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a Dep. of Biological Sciences, Florida International Univ., Miami, FL 33199
b Southeast Environmental Research Center, Florida International Univ., Miami, FL 33199
c South Florida Ecosystems Restoration Task Force, Department of the Interior, Miami, FL 33199
* Corresponding author (childers{at}fiu.edu)
Received for publication March 11, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: EAA, Everglades Agricultural Area • ENP, Everglades National Park • NWR, National Wildlife Refuge • WCA, Water Conservation Area
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Hydrologic modifications during the last 100 yr (including construction of more than 2500 km of canals and levees and hundreds of water control structures) have played an important role in the eutrophication of the Everglades (Light and Dineen, 1994; Sklar et al., 2001). These hydrologic modifications have had two important effects. First, the canal–levee network has segmented the remaining Everglades wetlands into several large impoundments, known as Water Conservation Areas (WCAs; Fig. 1A) . The WCAs were designed to store water, discouraging the natural tendency of water to flow south (downstream) through the Everglades landscape to the Gulf of Mexico. Everglades National Park (ENP), which is downstream end of the WCA network, is the only region of the remaining Everglades in which some natural flow regime exists. Second, hydrologic compartmentalization of the Everglades has caused the remaining wetlands to receive surface water mainly as point-source inputs at canal structures. The result is focal points in the landscape for water inputs, whereas predrainage water movement was diffuse and distributed across the landscape (Sklar et al., 2001). Increased nutrient concentrations coupled with these point-source inputs has led to the formation of defined enrichment gradients (Koch and Reddy, 1992; Urban et al., 1993; Qualls and Richardson, 1995; Doren et al., 1997).
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Soil nutrient levels are closely related to plant community composition in the oligotrophic Everglades, as in all wetlands (Mitsch and Gosselink, 2000). It seems intuitive that nutrients from enriched water inputs must first become part of the soil nutrient pool before they can affect plant productivity, competitive interactions, and ultimately community composition, though some wetland plants are able to use adventitious roots to take up nutrients directly from the water column (Howard-Williams, 1985; Noe et al., unpublished data, 2002). Unimpacted Everglades peat soils (with bulk densities < 0.15 g cm-3) typically have soil P concentrations of 200 to 400 mg P kg-1 dry wt. soil (McCormick et al., 2001). At concentrations above about 600 to 700 mg P kg-1 dry wt. soil, cattail begins to dominate the plant community (Noe et al., 2001). It seems clear that improvements in the quality of water flowing into Everglades wetlands may be reflected in reduced expansion of impacted, cattail-dominated areas. However, reductions in the overall extent of these areas will necessitate P reduction in soils, and thus will require a considerable length of time.
Our objective was to resample a series of wetland transects, representing a range of Everglades wetland environments, that were originally sampled in 1989 by Doren et al. (1997). Our goal was to determine the impacts of waterborne nutrient inputs to these wetlands during the intervening decade. In this paper, we present plant biomass, community composition, and soil P data from our 1999–2000 sampling and compare our results with those of Doren et al. (1997). We analyzed our plant community data with statistical techniques that were comparable with those of Doren et al. (1997). By comparing our species-specific information with similar data from WCA-2A (Vaithiyanathan and Richardson, 1999) and Shark River Slough, ENP (Busch et al., 1998), we were able to address long-term changes in plant community composition across the greater Everglades landscape. Our analysis also included decadal comparisons of native and exotic plant species. In addition, we quantified plant aboveground biomass by species, plant tissue nutrient content, soil bulk density, and soil organic content. We present these additional data to establish an expanded baseline dataset for future decadal monitoring of these transects. We hypothesized that we would find relatively minor spatial changes along these transects since they were last sampled, given that the quality of water entering most of these Everglades marshes has improved during the intervening decade. Specifically, we expected to see these spatial changes as the expansion of elevated soil P and cattail-dominated marsh. As a corollary to this hypothesis, we also did not expect to detect any reductions in soil P or improvements in plant community composition (e.g., a return to sawgrass or slough marsh species composition and relative abundance) in the impacted portions of these transects.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Hydrologic modifications have dissected the region into numerous subbasins. The 3059-km2 Everglades Agricultural Area (EAA) has been completely drained. To the south, Everglades National Park (ENP) includes more than 4300 km2 of the watershed; surface water flow in the park is regulated primarily by four water control structures along its northern border (known as S-12A–D). In between are five large Water Conservation Areas (WCA) that impound more than 3500 km2 of Everglades marsh (Light and Dineen, 1994; Fig. 1A). Water management has increased the frequency and intensity of disturbance events, such as floods, droughts, and fire (DeAngelis and White, 1994; Gunderson, 1994). Canals, levees, and unnaturally deep water have greatly reduced wetland connectivity in the WCAs. This highly modified hydrologic regime has also altered patterns of flow, inundation, and P delivery in Everglades marshes. The EAA is a major source of P to the oligotrophic marshes, often by point-source canal inputs (Coale et al., 1994; Davis, 1994; Doren et al., 1997).
In this study, we revisited the four marsh transects sampled by Doren et al. (1997) in 1988–1989 to quantify how vegetation patterns had changed in the intervening 10 to 12 yr. We also sampled soils along the transects to investigate relationships between soil P and vegetation patterns. In addition to returning to their WCA-1 (Arthur R. Marshall Loxahatchee National Wildlife Refuge), WCA-2A, WCA-3A, and Shark River Slough (Everglades National Park) transects, we established and sampled a transect in Taylor Slough, the other major marsh watershed in ENP (Fig. 1A). As with Doren et al. (1997), our transects were anchored on major canals at sources of water input. In some cases, our transects extended further from the canal input point than did those of Doren et al. (1997): 16 km compared with 6 km in Shark River Slough, 16 km compared with 5 km in WCA-3A, and 16 km compared with 8 km in WCA-2A (Fig. 1A). As with Doren et al. (1997), we sampled approximately every kilometer along these transects during the 1999–2000 wet season (Table 1).
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To estimate total P pools in plant tissues, we analyzed samples for tissue nutrient content. During the 1999–2000 wet season sampling, triplicate individuals of the dominant species (sawgrass in C quadrats, spikerush species in S quadrats, and cattail in T quadrats) were subsampled for live aboveground tissue. These tissue samples were analyzed for P content in the SERC Analytical Lab at Florida International University as per Daoust and Childers (1999). We report these P data in g P kg-1 dry weight. In a separate analysis and manuscript, we combined these values with species-specific dry weight biomass data to budget P into various ecosystem components at each transect location (Noe et al., unpublished data, 2002).
Soil Sampling
We quantified soil nutrient and physical parameters at each transect site shown in Table 1 by collecting a soil core adjacent to each macrophyte quadrat. These cores were collected by inserting a 2.36-cm-i.d. cellulose–acetate–butyrate (CAB) plastic tube with a cutting edge (to reduce compaction) to 10 cm below the soil surface. Intact cores contained the water column and the flocculent detrital layer (floc) as well as the 10-cm soil core, which were decanted off after inserting a plug to the soil surface. The soil core was extruded into a sampling container.
Soil samples from the five quadrats at a given site were combined to result in one composite sample for each community type (C, S, or T) at each sampling site. Upon return to the laboratory, soil samples were homogenized and large anomalous material (white roots, snail shells, etc.) was removed. The homogenized soil samples integrated the upper 10 cm of the soil profile in all of our samples. We determined field bulk density (Mg m-3) from sample dry weight and core volume. Otherwise, our sample analysis followed Doren et al. (1997). Samples were dried at 80°C until constant weight (48–72 h), ground to a fine powder with a Wiley mill, and stored until analyzed for total P. Total P was measured with the ashing–acid hydrolysis method of Solorzano and Sharp (1980). The resulting soluble reactive phosphorus (SRP) was measured colorimetrically with an autoanalyzer. Values are expressed as mg P kg-1 dry weight of material (hereafter mg P kg-1).
Statistical Analyses
We used dendrogram analysis to identify significant plant species assemblages from species frequencies (PC Ord; McCune and Mefford, 1999). We used Sorensen's distance measure with group average method to generate the similarity matrix for each dendrogram, and generated one dendrogram for each transect (total = 5). Dominant species were determined to be significant indicators with indicator species analysis methods of Dufrene and Legendre (1997). We used an additional ordination analysis (nonmetric multidimensional scaling; global NMS–PC Ord for Windows Version 4.14) to support the dendrogram results, but did not report these analyses for the sake of brevity. This support came from comparing groupings and identifying similarities (Clark, 1993). This particular method is well suited for data that are nonnormal or discontinuous as it tends to linearize environmental relationships. The dendrogram ordinations used Sorensen's distance measure, the variance-regression endpoint selection method, Euclidean axis projection geometry, and Euclidian residual distances. In these vegetation analyses, we assumed that locations with cattail communities were represented only by this community type while all other locations were assumed to have both sawgrass marsh and wet prairie slough communities, both of which we sampled. We described broad patterns of change in the frequency of dominant species along the transects without relying on inferential statistical tests of these changes because (i) stem densities were measured in 1999 but not in 1989 and (ii) tests of differences in species presence or absence among the five plots at each location had very low statistical power.
To determine if soil P varied among the plant community (dendrogram) groups, we used analysis of variance (ANOVA) to compare soil P among the plant species assemblages within transects. Tukey's Honestly Significant Difference post-hoc tests identified differences among plant assemblages when the ANOVA results were significant (p 
0.05). In all cases, we log-transformed soil P data to achieve homoscedasticity and normality of the residuals.
We tested if soil P differed significantly between oligotrophic community types (C and S) along the transects with twoway ANOVA with community type and transect as main factors (Systat; SPSS, 1998). Upon finding no difference in the soil P between community types, and no community type X transect interaction, we opted to not distinguish soil P values between C and S community types within a given site in further analyses. Data were plotted as total P (mg kg-1) versus distance down-transect (km) and were fit to the equation:
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Water Conservation Area 2A Transect
Water Conservation Area 2A is most proximal to the EAA, and as a result virtually all of the nonatmospheric inputs of water to this area have elevated nutrient concentrations. In 1989, Doren et al. (1997) found cattail in 100% of their sampling quadrats up to 2 km from the canal, and cattail occurred at sites as far away as 6 km. Sawgrass occurred in 10% of their sampled quadrats at the 2-km site, and was found in 100% of their quadrats at sites 3 to 6 km from the canal. In 1989, sawgrass marsh dominated at the 7- and 8-km sites. A decade later, our wet season data showed that the impacted cattail marsh (100% cattail occurrence) extended to the 4-km site. Sawgrass was no longer found within 2 km of the canal, and occurred at low densities and frequency (60%) at the 4-km site (Fig. 4)
. Sawgrass marsh or spikerush slough was only found at the 8- and 15-km sites (Fig. 4A). Total aboveground live biomass was roughly equivalent at the cattail and sawgrass sites, while the spikerush slough sites had roughly an order of magnitude less biomass. The two spikerush sites had virtually no standing dead plant material during the wet season while dead biomass at the two sawgrass marsh sites was more than double the live biomass (Fig. 4B). Spikerush and sawgrass plant tissues were consistently less than 0.005 g P kg-1 dry wt. whereas cattail plant tissues at the impacted sites were two to four times higher than this (Fig. 4C).
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Our soil P data showed high concentrations (1200–1600 mg kg-1) at the 0- to 4-km sites (vegetation Group 1), with the exception of the 0.5-km site, which had uncharacteristically marly soils (Fig. 4D). Marl soils have higher bulk density and lower organic matter content than peat soils, and thus generally have lower P content, when reported on a per-weight basis (Harris and Hurt, 1999). This pattern suggested that the soil P retention capacity was saturated within 4 km of the canal, with an exponential reduction to less impacted soils. From the 4-km site, we found a dramatic logarithmic decline in soil P toward the interior of WCA-2A (Fig. 4D). The exponential curve fit to data from these sites was marginally significant (p < 0.10, r2 = 0.89), probably because our WCA-2A transect had only three sites between 4 and 14 km. Phosphorus concentrations at the 8- and 14-km sites (vegetation Groups 2 and 3) ranged from 283 to 609 mg kg-1 and averaged 418 mg kg-1. Group 3 slough sites had lower soil P than the Group 1 cattail sites, but there were no significant differences in soil P between Groups 1 and 2 or Groups 2 and 3.
Water Conservation Area 3A Transect
Water Conservation Area 3A is managed as one of the wettest regions of the Everglades, particularly at the southern margin where our transect was located (Fig. 1A). Water tends to impound at this southern margin, and rarely flows north from Tamiami Canal (Light and Dineen, 1994; Fig. 1A). Thus, this transect is different from the other four we report here in that the transect is not oriented along a gradient of canal influence. We found cattail only adjacent to the canal (0 km, 40% frequency), the same pattern reported by Doren et al. (1997) for their 1989 sampling. We found a total of 24 plant species along our WCA-3A transect in 1999, only 11 of which were also present in 1989 (roughly 46% temporal coherence). Although total plant species richness was 24 species in 1999, we never found more than five species per square meter at a given site. Total aboveground live biomass was about fivefold greater at the sawgrass sites compared with the spikerush sites, and neither showed any clear patterns with distance (Fig. 5A)
. The spikerush sites had virtually no standing dead plant material, while dead biomass at the sawgrass sites was often five times live biomass (Fig. 5B). Spikerush and sawgrass plant tissues were consistently about 0.005 g P kg-1 dry wt. Notably, this WCA-3A transect was the only one in which we found no elevated plant tissue P values (Fig. 5C).
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Because Tamiami canal water does not regularly flow north into WCA-3A, we did not see the same exponential decline in soil P with distance from the canal that characterized the WCA-2A and even the Loxahatchee National Wildlife Refuge transects (Fig. 5D). We found divergent soil P values at the 0-km sites (C = 799 and S = 413 mg kg-1, respectively). With the exception of the high P concentration at this 0-km sawgrass site, soil P concentrations in WCA-3A averaged 362 mg kg-1 and ranged between 260 and 479 mg kg-1. In addition, soil P was not significantly different among the three plant assemblages (Fig. 3C).
Shark River Slough Transect
The Shark River Slough transect in Everglades National Park was located immediately across the Tamiami Canal from the WCA-3A transect. In 1989, Doren et al. (1997) found cattail in 90% of their sampling quadrats at the 0-km site, adjacent to the Tamiami canal, but they did not find it at any other site. Sawgrass was found in 100% of their quadrats at sites 0.5 to 5 km from the canal. A decade later, we found that cattail was still present at the 0-km site nearest the canal (at 100% frequency). We located and sampled both sawgrass or spikerush marsh plots at all other sites along our 16-km transect (Fig. 6)
. A total of 27 plant species were found along the Shark River Slough transect, compared with 26 species in 1989 and 14 species present both times (Table 2). However, species richness at a given site never exceeded five species per square meter. Total aboveground live biomass ranged from 400 to 800 g dry wt. m-2 in the sawgrass plots compared with less than 200 g dry wt. m-2 in the spikerush plots, and neither showed any clear patterns with distance (Fig. 6A). Notably, live biomass at the 0-km cattail site was less than at the 0.5 slough site, because this particular slough habitat was dominated by dense maidencane (Panicum hemitomon J.A. Schultes) (Fig. 6A). The rest of the slough sites had virtually no standing dead plant material, while dead biomass at the sawgrass sites was roughly equal to live biomass (Fig. 6B). Spikerush and sawgrass plant tissues were consistently less than 0.005 g P kg-1 dry wt. (Fig. 6C), with the exception of sawgrass 0.5 km from the canal, where tissue P content was 0.013 g P kg-1 dry wt., or nearly as high as the P content of the cattail at the 0-km site (Fig. 6C).
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The plant assemblages segregated based on soil P. Group 1, which included a cattail site and an enriched sawgrass site, had significantly higher soil P compared with Groups 2 and 4 (Fig. 3D). The slough (Group 4) and sawgrass assemblages (Group 2) had similar and lower soil P. The single site in Group 3, a Panicum spp.–dominated slough, had moderately enriched soil. We found elevated P in soils only at sites less than 1 km from the canal (Fig. 6D). At sites 1 km from the canal inflow and further, soil P concentrations were relatively constant, averaging 218 mg kg-1 and ranging between 120 and 346 mg kg-1. Our exponential model of soil P concentrations with distance was highly significant (r2 = 0.82, p < 0.001) and predicted a baseline, unimpacted soil total P concentration of 210 mg kg-1 (Fig. 6D).
Taylor Slough Transect
The Taylor Slough transect was established in this study. While we were not able to make comparisons with 1989 transect data, we felt these data should be presented here to facilitate future transect comparisons. Taylor Slough is hydrologically separate from the Central Everglades drainage, which includes our other transects (Fig. 1A). It is part of a much smaller and more localized watershed, and the freshwater marshes here have shorter hydroperiods and marly soils. Both of these features complicate comparisons with our other 1999 transect data. However, the Taylor Slough transect is anchored at the main canal water input point. Notably, until late 1999, water entered Taylor Slough through the S-332 pump structure (Light and Dineen, 1994). Since then, water has been introduced to Taylor Slough via overbank flooding from the L-31W (from which the S-332 once pumped water) that is driven by the new S-332D pump structure located upstream from the S-332 (Sklar et al., 2001).
Some cattail occurs in the deep-water pool immediately downstream of the S-332 pump structure, but because we sampled only marsh sites of typical water depth, we did not find cattail at any of our Taylor Slough transect sites. Overall, we found 25 plant species in Taylor Slough, and up to eight species per square meter at a given site. This was the highest species richness per square meter of the five transects. High diversity is typical of shorter hydroperiod Everglades wetlands, most of which are now found only in Everglades National Park (Gunderson, 1994; Busch et al., 1998). Total aboveground live biomass was less than 600 g dry wt. m-2 in the sawgrass plots and 200 g dry wt. m-2 in the slough plots, although the spikerush site adjacent to the canal had considerably higher biomass (Fig. 7A) . As with most of our slough sites, we found virtually no standing dead plant material in Taylor Slough sites, but dead biomass at the sawgrass sites was approximately double the live biomass (Fig. 7B). Phosphorus in spikerush and sawgrass plant tissue was less than 0.005 g P kg-1 dry wt. except in spikerush immediately adjacent to the canal, where tissue P content was 0.011 g P kg-1 dry wt. (Fig. 7C). Because tissue P content in spikerush is highly sensitive to P enrichment (Daoust and Childers, 1999), this may be an early indication of P impacts associated with L-31W canal water inputs. Our soil P data at this 0-km site do not support this, however (see Fig. 7D).
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Soil P did not vary among the plant assemblages (Fig. 3E). We found soil P levels in Taylor Slough to be among the lowest measured for all transects (Fig. 7D). These soils were predominantly marl, while the other four transects were dominated by peat soils. The mean ash content of these soils exceeded 66%, compared with generally <20% for most of the other soils studied. The P content of soils along this transect were generally between 110 and 150 mg kg-1 and averaged 131 mg kg-1, with the exception of the 4-km site (Fig. 7D). At this site, soils were an organically enriched marl with an ash content of 49% by weight, making this particular site more analogous to the peat soils of our other transects than to the marl soils typical of Taylor Slough. There was no canal effect, so this transect did not conform to the exponential model.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Although the peripheral canals do not represent large point sources of water to the Loxahatchee NWR, water quality impacts have been noted in wetlands near these canals (Doren et al., 1997). These effects are more pronounced in western Loxahatchee, along the L-7 and L-39 canals, which are closest to enriched agricultural water from the EAA (Fig. 1A). Between 1960 and 1987, the area of WCA-1 impacted by cattail-dominated marsh increased from 1 to 4%, and this western area accounted for much of this change (Richardson et al., 1990; Newman et al., 1997). A spatially intensive sampling of soil P levels throughout the Loxahatchee NWR showed the same pattern (Newman et al., 1997). Our macrophyte data showed that, in the past decade, these impacts have continued to worsen along the southwestern fringes (Fig. 2). Doren et al. (1997) found a total of 41 macrophyte species along their 1989 Loxahatchee transect. By 1999, species richness had dropped to 30 (a 27% decline), of which only 11 were found in both 1989 and 1999 (Table 2). This decadal decrease in species richness and shift in species composition was even more dramatic for native plant species (Fig. 8) . Some of these changes may be associated with floristic variability in plant presence depending on the time of year samples were collected, or with the microscale heterogeneity that typifies Everglades marsh plant community composition. However, we feel that these large-scale changes in community richness were real because we sampled a large number of vegetation quadrats along each transect, then pooled species presence data by transect. Few other Everglades studies have shown such marked changes in wetland plant community structure during only a 10-yr period of time (Davis and Ogden, 1994).
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Virtually all of the nonatmospheric inputs of water to WCA-2A have elevated nutrient concentrations because of its proximity to the EAA. This 43 281-ha conservation area was first impounded with both canals and levees in the early 1950s (Light and Dineen, 1994). In the 1970s, it was decided that excess water in the EAA would be pumped south, into these canals, rather than north, into Lake Okeechobee (Sklar et al., 2001). At this point, P loading to WCA-2A increased dramatically (Walker, 1991). In the years immediately following this change in water management, the rate of change of sawgrass marsh or spikerush slough to impacted cattail marsh increased from 1% per year in 1973 to 4% in 1987, and the total area of impacted cattail marsh increased from 2054 ha (5% of total area) in 1973 to 16017 ha (>33% of total area) in 1987 (Wu et al., 1997). This impacted area has continued to expand, and water quality is largely acknowledged to now be the primary control on vegetation change in WCA-2A (Urban et al., 1993; Rutchey and Vilchek, 1999; Wu et al., 1997). Soil P concentrations range from 400 to as high as 1650 mg kg-1 dry wt. (Koch and Reddy, 1992; Debusk et al., 1994). Although some interior areas of WCA-2A are often called unimpacted, it is widely recognized the interior of WCA-2A may no longer be considered oligotrophic (Davis, 1994; Wu et al., 1997).
While WCA-2A is the most impacted region of the remaining Everglades, it is also the best studied. It is not possible in this venue to review all of the research conducted in WCA-2A. Numerous investigators have quantified soil P content with spatially articulate sampling regimes (Koch and Reddy, 1992; Craft and Richardson, 1993; and Debusk et al., 1994, among others). Vegetation change has been well documented, and the interacting effects of eutrophication, hydroperiod, and fire have been investigated here (e.g., Urban et al., 1993; Newman et al., 1996; Wu et al., 1997). Our focus was on revisiting a single transect oriented along the topographic flow trajectory from a key canal input (Fig. 1A). Most canal water enters WCA-2A along the northeast margin, through the S-10 spillway structures. The topographic gradient of nearly 1.5 m runs north–south, although some water does enter the system at the western and southern margins. Our transect macrophyte data showed that the impacted cattail zone had roughly doubled its width, from 2 km in 1989 to 4 km in 1999 (Fig. 4). Even more striking, within 4 km of the canal, where cattail dominated, we found that sawgrass had been completely extirpated from the community.
A more detailed look at the macrophyte community in WCA-2A showed a 33% decline in species richness from 1989 to 1999 (from 24 to 16 species; Table 2). Of the 24 plant species found by Doren et al. (1997), we found only eight in 1999, suggesting an even more dramatic elimination of 66% of plant species and an even more dramatic decline in native species (Fig. 8). Vaithiyanathan and Richardson (1999) found a total of 39 macrophyte species at the 51 sites in WCA-2A that they spot-sampled in 1994. Of these, only 20 species also occurred on our WCA-2A transects, in either 1989 or 1999. They identified 6 of these 39 plant species as being indicative of unimpacted marshes, and 6 as being indicative of impacted marshes, and our comparison of these 12 species with Table 2 showed good coherence, with only two impacted species [inland leatherfern (Acrostichum danaeifolium Langsd. & Fisch.) and denseflower knotweed (Polygonum densiflorum Meisn.)] not found in our WCA-2A transects in either 1989 or 1999. Interestingly, 11 of the 39 species reported by Vaithiyanathan and Richardson (1999) for WCA-2A in 1994 did not occur in any quadrat at any of our sites anywhere in the Everglades, in 1989 or 1999.
Soil P concentrations in WCA-2A showed an exponential decrease with distance beginning 4 km from the canal (Fig. 4D), while in 1989 this decline began 3 km from the canal (Doren et al., 1997). In both cases, soil P content was consistently high at sites 3 to 4 km from the canal, suggesting that (i) the soil had reached P saturation within this zone in 1989 and (ii) this region of saturation had expanded 1 km further by 1999 (Fig. 4D). Similarly, Reddy et al. (1998) found that this impacted area of WCA-2A had increased in extent between 1990 and 1997.
Water Conservation Area 3A is the largest of these impounded and managed regions. It is in the heart of the "ridge and slough" portion of the Everglades landscape, where sawgrass ridge strands and tree islands alternate with deeper sloughs in an orientation parallel to the direction of historical water flow (Davis, 1943; Loveless, 1959). Water Conservation Area 3A is hydrologically divided into northern and southern regions by "Alligator Alley" (Interstate Highway 75; Fig. 1A). The northern area of WCA-3A has inundation frequencies ranging from 32 to 61% of the year and mean water depths ranging from 10 to 18 cm (1972–1984 data; David, 1996). In contrast, southern WCA-3A is managed as one of the wettest regions of the Everglades, with the marsh inundated more than 96% of the time to mean depths of more than 60 cm (1972–1984; David, 1996) near where our transect was located (Fig. 1A). Water no longer flows from north to south through the WCA-3A region. Water tends to impound at the southern margin, and there is essentially never flow from Tamiami Canal north into WCA-3A (Light and Dineen, 1994; Fig. 1A). Thus, this transect was different in that it was not oriented along a gradient of canal influence. Our soil P data reflect this lack of canal nutrient loading (Fig. 5D). One site in direct proximity to the canal had elevated soil P compared with the remaining sites. This was probably a canal effect on canal bank soils that did not extend into the marsh.
The macrophyte data also suggested an abatement of nutrient loading impacts to southern WCA-3A. In 1989, Doren et al. (1997) found cattail in 100% of their sampling quadrats at the site adjacent to the Tamiami canal. A decade later, we still found cattail at the slough site nearest the canal (0 km), but it did not occur at any other site (Fig. 5). Plant community composition changes from 1989–1999 were very different in WCA-3A compared with WCA-2A or Loxahatchee NWR. Doren et al. (1997) found a total of 21 plant species in southern WCA-3A in 1989, and by 1999 this had increased to 22 species (Fig. 8). It is difficult to conclude a trend of increasing biodiversity in the WCA-3A plant community from these data, but it is clear that the 28 and 33% decline in species richness seen in the Loxahatchee NWR and WCA-2A, respectively, did not happen here. Interestingly, this slight increase in species richness was a result of 13 new species in 1999 while 11 of the 1989 plants were not found a decade later (Table 2).
The Shark River Slough transect (ENP) was located across the Tamiami Canal from the WCA-3A sampling, in both the Doren et al. (1997) study and our work. Between 1989 and 1999, water levels and water flow into Shark River Slough were controlled mainly by four large gated structures along this canal: the S-12 structures (Light and Dineen, 1994; the 1989 and 1999 transects were anchored at the S-12C structure). Management of the S-12 structures has decreased water levels, hydroperiods, and flow velocities in Shark River Slough compared with predrainage estimates (Light and Dineen, 1994). Although such hydrologic changes are expected to influence all biotic components of the Everglades ecosystem, this influence may be manifest in complex ways (Gunderson, 1994). For example, Busch et al. (1998) were unable to find a clear relationship between overall marsh macrophyte cover and water depth at three Shark River Slough sites, and they attributed this to complexities associated with hydrologic history, fire frequency, and nutrient effects. This transect study was designed to address changes in the latter between 1989 and 1999.
In 1999, cattail was still present at the 0-km site nearest the canal, but this community had not expanded to the 0.5-km site (Fig. 6). Shark River Slough was not characterized by the same aggressive advancement of cattail that we documented in WCA-2A and along the western margin of the Loxahatchee NWR. Of the 27 plant species found along the Shark River Slough transect, 14 were also found in 1989, when Doren et al. (1997) found a total of 26 plant species (Table 2). Thus, neither Shark River Slough nor WCA-3A showed the large decline in plant species richness, from 1989–1999, that we observed in WCA-2A and the Loxahatchee NWR. Notably, none of the 13 plant species found in 1999 but not in 1989 were newly invaded exotics (Table 2, Fig. 8). Busch et al. (1998) conducted tri-annual plant surveys of selected sites in Shark River Slough from 1985–1995. Of the 30 species identified in their surveys, 20 occurred in the species list of either this study, the 1989 study, or both. The remaining 10 species tended to either be submerged aquatic plants (e.g., Chara spp., Valisneria spp., and Potamogeton spp.), which we did not sample, or had only one occurrence in the Busch et al. (1998) dataset (e.g., Phyla spp., Thalia spp., and Muhlenbergia spp.).
Phosphorus in plant tissues was elevated at only the 0-km cattail site (0.017 g kg-1 dry wt.) and the 0.5-km sawgrass marsh site in Shark River Slough (0.013 g kg-1 dry wt.). Background (unimpacted) tissues from both spikerush and sawgrass were less than 0.005 g P kg-1 dry wt. (Fig. 6C). We found similar and consistent tissue P values at unimpacted sites of all four transects. Sawgrass is quite sensitive to increases in P availability, and its tissue P content increases rapidly with P enrichment (Steward and Ornes, 1983; Daoust and Childers, 1999; Lorenzen et al., 2001). It is thus possible that sawgrass tissue P content may be an early indicator of enrichment conditions that will (eventually) lead to cattail invasion. The elevated tissue P in sawgrass at our 0.5-km site corresponded with soil P concentrations of approximately 660 mg kg-1 at this site (Fig. 6C,D), suggesting canal impacts, although cattail has not yet advanced this far into the marsh. In 1999, we found elevated P soils only at sites less than 1 km from the canal. These values contrast with the 1989 pattern that showed soil P above background levels (about 300 mg kg-1) at sites up to 4 km from the Tamiami Canal (Doren et al., 1997). This area of Shark River Slough, like many portions of the southern Everglades, contains a mosaic pattern of peat soils mixing with marl soils, and marl soils tend to have lower P concentrations per unit dry weight. Data on the ash content of our 1999 Shark River Slough soils (not shown) suggested that several sites had soils with relatively high mineral matter. We do not know the number of 1989 samples that were peat versus marl soils. However, in such spatially heterogeneous conditions, even very small differences in sampling locations between 1989 and 1999 could account for substantial differences in soil P.
In 1999, we encountered most exotic plants near canals in areas with elevated soil P levels and cattail. However, small-leaf climbing fern [Lygodium microphyllum (Cav.) R. Br.] (an exotic viny fern) was found at several sawgrass sites in mid- to eastern portions of Loxahatchee NWR. Small-leaf climbing fern was first reported in 1989, and had spread to more than 12% (9175 ha) of Loxahatchee NWR by 1997 (Brandt and Black, 2001). Most exotics tended to occur in Loxahatchee NWR and WCA-2A, the northern WCA most proximal to the EAA. Five of the six exotic species found in either 1989 or 1999 occurred in WCA-1 or WCA-2A (Table 2). Among all four transects, we found a total of four exotic species in 1989 (barnyardgrass [Echinochloa crusgalli (L.) Beauv.], Peruvian primrose-willow [Ludwigia peruviana (L.) Hara], napiergrass (Pennisetum purpureum Schumach.), and curly dock [Rumex crispus L.]) and five in 1999 (barnyardgrass, common water hyacinth [Eichhornia crassipes (Mart.) Solms], small-leaf climbing fern, nakedstem dewflower [Murdannia nudiflora (L.) Brenan], and curly dock) (Table 2, Fig. 8). Water hyacinth, a floating aquatic plant that was only observed at the 0-km site at WCA-1 in 1999, may have been introduced into the cattail marsh from the adjacent canal. Overall, while the number of exotic species increased along most transects, this increase was not statistically significant and may not have been directly tied to nutrient additions. However, continued environmental alterations have clear implications for future increases in both the number and extent of exotic species invasions (Cuddigy and Stone, 1990; Cronk and Fuller, 1995).
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Our hypothesis tested true along both northern Everglades transects. The impacted cattail marsh along the southwest margin of the Loxahatchee NWR, adjacent to the S-6 pump structure, had expanded from 0.5 to 1 km into the marsh from 1989–1999. Sawgrass was extirpated within 1 km of the canal during this time, and at our 2-km site we found a cattail–sawgrass mixed community. In WCA-2A, the impacted cattail marsh expanded from 2 to 4 km from the canal during this decade, and sawgrass was no longer present within 4 km of the canal in 1999. This pattern did not follow in our more southern transects. Along both the WCA-3A and Shark River Slough transects, we found cattail only immediately adjacent to the canal (at the 0-km sites), and were able to sample both sawgrass and spikerush marsh plots at all sites along our 16-km transects. Doren et al. (1997) reported the same patterns, suggesting little change in plant community status in northern Everglades National Park during the 1990s.
Differences in sampling intensity (and thus sampling area) between the 1989 and 1999 surveys complicated interpretation of decadal changes in species richness in the northern Everglades. However, in our 1999 sampling we either sampled a greater area (e.g., Loxahatchee NWR: 27 sites in 1989 vs. 45 sites in 1999) or a nearly equivalent area (e.g., WCA-2A: 13 sites in 1989 vs. 9 sites in 1999), and still found that plant species richness declined significantly (p < 0.001) in both areas during this time. A reduction of species richness due to nutrient enrichment of wetlands, particularly oligotrophic wetlands such as the Everglades, has been widely reported (Odum et al., 1975; Whigham and Simpson, 1978; Guntenspergen and Stearns, 1981). We observed little change in species richness in the more southerly areas of the Everglades, although we sampled more sites in 1999 (e.g., WCA-3A: 13 sites in 1989 vs. 24 sites in 1999; Shark River Slough: 15 sites in 1989 vs. 23 sites in 1999). The decline in species richness in WCA-2A and Loxahatchee NWR from 1989 to 1999 did not appear to be an artifact of different sampling intensities.
We also saw dramatic changes in species composition along all four transects from 1989 to 1999. Only three species were present in all locations in all years: sawgrass, spikerush, and cattail. Such changes in plant community composition is also widely recognized as an effect of nutrient additions in wetlands (Gorham, 1956; Gosselink and Turner, 1978; Aerts and Berendse, 1988; Hayati and Proctor, 1990). While no trends or patterns to the vegetation shifts are yet discernable from our two samplings, these dramatic shifts in species composition have huge implications for southern Florida restoration efforts. The widespread conversion of slough communities to sawgrass marsh since the 1960s represents a similar shift in species composition within communities that has been attributed to alterations in hydroperiod and fire regimes (Davis et al., 1994) and to dramatic reductions in water flow (McVoy and Crisfield, 2001). These shifts seem to be tied to anthropogenic perturbations rather than environmental stochasticity. As such, the trajectories of these vegetative community changes are unpredictable and may lead to shifts in species dominance that will dramatically alter the aspect and ecology of the Everglades.
Soil P patterns generally mirrored the vegetation patterns. We found elevated soil P values up to 1 km from the canal along our Loxahatchee transect, as did Doren et al. (1997). Cattail marsh had expanded to this distance by 1999, but the community at this site in 1989 was a cattail–sawgrass mix, suggesting that vegetation changes lagged soil P increases here. In WCA-2A, the saturated zone of high soil P had moved from 3 km in 1989 to more than 4 km in 1999, which was consistent with the expansion of cattail and extirpation of sawgrass during the same time period at these locations. We found no canal impacts on soil P patterns along the southern margin of WCA-3A. We found Shark River Slough soils to be impacted within 1 km from the canal and our Taylor Slough transect data suggested that variable organic matter content had a stronger influence on soil P than did canal impacts. Soil P patterns in WCA-2A were similar between 1989 and 1999, and the zone of P saturation increased. This suggested that, although water column total P concentrations decreased in the 1990s in the northern Everglades (e.g., WCA-2A, Loxahatchee NWR) as a result of higher rainfall, greater water depths, and agricultural best management practices (Walker, 1999; Smith and McCormick, 2001), there was no reduction in the spatial extent of soil P impacts. One recent report suggested a synoptic decrease in soil P levels throughout the Everglades landscape during the 1990s (Stober et al., 2001), but we were unable to substantiate this decrease with any other published studies. In fact, DeBusk et al. (2001) documented an increase in soil total P concentrations in the northern Everglades (WCA-2A) from 1990 to 1998, both in enriched cattail marsh and interior sawgrass marsh. Regular sampling of these transects in the future, and consistency in field and analytical techniques in these samplings, will continue to add data that are valuable for critical management decisions and the adaptive management of Everglades restoration efforts.
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