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the role of flow in the everglades ridge and slough landscape >
possible mechanisms of ridge and slough formation and maintenance
Possible mechanisms of ridge and slough formation and maintenance
The central geomorphological issues regarding the pre-compartmentalized ridge and slough landscape are the mechanisms by which the landscape pattern arose, and the dynamics of landscape pattern over time. While the first issue certainly is of importance, the immediate needs of restoration planning draw attention primarily to the second. It will be necessary to ascertain whether or not landscape patterns naturally shifted over time, and if so, what temporal patterns existed. Also, it will be important to know how human-induced changes in the landscape have altered the natural temporal pattern of landscape dynamics. Regardless of the long-term dynamics, maintenance of the remaining ridge and slough pattern, particularly preventing sloughs from becoming dense stands of sawgrass, is critical for sustaining habitat and dispersal pathways for aquatic animals.
The key geomorphological characteristics of the Everglades are: its peatland base; outcroppings of limestone; a highly cohesive substrate with high clay and organic content; and a floc layer that is more commonly associated with muddy estuaries. These characteristics are in contrast to alluvial systems that have no near-surface bedrock control, very low organic content of sediments, and that beds and banks composed of unconsolidated sand and silt (Gregory Pasternack, personal communication). In addition, the role of vegetation in shaping landscape pattern is important, as well as the feedback mechanisms that likely exist between topography and vegetation.
As in classical geomorphology, there is almost certainly a physical and biological explanation for the particular landscape patterns found in the pre-drainage Everglades and topography in the present Everglades, and they are tied to physical driving forces. On one hand, it is possible that the mechanisms may never be known for certain, particularly given their likely complexity and the long time scale over which they may operate. On the other hand, geomorphic theories have provided reasonable explanations for landscape processes over the full range of time scales, from raindrop erosion to mountain building (Gregory Pasternack, personal communication).
It also is possible that alterations in the ridge and slough landscape result from changes in water quality. In fact, CERPs underlying premise includes water quality as a very important component of Everglades restoration. However, this paper focuses on the hydrological rather than the water quality aspects of ridge and slough formation and maintenance. CERP assumes that restoration of natural flows and levels will be accomplished with water that is not poorer in quality than that of the unimpacted ecosystem.
In the following sections, possible mechanisms of ridge and slough formation and maintenance are presented and discussed. Note that flow is an important component of almost all of them. As mentioned previously, it is not possible to prove or disprove any of these mechanisms, given the present lack of data and analysis. The possible mechanisms are presented to stimulate discussion and future research. In addition, although the possible mechanisms are presented in separate sections, it is likely that a combination of several of these mechanisms operating together is responsible for formation and maintenance of the ridge and slough landscape.
One possible mechanism of slough maintenance is net downstream transport of sediments, primarily floc, by water flow prior to compartmentalization. For the purposes of this discussion, floc is defined as loosely consolidated particles that are larger than 0.45 µm in diameter, that are largely organic in nature, and that have densities very close to 1.0. Such a transport mechanism is consistent with the landscape directionality, the shape of ridge outlines, and preliminary indications of floc movement.
Sediment transport by water flow is responsible for maintaining many types of flowing systems. Flows can take multiple paths down gradient and these paths form in the areas of least resistance. As flows increase, these paths widen and there is a velocity distribution across each flow path, with higher velocities in the central (deeper) area and slower velocities on the sides due to friction. Suspended sediments fall out at the slower velocity edges thus the formation of ridges. The next flow event adds to the process and flow deepens in the center and the ridges grow more. During really large hydrological events, the flow can even be horizontal as the banks overflow and recede, also trapping sediments in the ridge area. As the ridges build, the channel depth increases and the deeper flooding tends to kill off vegetation, making the slough more efficient for water transport and the ridges relatively less efficient.
In the Everglades, because of the normally slow moving water and the peat substrate, floc is likely to make up the bulk of transported sediments. The floc layer can be up to 12 inches thick and is nearly structureless, especially in comparison with the peat soil substrate. Pre-drainage observations of floc in sloughs are available.
Downstream transport of floc presently found in the remaining ridge and slough landscape might occur either on a slow continual basis, incrementally with individual local storms, or as infrequent high-energy events. Bill Loftus (unpublished data) observed that after Hurricane Andrew in 1992, the floc layer, the periphyton/Utricularia mat, and dead vascular plant stems were blown into the sawgrass ridges from the adjacent sloughs. Ridges apparently acted as traps to capture the material. This observation demonstrates how infrequent high-energy events may serve to distribute and deposit materials on the ridges.
In any of these cases, unrestricted flow paths (sloughs) would be critical for uninterrupted downstream transport. The density of floc is very close to that of water, giving floc a low settling velocity. Considering how easily floc is resuspended by physical and biological processes such as gas bubble flotation, bioturbation, and temperature/density effects, it is possible for floc to be carried long distances downstream.
Sedimentation within sloughs subsequent to compartmentalization
Net accumulation in sloughs of organic sediments such as floc might be the process most responsible for the disappearance of open-water habitat, and the increase in the area of dense sawgrass. The shift from net transport of floc prior to compartmentalization to net accumulation of floc subsequent to compartmentalization could be caused by a number of mechanisms, acting separately or together. For example, lowering average water surface elevations decreases water depths, which in turn promotes emergent plant species. Increased emergent growth increases hydrologic resistance to flow, decreasing velocities and floc transport. Increased emergent growth, more so if periphyton coatings are present, also increases filtering of suspended floc. Both decreased water velocities and increased filtering will tend to increase in situ accumulation, further decreasing water depths and promoting increased plant growth and stem density. Together, these effects could lead to vegetative as well as topographic filling-in of sloughs.
The onset of such filling-in processes was remarked upon almost fifty years ago. The following is from a Florida Game and Freshwater Fish Commission progress report regarding the then new Water Conservation Areas 2 and 3.
Influence of barriers on flow and sediment transport
Barriers such as roads and levees disrupt continuous flow paths. Perforation by culverts or bridges can connect some sloughs, but the majority of sloughs remain completely blocked, forcing water and sediment laterally across ridges. Areas of stagnation can result between culverts. Suspended sediments likely are deposited in these areas of stagnation upstream and downstream of the barrier. Higher density sediments would be picked up in the area of accelerated flow velocities upstream of the culverts and that these sediments would be deposited immediately downstream as the water slows and spreads out. For example, this mechanism might explain the pockets of unnatural woody vegetation immediately downstream of the culverts under Tamiami Trail.
Along a linear flow barrier, creation of discrete drainage points connecting upstream and downstream wetland fragments also results in the creation of backwater regions between these discrete points. A backwater wetland receives inflow as water rises, and discharges water as the water surface falls, but backwater wetlands largely are disconnected from their sediment and nutrient sources, and also have reduced flushing. Linear flow barriers with discrete cross connections (e.g., culverts) will develop backwater lobes between these discrete points extending to a distance away from the barrier.
Influence of canals
Canals intercept continuous flow paths and provide short-circuits that connect flow paths that would otherwise have been isolated. Satellite images of areas around canals that do not have associated levees (e.g., Miami Canal, agricultural canals) demonstrate that vegetative shifts do occur across canal boundaries. Canals that are not exactly perpendicular to the flow direction provide preferential flow paths that divert water and cause different hydrologic conditions on either side of the canal. Canals also act to reduce the surface water gradient, and therefore reduce water velocities in adjacent wetlands.
Changes in water levels due to the water management system
Construction of Floridas extensive water management system altered other hydrological parameters in addition to flow. For example, it is possible that unusually high water levels in Water Conservation Area 3A resulting from water management operations upstream caused the loss of most of the tree islands and resulted in the expansion of sawgrass, and that degradation of vegetation patterns in many areas are due to changes in water quality (Terry Rice, personal communication). Regardless of which possible mechanism(s) for ridge and slough formation are correct, it is likely that flow interacts with water depth, hydroperiod, and other hydrological parameters in maintaining landscape heterogeneity in the Everglades.
Although extended periods of unnaturally high water levels can be detrimental, water levels that are too low also can have negative consequences. In shallower waters, emergent species, particularly Eleocharis cellulosa, species of Rhynchospora, and short and mid-sized sawgrass, replace the floating and submerged slough community. This sequence of wetland plant communities with changing depths is a familiar one found throughout the world. The emergent communities, besides being associated with shallower depths, experience shorter hydroperiods and are subject to increased fire frequencies, as evidenced by the frequency of charcoal in sediments cores. Collectively, these are environmental conditions unfavorable for peat accumulation and consequently this wetland plant guild usually is associated with marl deposits formed by the abundant periphyton found in the same habitat rather than with peat.
The water depths needed to sustain Nymphaea-dominated slough communities are hard to define precisely in a variable environment such as the Everglades because water flow and low nutrient-levels also are important factors. However, when water levels began to regularly exceed 2.5 feet in the central Everglades, Nymphaea communities began replacing the emergent wet prairies (Goodrick 1984). Cohen et al. (1984) and Herdendorf et al. (1986) suggest that Nymphaea is characteristic where year-round water depths range from 1.0 - 3.3 feet and 0.8 - 3.3 feet, respectively. In Shark River Slough, Olmsted and Armentano (1997) and Ross et al. (2001) found that the wettest sites were occupied by Eleocharis stands and that slough vegetation was too rare to be mapped. Insights into the influence of water depths can be gained from knowing what depths are too shallow for floating plant communities. Ross et al. (2001) found that in the Eleocharis stands, the greatest water depths in the study areas averaged 1.3 - 1.8 feet year-round in an average rainfall year of the 1990s, with maximum 30-day depths being 2.0 - 2.6 feet. In the wetter years, of course, depths were higher.
Variations in water depth also can influence decomposition rates. If peat on ridges experience shorter hydroperiods due to changes in water management, the ability of fungi to serve as the dominant agents of vascular plant decomposition may increase. Fungi are important decomposers of vascular plant material in terrestrial habitats, and are less important in submerged freshwater habitats. Because fungi possess enzyme systems necessary for decay of structural plant material such as lignin, they may out-compete bacteria in these circumstances, and increase the rate of plant decomposition on ridges. This increased rate may contribute to the flattening of the ridge and slough landscape under altered water depth patterns (also, see following section on differential rates of peat accumulation and decomposition).
Although changes in water depth under managed conditions have been suggested as an alternative explanation for decreases in the ridge and slough landscape, changes in water depth alone are not sufficient to account for the observed landscape changes. Comparison of the original and current patterns suggests that a directionally neutral change, such as changes in water depth due to water management practices, while certainly important, would not be sufficient alone to explain the observed changes. Ridges and sloughs exhibit a regional pattern that corresponds with the surface gradient in the Everglades, and the direction and extent of pre-drainage flow patterns. This pattern alone is strong evidence for the role of water movement.
In addition, an examination of water level and vegetation in Shark River Slough and Water Conservation Area 1 suggest that degradation of landscape pattern is not related to changes in water levels. A similar increasing trend of amorphous, non-directional polygons is illustrated in Plate 9 of Davis et al. (1994), both in Shark River Slough and in Water Conservation Area 1 between 1968 and 1984. The vegetation transformations observed in both of these locations are not due to changes in managed water depth; nearby stage gauges indicate no change in water depth patterns or mean water depth over the observed period. All of these examples illustrate a transformation from an originally flowing, directional system to a presently non-flowing, or insufficiently flowing, impounded system.
Differential rates of peat accumulation and decomposition
Peat is created by the accumulation of organic matter over time as organic matter production exceeds decomposition. Labile plant material decays quickly, while refractory plant material decays more slowly. Peat is composed of refractory components of the source vegetation material. If sawgrass decomposes more slowly than water lily and other slough species, this difference alone could account for maintenance of the elevations of the ridges and the lower elevations of the sloughs. Other physical and biological processes interact to control increases in peat elevation. White (1994) uses his process-oriented landscape description to show that peat elevation largely is a consequence of increases from peat accumulation balanced by decreases from fire.
Numerous decomposition studies have been conducted in the peatlands of North America and Europe. Northern bogs are dominated usually by Sphagnum mosses, which create the peat of these wetlands. Although different Sphagnum species are closely related, they differ in both net primary production and in decay potential. Species that grow in the hollows, the lower elevation and wetter microhabitats, produce biomass at rates up to twice that of mosses growing on the higher, dryer hummocks (Nungesser 1997, Table 2, p. 35). Johnson and Damman (1991) report that in transplant studies, Sphagnum moss species that inhabit elevated positions decay more slowly than those that grow in the wetter microhabitats. Labile fractions of Sphagnum mosses range from 18% in hummock species to 28% in hollow species (Clymo 1983, Table 4.7, p. 177). The different decay rates reinforce the differences in microhabitat peat depths because peat accumulates faster in the hummocks in spite of a two-fold higher net primary production in the hollows. Therefore, under identical conditions, the hummock species decompose more slowly than the hollow species, producing greater peat accumulation for the same unit of biomass produced, in spite of lower net primary production.
Other peatland research has shown similar results species that grow in the lower, wetter microhabitats decay at rates two to six times that of species that grow in the elevated, dryer microhabitats (Heal et al. 1978, Rochefort et al. 1990, van Dierendonck 1992, Johnson and Damman 1993). A peatlands microtopography model developed to predict hummock and hollow features indicates that the differences between production and decay of the Sphagnum species contributes greatly to generating bog microtopography (Nungesser 1997, Nungesser in press). Under equilibrium conditions, the higher decomposition of the hollow species accumulates less peat than the hummocks species with lower decomposition rates in spite of substantially different productivities. Innate differences in decay rates of peatland species have been reported for other peatlands, as well (Rosswall 1973, Clymo 1983, Morris and Lajtha 1986, Johnson and Damman 1991, Harris et al. 1995).
In the Everglades, similar processes may produce the distinct differences between the ridge and slough microtopography. The dominant ridge species, sawgrass (C. jamaicense), is highly resistant to decay relative to the slough species Nymphaea odorata. Sawgrass contains much higher proportions of lignin than the floating and submerged slough species. In field litterbag studies, 60% of sawgrass litter in unenriched areas remained after one year (Davis 1991), whereas the Nymphaea litter disintegrates within a few months (Shili Miao, personal communication). Harris et al. (1995) reported much lower annual loss of sawgrass mass, 8.7% to 10.8%, in contrast to higher annual loss rates of other plant species found in sloughs (E. cellulosa, 45.7% - 84.0%; P. repens 36% - 46.5%). These differences would enhance peat accumulation on the ridges relative to the sloughs.
In addition, the presence of unimpeded flow in the pre-drainage Everglades could increase the oxygen concentrations in the bottom of sloughs. More stagnant conditions resulting from decreases in flow could decrease oxygen concentrations in the bottoms. Increased oxygen concentrations would further enhance the rate of decomposition of the more labile submerged vegetation in the slough, resulting in increased differences between ridge and slough bottom elevations. Also, decomposition release nutrients into solution. Increased availability of soluble nutrients and the transport of those nutrients in the presence of unimpeded flow could further increase decomposition rates of slough vegetation (Newman et al. 2001).
Erosional formation of ridge and slough habitat
In contrast to the effects of flow on deposition of organic matter such as floc, it is possible that the ridge and slough habitat was formed originally via erosional processes occurring on a recently uplifted surface (Gregory Pasternack, personal communication). This possibility was first raised by Parker et al. (1955) in a quote appearing previously in this paper.
The emphasized text contains very important ideas, particularly regarding the term consequent. It is a term that originates with W. Davis, a forefather of geomorphology, in his classic essay entitled, The Geographical Cycle (Davis 1909). While Davis ideas about landscape origins and evolution recently have been displaced by the Gilbert/Hack dynamic equilibrium theorem (Hack 1960), Davis ideas did dominate the early and middle 20th century. His ideas still are viewed as useful, though not generally correct. It is possible that Parker et al. were using their Davisian point of view to suggest a specific hypothesis about the origin and processes at work in the ridge and slough landscape.
A consequent stream is one that forms in proto-troughs on a recently uplifted (or in this case, perhaps, on a peninsula recently exposed by sea level drop) fresh surface. According to Davis, all the changes which directly follow the guidance of the ideal initial forms may be called consequent; thus a young form would possess consequent divides, separating consequent slopes, which descend to consequent valleys, the initial troughs being changed to consequent valleys in so far as their form is modified by the action of the consequent drainage. Thus, Parker et al. (1955) were suggesting that the regional surface had an initial roughness of non-aligned proto-troughs that, through the action of water-induced erosion, became connected to form sloughs directed down the regional gentle gradient. It is central to this possible mechanism that the ridge and slough landscape is of an erosional origin, not a depositional origin.
Extreme hydrological events
The Everglades, as are most ecosystems, is influenced by extreme hydrological events. The subtropical setting guarantees occasional impacts from hurricanes, tropical storms, and tropical waves, all of which are capable of producing rainfall events in excess of 20 inches over a relatively short period of time. Because these events typically occur during Floridas wet season (May through September), their impacts may be exacerbated by already saturated soils and existing high water levels. As an example of the potential magnitude of such events, Taylor Alexander relates part of an oral history taken from Fred Dayhoff an Everglades hunter and angler who later became an Everglades National Park ranger.
These extremes in flow and water depths likely had a profound impact on the Everglades landscape. Increased flow volumes and rates undoubtedly transported particulate and dissolved materials, affecting both upstream source areas and downstream receiving areas. The impact of these events is difficult to assess, particularly without data from a detailed paleoenvironmental study. However, it is certain that following construction of the water management system in the mid-1900s, the magnitude and frequency of extreme hydrological events was decreased. What role these water management changes have on the impact of extreme events is unknown, but worthy of further attention.
Fire was an important component of the pre-impacted Everglades. Drainage in the Everglades beginning in the late 1800s resulted in a substantial increase in the occurrence of fire in the ridge and slough landscape. It is possible that the linear slough pattern was created by tongues of fire that followed dryer (overdrained) peat lines in a sawgrass-dominated landscape (Craighead 1971). These fires burned out sawgrass rhizomes and peat to create linear pools (sloughs) throughout the ridge and slough system. Fire may explain the sharp separation in vegetation between ridge and sloughs, and the current differences in elevation between ridges and sloughs. More recently, sawgrass is re-invading sloughs where ponded water (levees) has reduced fires.
Underlying bedrock patterns
Another mechanism that has been suggested is related to changes in sea level. During the past interglacial epoch, when the sea level was higher than present, flow from the Gulf of Mexico likely crossed the southern peninsula of Florida, thus leaving ripple deposits. These deposits eventually, through sediment deposition, could have led to elongated ridges in the bedrock. It is possible that underlying bedrock undulations are the base upon which peat deposits are built. Surface geophysics can provide conclusive evidence regarding bedrock configuration and its role in ridge formation and maintenance. Regardless of the specific mechanism(s), the regularity of the ridge and slough pattern suggests the presence of some organizing force acting over the full breadth of the landscape.
Preliminary measurements of peat and underlying bedrock elevations have been made in several transects conducted in ridge and slough and sawgrass plain habitats (Chris McVoy, personal communication). McVoy compared measurements of peat and bedrock elevations to aerial images of the landscape. As expected, peat elevations were higher in areas that would be visually described as sawgrass ridges, based on the presence of sawgrass stands. Conversely, peat elevations were lower in areas that would be visually described as sloughs, based on the presence of open water habitat having sparse emergent vegetation. However, bedrock elevations underneath the peat were variable but generally flatter than peat elevations, and did not correspond to changes in peat elevation. It is possible that bedrock elevations directly beneath the transects would not necessarily correspond to peat elevations along the transects. It may be that, in the presence of unimpeded flow, the bedrock pattern is manifested in the peat pattern at some distance downslope. While these preliminary data do not support the possible mechanism of underlying bedrock influence, more data need to be collected across a broader range of habitats, including degraded and undegraded ridge and slough habitat, to confirm the initial results.
Microhabitat differences in water chemistry due to differences in flow
Anecdotal evidence from northern peatlands suggest the possibility that microhabitat differences in flow may result in microhabitat differences in water chemistry, thereby influencing Everglades vegetation patterns. The majority of the remaining ridge and slough habitat in the Everglades is in areas in which water quality has not been degraded by inflows from the north. Therefore, small differences in water chemistry could have large effects on biology.
A common form of northern peatlands are raised bogs (the lowest nutrient peatlands), in which the surface is elevated slightly above the surrounding landscape. This elevation results in a very nutrient-poor wetland, with precipitation providing the only water source. In contrast, seepage areas often are found on bogs, and these areas exhibit vegetation tolerant of more eutrophic conditions than vegetation of the main bog expanse. Vegetation in these seepage areas is more typical of nutrient-enriched peatlands than of the raised bogs, but the source water for the seeps is exclusively the upslope parts of the bog. Nutrients are elevated in these seeps (Malmer and Sjors 1955, Heinselman 1963, Ingram 1967, Siegel 1992). Ingram (1964) reported that phosphorus levels in a water track (seep) were 0.148 g m-2 compared to 0.057 g m-2 from the source bog expanse (upslope). Similarly, potassium was 1.51 g m-2 in the water track and 0.99 g m-2 in the mire expanse. Malmer and Sjors (1955) similarly reported water tracks that contained higher potassium and calcium levels than the source drainage area of the bog. Ingram (1967) concluded that moving water supports poor fen vegetation while the same water when still supports only bog vegetation. Kulczynski (1949) reported similar findings. Similar relationships were reported by Ingram (1967), who found that the fluctuation of bog water levels is significant in determining site quality; the relationship of his site index with maximum water fluctuations was r = 0.73.
These observations suggest that water movement in peatlands provides a means to transport nutrients and to support a richer vegetation type than still water. If this mechanism operates in the Everglades ridge and slough habitat, the slight nutrient enrichment of moving slough water may contribute both to faster growth of slough species and more rapid decomposition of slough litter. Enrichment may occur for a variety of reasons, including groundwater seepage, contact with mineral soils or bedrock, and scavenging of nutrients from decaying peat. Davis (1991) showed that nutrient enrichment in the Everglades increases the decomposition rates of both sawgrass and cattail. With higher decomposition rates of slough species, this slight nutrient enrichment may be one of the feedbacks increasing slough/ridge differentiation. Such a potential mechanism can be tested readily through controlled experiments in slough habitats that examine nutrient levels in flowing water versus still water.
U.S. Department of the Interior, U.S. Geological Survey
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