Abstract Introduction Study Area Material & Methods Results >Discussion Acknowledgements Literature Cited Figures Tables

A zone of low hydraulic conductivity limestone is present at the top of the Fort Thompson Formation (Genereux and Guardiario 1998). Vertical head differences in surface water and ground water measured by Sonenshein (2001) indicate that this zone can restrict vertical flow between the surface water in the wetlands west of Levee 30 in Miami-Dade County (Fig. 1) and the ground water. The results of our hydrogeological study indicate that in the study area, the zone of low hydraulic conductivity includes the Q4 HFC and the upper part of the Q3B HFC, i.e., below the 3 meter sampling depths (Fig. 2). The decrease in copepod numbers and species richness below the Q3B HFC recorded in this study might be due to high permeability in Q5, which could act as preferential passageways for epigean forms, and to the presence of the semipermeable layer in Q4 and upper Q3B HFCs, that could act to retard vertical water exchange and, therefore, organism migration.

Wells in the study area were located along canal banks. The karstic area around the canals is mainly recharged by horizontal seepage from canals when water levels in canals are rising, as well as by vertical infiltration of rainfall. During the wet season differences in water head may cause ground water to seep back into the canals.

Studies on seepage to and from canals in the southern Everglades are recent but few. Several canals in the study area have been investigated (Nemeth et al. 2000, Solo-Gabriele et al. 1998, Sonenshein 2001). From Everglades wetlands, water seeps into the Biscayne aquifer, which is about 20 m thick beneath L31N and, due to the high permeability of the aquifer, groundwater flows relatively fast from west towards urban and agricultural area to the east (Fish and Stewart, 1991). The transmissivity of the Biscayne aquifer in the Everglades area west of L-31N was estimated to be 126,000 m²/d (Nemeth et al. 2000). The rate of ground- water seepage into the canal is controlled by the head difference between the aquifer and the canal. Modeled seepage rates from L-31N usually ranged between 0.9 and 3.6 m³/d per meter of levee, but extreme values ranged between -1.8 and 4.5 m³/d (Nemeth et al. 2000). All canals we investigated on L-31N were located on the east side of the canal, except A7. Therefore, ground water seepage directed eastward from the canal could cause passive dispersal of surface copepods horizontally into ground water along the canal bank where we collected them. The seasonal changes we recorded in conductivity, salinity, temperature, and oxygen concentration, suggest considerable surface water infiltration and mixing with groundwater in the shallow aquifer especially during the wet season.

The seasonal variations in water level and in the physical and chemical characteristics of the water did not correlate with the number of copepods that was collected. Nonetheless, higher numbers of copepods were collected in the dry season (low water levels), when the lowering of water levels in L-31N canal increases groundwater flow eastward (Nemeth et al. 2000), and surface copepods may have entered the shallow aquifer, moving horizontally from canals. Several studies reported that the ratio of stygoxenes to stygobites and stygophile individuals in ground water reflects the amount of surface water influence and its seasonal variations (Danielopol et al. 1997, Gibert et al. 1995). When surface water infiltrates the aquifer, stygoxenes penetrate ground waters (Mosslacher 1998).

The nine most abundant species of copepods collected in ground water in the present study are usually reported from surface habitats in the literature and have been collected from surface waters in ENP (Bruno et al. 2000, 2001, 2002a, b; Loftus and Reid 2000; Reid 1989, 1992a). These species, therefore, are stygoxenes, with two exceptions (see below). The temporal presence and abundance of these species in our ground water samples corresponded with their temporal distribution in surface- and ground water in ENP (Bruno 2001, 2002a, in prep.). The number of individuals and species was higher near the surface, where exchange with surface water is higher, because the shallow aquifer is in direct contact with the unsaturated zone. Several species that are rare in the area (M. cushae, Homocyclops ater) (Bruno et al., 2003) appear to be able to enter ground water as a defense against environmental and biological constraints on the surface. These stygoxene organisms were able to disperse into ground water, but they were unlikely to establish local populations in ground water. Unlike stygobite and stygophile organisms, stygoxenes are not specialized for ground water habitat, can die or migrate towards the surface water between flood periods (Dole-Olivier and Marmonier 1992, Mosslacher 1998). In our study, numbers of individuals were high in Q5 during the wet season, whereas high numbers were recorded at deeper samples, Q4, Q3A, and Q3B HFCs during the dry season. At this time of year, stygoxenes appeared to remain in ground water, probably finding a refugium there from the drought. It is possible that these stygoxenes were unable to survive and reproduce in these deeper layers, causing a decline in the number of individuals that were collected later in the study.

Surface copepods that entered the ground water were mostly present at shallow depths, (Q5 HFC), and the shallow samples had high species richness. The surface- and ground water ecotones are sites with considerable hydraulic exchange and, therefore, high biogeochemical activity (Gibert et al. 1997), which usually sustain highly diverse communities (Danielopol et al. 1997). Oxygen content was always higher near the ground water surface in this study. For many surface water copepods, deep ground water might be beyond their tolerance limit for oxygen content.

The effect of canals on dispersal of planktonic copepods was strong, especially because these organisms are able to spend part of their life cycle in ground water. The similarities in species composition between test wells along canal reaches suggest that copepods mainly enter ground water horizontally along canals via active and passive dispersal. The highest number of individuals recorded in the study was associated with the species Orthocyclops modestus, found in well B1 (Fig. 1). Orthocyclops modestus is a surface water species; however, in ENP it is abundant in ground water in the Rocky Glades, where it is one of the dominant species in ground water copepod communities (Bruno, in prep.), and was collected in shallow wells in the alluvial floodplains of two rivers in Montana (Reid et al. 1991).

Diversity of freshwater copepod fauna in southern Florida is comparable to diversity of other subtropical wetlands, and to adjacent regions. For example, Reid and Marten (1994) report 25 species of cyclopoids from marshes and temporary waters in Louisisana and Mississippi; Reid (1993a) reports 17 species of cyclopoids and 15 species of harpacticoids from temporary and perennial wetlands in Central Brazil; Suarez-Morales and Reid (1998) recorded 5 species of calanoids, 28 species of cyclopods, and 12 species of harpacticoids from Quintana Roo and Yucatan in Mexico. However, freshwater copepod richness in the Everglades is lower than in North America (Reid 1994). An updated checklist of species collected in freshwater in ENP and adjacent regions (Bruno et al. 2003) recorded 2 species of calanoids, 24 species cyclopoids, and 13 harpacticoids. Even though surface and groundwater habitats in ENP have been extensively investigated by Bruno for the past 5 years, the number of endemic species recorded for ENP is low, and most of the taxa are generalist and widely distributed in North or Central America (Bruno et al. 2003). Also, the number of stygophile and stygobite species in ENP is low when compared with other areas in North and Central America (Strayer 1988, Strayer and Reid 1999, Suarez-Morales and Rivera-Arriaga 2000). Groundwater habitats in ENP may be a result of historical factors, i.e., the relatively young age (approximately 5,000 years) of the Everglades, which might not have allowed some taxa enough time to disperse here (Bruno et al. in prep, Loftus and Reid 2000, Reid 1992a). Also, 5,000 years might not have been sufficient time for ground water colonizers to evolve adaptations to life in subterranean habitats, since it is hypothesized (Rouch and Danielopol 1987) that generalistic eurytopic species, such as many of the stygoxenes found in ENP, will eventually become hypogean forms, given enough time. It is interesting to notice the surprisingly low diversity of harpacticoids in groundwater habitats in ENP, since harpacticoids are a taxon with the highest number of stygobitic forms within the copepods (Galassi 2001). In North America, there are only a few harpacticoid genera with exclusively stygobitic forms (i.e. Nitocrellopsis, Stygonitocrella, Psammonitocrella, Parastenocaris). This provides additional evidence of the epigean affinity of the subterranean communities; it is also possible that the distribution of harpacticoid populations in South Florida is very spotty and that the population is composed of few individuals.

The closest area in which true troglobitic copepods have been known to occur is the northern part of Florida (Reid and Strayer 1994, Strayer and Reid 1999), which has an older geological history (Davis and Legrand 1972). Genera such as Acanthocyclops, Diacyclops, and Mesocyclops have stygobitic forms that are morphologically very closely related to the epigean forms in the same area (Reid (1992c, 1998), Reid and Strayer 1994).

The only stygophiles collected in the present study belong to the genus Diacyclops, a genus widely distributed in groundwater related habitats. The most abundant of the two, Diacyclops nearcticus, is probably one of the few stygophile species in ENP. The male of this species was described by Bruno and Reid (Bruno et al. 2000) from specimens collected in a well in the Rocky Glades, and was subsequently collected from wells there (Bruno 2003). The second stygophile taxon was Diacyclops crassicaudis brachycercus (1 specimen collected at a depth of 7.5 m). This species is present in Europe and in North America. In both continents, records are mostly from small ephemeral waters or puddles, but ground water records (such as caves, river heads, springs, seeps, and stream hyporheos) are also numerous (Reid, 1992b). This species is apparently extremely rare in South Florida, though it is adapted to ground water.

Results from our study suggest that canals can influence ENP copepod communitites in surface and ground waters. Surface populations of organisms in the Water Conservation Areas north of ENP could be easily transported into canals and carried through the canal system that borders ENP. As a consequence, high seepage rates to and from canals can change the community composition in ground water and in adjacent surface waters, and canals might be a potential means of species introduction in ENP. Faunistic data agree with the modeled seepage directions for ground water along canals along the eastern border of ENP by Nemeth et al. (2000). Thus, the presence of surface copepods in ground water along the canal systems can be used as an indicator of water seepage; the knowledge of hydrologic exchange between surface water and ground water is critical to understanding the movement of water and solutes in ENP, given that the Comprehensive Everglades Restoration Project is aimed at partly restoring water quality and quantity for the Everglades. One of the needs of the restoration effort is to account for all significant hydrologic inflows and outflows to the Everglades ecosystem. The seepage of ground water under Levee 31N constitutes a substantial outflow of water from this system (Nemeth et al. 2000).