Seasonal fish community variation in headwater mangrove creeks in the southwestern Everglades: an examination of their role as dry-down refuges

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Disturbance from recurrent, seasonal dry-down events has a strong structuring effect on freshwater fish communities inhabiting Everglades marshes (Kushlan, 1974; Loftus and Eklund, 1994; Trexler et al., 2001, 2005; Chick et al., 2004). In response to dry-down, fish move from marshes to deeper habitats such as alligator holes, solution holes, and canals (Nelson and Loftus, 1996; Trexler et al., 2001; Kobza et al., 2004; Rehage and Trexler, 2006). Thus, access to dry-season refuges is a key element underlying long-term population dynamics in freshwater fishes (DeAngelis et al., 1997). Our results indicate that mangrove creek headwaters in the southern part of the ecosystem can also serve as important dry-season refugia, particularly for large-bodied species, whose abundance is strongly limited by seasonal dry-down (Trexler et al., 2001, 2005; Chick et al., 2004). A pulse of freshwater fishes was detected in RB creeks in February as marshes upstream began to dry periodically, which resulted in marked seasonal variation in patterns of abundance and composition in RB headwaters. The pulse was composed of both predatory species, such as Florida gar, bowfin, and centrarchids; and of the small cyprinodontoids, although some species (e.g., mosquitofish) appeared to reside in creek all-year around. No evidence of a similar pulse was noted in the North and Watson rivers, where the contribution of freshwater taxa to the community was consistently small (0%-24%) and showed lower seasonal variation.

Mangrove fish communities are highly variable in both short (tidal) and longer time scales (seasonal) because of pronounced environmental fluctuations (Kupschus and Tremain, 2001). Seasonal changes in the abundance and composition of tropical and subtropical fish communities have been reported in mangrove systems throughout the world, including Madagascar (Laroche et al., 1997), Brazil (Barletta et al., 2005), Australia (Loneragan et al., 1986), the Solomon Islands (Blaber and Milton, 1990), Taiwan (Lin and Shao, 1999), and Mexico (Yanez-Arancibia et al., 1988). Of these examples, mangrove creeks of the Caeté River estuary in Brazil (Barletta et al., 2005) exhibit the greatest freshwater inflow and may closely resemble the mangrove creeks at our Everglades sites. However, the directionality of seasonal variation in RB and NW creeks is opposite that of Caeté, where the influx of freshwater species occurs during the wet season when salinities are low. Seasonal community dynamics have also been shown in Everglades mangrove regions (Thayer et al., 1987; Ley et al., 1999; Lorenz et al., 1999; Faunce et al., 2004). In Florida Bay and Whitewater Bay, Thayer et al. (1987) reported increases in both fish numbers and biomass during the wet season. Our results from NW fit their findings. Fish abundances varied monthly in mangrove creeks of southeastern Florida Bay, with those of freshwater species increasing during February and March (Faunce et al., 2004), as seen in our RB sites.

Several factors may be responsible for the lack of a freshwater species influx in NW headwaters. Marshes upstream of NW creeks have shorter hydroperiods than those upstream of RB sites, and consequently may contain lower densities of fishes, particularly of the large species (Lorenz, 1999; Trexler et al., 2001, 2005; Chick et al., 2004). Freshwater fishes may be absent from NW creeks simply because the pool of potential marsh migrants is small. Secondly, salinity levels are higher in NW than RB headwaters, and may approach or exceed the physiological tolerances or preferences for some of the freshwater species such as centrarchids (Loftus and Kushlan, 1987). However, other marsh inhabitants exhibit high salinity tolerances (Lorenz and Serafy, 2006; Nordlie, 2006), and should find suitable salinity conditions in NW, despite the fact that there were rarely caught there.

Thirdly, the pattern of marsh dry-down differed between regions, and marshes upstream of NW remained flooded beyond our dry-season sample. A pulse of freshwater species could have possibly occurred later in the season, and would have been missed by our sampling. Other studies, however, suggest that marsh fishes move into deep-water refugia well in advance of low-water conditions (Chick et al., 2004; Rehage and Trexler, 2006). Even in long-hydroperiod marshes that rarely dry, and where direct mortality due to dry-down conditions is unlikely, large-fish densities decrease significantly in the open marsh during the dry season and concentrate in deep-water refuges. Marsh water-levels upstream of NW were low (close to 5 cm); therefore, a pulse of migrants should have occurred by our April sample. Furthermore, salinities in later months exceeded 15 in NW, which is too stressful for many of the potential migrants. Another explanation may be a higher abundance of alternative dry-down refuges (e.g., solution and alligator holes) in marshes upstream of NW relative to RB. However, abiotic (high ammonia and low oxygen) and biotic (high predation) conditions in these alternative refuges are often stressful (Nelson and Loftus, 1996; Kobza et al., 2004), and could make these refuges less preferred relative to creeks. Lastly, small differences in local topography (e.g., the presence of berms along creeks) could limit fish movement in and out of creeks, perhaps to a greater extent in NW than RB sites (Green et al., 2006).

The pulse of freshwater species in RB occurred early in the dry season, and despite the fact that RB assemblages remained dominated by freshwater species in the later sample, their abundances decreased considerably. This decrease could be explained by a large-scale return of the freshwater taxa to marshes, if marshes had reflooded. However, this was unlikely in 2005 since water levels in upstream marshes at the time of our transition and dry season samples were relatively low and similar (11 and 13 cm, respectively). Alternatively, the increase in salinity between the transition and dry samples in RB (from < 1 to 5) could explain the decline in freshwater species through mortality or movement to more suitable salinity environments. An important source of mortality could also be predation. Mangrove habitats provide important foraging grounds for marine and estuarine piscivores (Blaber and Milton, 1990; Chong et al., 1990). Even though mangroves can provide a refuge from predation because of their high habitat complexity (Primavera, 1998; Acosta and Butler, 1999), the abundance of predators is not necessarily lower in these shallow coastal habitats (Sheaves, 2001). In RB creeks, piscine predators such as snook, Florida gar, largemouth bass, and bowfin were abundant early in the dry season and could account for the declines in the cyprinodontoids between February and April. Top predators, such as alligators, wading birds, and bull sharks could possibly account for the decreases in the abundance of the larger freshwater species. More extensive, paired sampling in creeks, marshes (including other dry-down refuges), and downstream portions of the estuary is needed to discriminate between these and other plausible explanations for the timing and extent of pulsing of freshwater taxa into headwater creeks.

Results from studies that rely on a single gear type to sample mangrove fishes may be restricted in their applicability because of gear selectivity (Rozas and Minello, 1997). CPUE in gill nets was appreciably lower than that for electrofishing, and catch composition differed between gears. This suggests that gill nets with the 30- min soak times used in our study do not provide a reliable index of abundance, nor detect seasonal variation in community structure, even if previous studies have shown that gill nets may be better at capturing certain aspects of target fish populations, such as size structure, relative to electrofishing (Colvin, 2002). Comparison of the small-fish CPUE showed that minnow-trap placement in the water column strongly affects catch numbers and species composition. This is likely explained by variation in microhabitat use among small-fish species; a factor that needs to be considered if sampling is targeted to multiple species.

The influx of freshwater species into RB headwater creeks may enhance estuarine fish abundance and richness, and should provide an important prey source for marine and estuarine piscine predators, as well as for avian predators. Interannual variation in drying patterns may create circumstances in which other creek headwater habitats (including NW) may serve as dry-season refugia. In other parts of the ecotone, factors such as high salinity and local topography could limit the connectivity between mangrove and upstream marsh habitats (i.e., Green et al., 2006). Ongoing restoration of the Greater Everglades ecosystem aimed at re-establishing historical freshwater flows (CERP, 1999), could greatly enhance this connectivity. Increased freshwater flows are expected to result in reduced salinities, prolonged pooling of freshwater, and a spatially-expanded and seasonally-extended oligohaline zone at that marsh-mangrove ecotone, including our study creeks (Davis et al., 2005). These conditions should make large portions of the mangrove region suitable for freshwater species. In northern parts of Florida Bay, increased freshwater flow has resulted in higher abundance and biomass of small-bodied freshwater taxa, and thus the recovery of the demersal forage fish community (Lorenz, 1999; Lorenz and Serafy, 2006). Similar effects could occur in our study area in southwestern Everglades, but overall responses of freshwater fishes are somewhat uncertain. Increased freshwater flow and decrease salinity are expected the influence multiple components of marine, estuarine, and freshwater food webs and how these interact over a complex and heterogeneous ecotonal landscape. Further research is needed to develop predictions and gain a better understanding of the net effects of hydrological restoration on the freshwater fish community of mangrove headwaters.