 2008) and ambitious ecological restoration ever undertaken, but it involves multiple agencies and organizations analyzing natural and ecological attributes to recover the natural system. A critically important but often overlooked aspect of the Everglades is the role of flow—the actual movement of water across the landscape—in creation and maintenance of habitats for Everglades plants and animals. Successful restoration of the Everglades landscape requires a thorough understanding of the role of flow (SCT 2003).
Ridge and Slough Landscape
Two of the main ecosystems formed in the Everglades marsh system were the sawgrass plains in the north and the ridge and slough landscape in the center and south (Figure 1). Extensive communities of freshwater wetland vegetation including somewhat regularly distributed tree islands made the ridge and slough landscape, resemble a patterned peatland. The slight elevation gradient from Lake Okeechobee to Florida Bay (less than 4.5 cm per km) made a 48km-wide sheet of water flowing over and channeling through the peat. Ridges were alternately spaced and a lack of drainage channels implies that water spread out over the entire 48km breadth. Early measurements reveal that the water averaged 15cm–1m deep and was rarely stagnant traveling 34m per day (NPS 1997). The sheet flow's spatial extent was vast and dominated the pre-drainage Everglades hydrology. This pattern of water flow is thought to have formed and maintained the ridge and slough landscape (Kushlan 1993). "The water is pure and limpid and almost imperceptibly moves, not in partial currents, but, as it seems, in a mass, silently and slowly..." (Smith 1848 in Trustees 1881). Though the historical documentation of the ridge and slough landscape is limited, there is a strong similarity between the original direction of flow, alignment of ridge and sloughs, and tree-island orientation (SCT 1997).
Peat heights between ridges and sloughs differed by about 0.5–1m, resulting in an ecosystem filled with water throughout the year, with areas flooded or dry depending on the season. A cross sectional diagram (Figure 2) is drawn to scale to demonstrate the landscape. Although debate remains, as indicated in the Natural Systems Model (Fennema et al. 1994, VanZee 1999), historical records suggest that sloughs were covered by water approximately nine months of the year, and some tree islands were flooded for perhaps three months each year (McVoy et al. in review). The interaction of habitat heterogeneity with dynamic water storage and sheet flow made a unique and diverse habitat for native flora and fauna.
Development in South Florida
With the beginning of human habitation in the mid-1800s, about 200 km2 north and west of Lake Okeechobee were ditched and drained for agriculture (Trustees 1881). Development continued, and with the floods of 1947-1948, the federal government authorized the Central and Southern Florida Flood Control Project. Massive public works construction projects created the East Coast Protective Levee destroying about 2,500km2 of the Everglades ecosystem (Lord 1993). A levee was also built to buffer Lake Okeechobee degrading around 11,650km2 of Everglades watershed (Lord 1993). With development and the creation of Water Conservation Areas 1, 2, 3 and the Everglades Agricultural Area (EAA), just over 15,000 km2 of marsh and watershed have been lost (Lord 1993). Although some hinted at the importance of the Everglades system and the need for its preservation in the 1920s, the economic push for development proved too strong. Marjorie Stoneman Douglas's book, The Everglades: River of Grass (1947) could do nothing to stop drainage and flood control for urbanization, agriculture, and development.
Construction begun in earnest in the early 1900s blocked water flow and began to impact the ridge and slough landscape. In 1910, debate ensued over construction of the Tamiami Trail, with Tatum (Tamiami Trail Commissioners 1928) warning that the road would serve as a dam for southward flowing water. Documentation exists of water flowing over the trail in years of flooding. Aerial photographs just 12 years after completion revealed the drastic difference in landscapes that may have been caused by the barrier to flow. Levee and dam construction continued throughout the 1900s and further fragmented and compartmentalized the ridge and slough landscape as exemplified with Alligator Alley (I-75). The effect of channeling the landscape is demonstrated in Figure 3.
A Changing Ridge and Slough Landscape
Loss of ridge and slough landscape is defined by the Science Coordination Team (2003) as a loss of elevation difference between ridge height and slough depths resulting in a flattening of the landscape and a loss of distinct ridge and slough vegetation growth aligning with flow direction. Early predrainage documentation and photographs reveal an organized pattern of congruent landscape, vegetation, and flow similar to Figure 4, while post-drainage documentation reveals a scattered, blurred, and unstructured landscape similar to Figure 5.
It is undeniable that the ridge and slough landscape is disappearing and though causes of these changes are not known definitively, initial evidence points to direction, speed, and rate of flow as having large effects on parallel ridges, sloughs, and tree islands (NRC 2003). Greatest ridge and slough degradations are in areas like WCA2 and 3B where water flow has been affected the most (Armentano pers. comm.). The SCT (2003) argues that ridge and slough degradation likely results from changes in water depth and seasonal variations along with flow redirection as a result of constructed barriers (NRC 2003). Though mechanisms of creation and maintenance of ridge and slough landscapes have not yet been documented, it is imperative that the remaining ridge and sloughs be preserved. Current scientific understanding supports flow as a fundamental component of ridge and slough restoration.
There also is strong evidence that loss of the ridge and slough landscape is having a negative effect on Everglades plants and animals. Loss of landscape diversity results in loss of habitat, foraging areas, and refugia for wading birds, fish, and other animals (SCT 2003). Wading bird nesting numbers have decreased by 75–90% of original population (Ogden 1994) and alligator holes are now absent where they were once prevalent (Craighead 1968, Mazzotti and Brandt 1994). Barriers to flow also represent barriers to migration of animals and barriers to gene flow between adjacent regions of the Everglades (SCT 2003). And drier and more even landscapes also result in altered fire regimes; longer dry seasons resulting in more intense fires (Gunderson and Snyder 1994).
Loss of Water Flow
Loss of water flow is largely attributed to water management and development and has severely altered the ridge and slough landscape. Elongated ridges aligning with the direction of water flow are evident in Figure 4, which is consistent with pre-drainage patterns, while Figures 5 & 7 depict a degraded pattern with amorphous vegetation clusters having no directional alignment. Although some construction attempted to retain the integrity of the flow pattern using canals along the road connecting the ridge and slough landscape every 4km, the landscape has been greatly degraded. Culverts were put into Alligator Alley (I-75) to try to maintain some water flow. Redirection of water flow severely impacted the ridge and slough landscape upstream and downstream of the Alley (Figures 5 & 6).
Causes of Ridge and Slough Degradation
Interruption of Water Flow
There are several hypotheses as to why the ridge and slough landscape is disappearing. With dams and levees interrupting water flow, it is possible that loss of flow results in loss of sediment transport, resulting in organic material accumulation and filling in of open water habitat (Figure 7). A larger sediment surface area exposed for vegetation growth also could be created by lowering water levels. Lower water levels would result in an increase in vegetation, resulting in increased nutrient uptake and storage.
Loss of Peat Soils
A flattening of the landscape can also occur with the loss of peat soils from ridges. The shortened hydroperiod and lowered ground water levels have resulted in areas losing 1cm–1m of peat soil (Stober et al. 1996, Sklar et al. 2000). Overdrained ridge and sloughs has lead to the oxidation of top peat layers and more frequent and intense fires. This change in the landscape continues to alter water flow.
Altered Water Level and Quality
Changes in water levels and water quality also can have adverse effects on ridge and slough landscape. Each vegetation species has optimum ranges of water depths and quality. Therefore when water levels are lower and hydroperiods are shortened, vegetation composition and growth changes. Flow was considered responsible for the vegetation communities of sawgrass (Cladium jamaicense Crantz) growing on ridges and water lilies, spikerush (Eleocharis sp), and beakrush (Rhynchospora sp.) in sloughs and prairies (Sklar et al. 2000). However, altering water level has resulted in sawgrass invasion into sloughs and beakrush dying off. Reduced water quality has resulted in an increase in cattails given their tolerance range. The vegetation species composition has been completely altered (Kolopinski and Higer 1969, Davis and Ogden 1994, Sklar et al. 2000). Natural patterns of succession occur in response to natural variation in hydrology and water quality; however present hydropatterns are altered and eutrophication has exacerbated the problems resulting in unnatural patterns of succession.
Altered Vegetation Decay
Changes in topography also may be attributed to different rates of vegetation decay. Research has shown that plants that grow in sloughs, such as Nymphaea odorata, decay much faster than plants that grow on ridges, such as Cladium jamaicense (Davis 1991, Shili Miao, pers. comm.). When comparing field litter, 60% of sawgrass remained after a year (Davis 1991), while slough species decayed within a few months (Shili Miao, SCT, pers. comm.). The rate of decomposition in sloughs is so great under normal flow conditions that less organic matter accumulates in wet areas like sloughs than on ridges (Heal et al. 1978, Rochefort et al. 1990, van Dierendonck 1992, Johnson and Damman 1991). These decay rate differences could help maintain elevational differences between ridges and sloughs. More stagnant conditions resulting from decreases in flow could decrease dissolved oxygen concentrations in sloughs, slowing down decomposition rates and resulting in faster peat accumulation and eventual filling in of sloughs.
Research continues on processes of ridge and slough creation and maintenance, and effects of water flow and its alteration on the greater Everglades system. Florida International University (FIU) scientists are relating flow to particle production, fate, transport, and deposition (Mitchell-Bruker et al. 2002) and are preparing a model focusing on simulating ridge and slough development and health under varying water management scenarios. This model will be extremely useful in dealing with restoration alternatives. FIU is also determining the hydrological requirements of several major aquatic slough species to evaluate present ridge and slough habitat. Development and survival will be tested at different locations in Shark Slough. The University of Florida is studying soil and water chemistry in relation to how ridge and slough patterns are established and maintained in Shark River Slough. The South Florida Water Management District (SFWMD) is tracking historic patterns in ridge and slough patterns with aerial photography from 1940 to 2003 and relating vegetation growth and decay to flow.
SFWMD scientists seek to understand differing landscape features and how they are maintained and formed through differing vegetation growth, establishment, and habitat requirements. Vegetation decay is being compared among sloughs and ridges. The Loxahatchee Impoundment Landscape Assessment (LILA) is a landscape-scale project monitoring biological performance measures and their relation with hydrology to assist with CERP. Within the Everglades landscape, four impoundments have been constructed in which water depth and flow can be altered to induce responses from wildlife, tree islands, and ridge and slough communities. The project is a pilot study for hydrological scenarios under CERP to bring macrocosm effects to larger landscape restorations. LILA is likely to be highly effective because all critical responses (like hydrology) are controlled to replicate historical flow and water patterns. LILA also produces multi-disciplinary results in one study rather than many individual studies and will likely produce strong theories as to the role of flow.
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