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

Abstract

The connectivity between the fish community of estuarine mangroves and that of freshwater habitats upstream remains poorly understood. In the Florida Everglades, mangrove-lined creeks link freshwater marshes to estuarine habitats downstream and may act as dry-season refuges for freshwater fishes. We examined seasonal dynamics in the fish community of ecotonal creeks in the southwestern region of Everglades National Park, specifically Rookery Branch and the North and Watson rivers. Twelve low-order creeks were sampled via electrofishing, gill nets, and minnow traps during the wet season, transition period, and dry season in 2004-2005. Catches were greater in Rookery Branch than in the North and Watson rivers, particularly during the transition period. Community composition varied seasonally in Rookery Branch, and to a greater extent for the larger species, reflecting a pulse of freshwater taxa into creeks as marshes upstream dried periodically. The pulse was short-lived, a later sample showed substantial decreases in freshwater fish numbers. No evidence of a similar influx was seen in the North and Watson rivers, which drain shorter hydroperiod marshes and exhibit higher salinities. These results suggest that headwater creeks can serve as important dry-season refugia. Increased freshwater flow resulting from Everglades restoration may enhance this connectivity.

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Biological connectivity between fish communities of mangrove regions and those of other marine and coastal habitats (e.g., coral reefs, seagrass beds, sand, and mudflats), although deserving further attention, has been explored in a number of recent studies (see reviews by Beck et al., 2001, Gillanders et al., 2003; Sheridan and Hays, 2003; Mason et al., 2005; Sheaves, 2005; Faunce and Serafy, 2006). The presence of mangroves along coastal areas enhances the richness, abundance, and biomass of fishes in marine habitats (e.g., coral reefs; Nagelkerken et al., 2002; Dorenbosch et al., 2004; Mumby et al., 2004). Mangroves provide nursery grounds for larval and juvenile marine fishes and crustaceans (Robertson and Duke, 1987; Laegdsgaard and Johnson, 1995; Nagelkerken et al., 2000) due to their high prey abundance (Robertson et al., 1988; Sheridan, 1997) and their role as a predation refuge (Primavera, 1998; Acosta and Butler, 1999). Juvenile survival may be enhanced in shallow mangrove habitats where structural complexity, shading, and turbidity are relatively high (Laegdsgaard and Johnson, 2001; Ellis and Bell, 2004). Mobile marine fishes use mangrove habitats transiently as foraging grounds (Blaber and Milton, 1990; Chong et al., 1990), reproductive grounds (Chaves and Bouchereau, 2000), or move in when environmental conditions in diel or seasonal cycles are favorable (e.g., at high tide or with increased salinity or temperature; Ley et al., 1999; Barletta et al., 2005).

By comparison, the connectivity between fish communities in mangrove regions and upstream freshwater habitats has received much less attention. A reason for this is that in many mangrove systems, the freshwater influence is small, and the contribution of freshwater fishes to the estuarine community is limited (Pinto and Punchihewa, 1996; Laroche et al., 1997; Kuo et al., 1999; Nordlie, 2003; Hindell and Jenkins, 2004). In the Greater Everglades Ecosystem, shallow vegetated freshwater marshes transition into an extensive region of tidal mangrove forests (up to 15 km in width, over 60,000 ha of mangroves), which dominates the landscape along the southwest Florida coast (Smith et al., 1994). At the ecotone, mangrove-lined creeks drain upland marshes into a network of interconnecting estuarine rivers, bays, and mangrove forests. The ecosystem is rainfall-driven, marked by strong seasonality (high rainfall in the summer and fall, low in the winter and spring), which greatly influences the spatial extent of inundation of freshwater marshes, as well as the salinity regime of this broad estuarine region (Gunderson and Loftus, 1993).

As in other estuarine systems, salinity levels play an important role in structuring the plant and animal communities of the Greater Everglades Ecosystem (Montague and Ley, 1993; Serafy et al., 1997; Ley et al., 1999; Lorenz, 1999; Faunce et al., 2004). Historically, large volumes of freshwater reached estuarine areas, particularly during the wet season (Fennema et al., 1994). Today, drainage, channelization, and impoundment of marshes have greatly diminished the freshwater inflow into estuarine areas, resulting in substantially higher and more variable salinity regimes (Smith et al., 1989; Montague and Ley, 1993; Light and Dineen, 1994; McIvor et al., 1994). Fish community response to the natural and the anthropogenically-derived variation in freshwater inflow and salinity has been relatively well-studied in the southern and eastern parts of the ecosystem, namely, Florida Bay and Biscayne Bay (Thayer et al., 1987; Montague and Ley, 1993; Serafy et al., 1997; Ley et al., 1999; Lorenz, 1999; Serafy et al., 2003; Faunce et al., 2004; Lorenz and Serafy, 2006); but remains understudied along the southwest region (but see Green et al., 2006), where the mangrove zone is substantially more extensive than in the southern and eastern parts (Smith et al., 1994). Mangrove creeks along this area also drain generally longer hydroperiod marshes than the southern and eastern regions (Fenema et al., 1994). These marshes (Shark Slough) support more diverse and abundant fish assemblages than southern marshes (Taylor Slough) (Trexler et al., 2001; Chick et al., 2004; Green et al., 2006); thus high connectivity between the mangrove and freshwater fish communities may be expected.

In this study, we examined variation in the fish community of headwater mangrove creeks in response to seasonal fluctuations in freshwater flow and salinity in the southwestern region of Everglades National Park (ENP). In particular, we explored the role of low-order, ecotonal mangrove creeks as dry-season refuges for freshwater fishes. As marsh water levels drop, fishes are forced into deeper habitats such as alligator holes, solution holes, canals (Kushlan, 1974; Nelson and Loftus, 1996; Chick et al., 2004; Kobza et al., 2004; Rehage and Trexler, 2006), and presumably headwater creeks. We sampled the fish community in the uppermost stretches of creeks, where habitat may be most suitable for freshwater species because of proximity to marshes and low salinity regimes. A secondary goal of this study was to compare sampling efficiency among gears. Sampling with electrofishing and gill nets targeted large fishes, whereas minnow traps targeted small fishes. Sampling focused on two regions: Rookery Branch (RB) and the North and Watson rivers (NW) (Fig. 1). Headwater creeks in the RB region link the main freshwater drainage of the southern Everglades (Shark Slough) to Tarpon Bay, and the Shark and Harney rivers. Creeks in the NW area are headwaters of the North and Watson rivers which flow into Whitewater Bay. In neither system have the fish communities in the oligohaline reaches received enough attention to describe their seasonal and long-term dynamics beyond surveys that provided inventory data (Tabb and Manning, 1961; Odum, 1971; Loftus and Kushlan, 1987), despite their historical importance as a prey source for wading birds (Ogden, 1994) and their key role in the mangrove food web (Odum, 1971).

Figure 1. Map of southwestern Everglades National Park showing location of headwater creeks included in this study. Twelve creeks (filled circles) were sampled: six in the North and Watson rivers (NR1-3 and WR4-6) and six in the Rookery Branch region (RB7-12). Location of four reference NPS hydrological stations is indicated by open triangles (see Figure 2 for data from these stations). Distance from sampling sites to stations varies. CN and NR stations are located in creek channels downstream of sites sampled (CN is 4200 m downstream from RB7 and NR is 900 m downstream from NR3). P35 and P38 are located in freshwater marshes upstream of creeks (P35 is 300 m from RB9 and P38 is 6300 m from NR2). [larger image]

Related information:

SOFIA Project: Role of Marsh-Mangrove Interface Habitats as Aquatic Refuges for Wetland Fishes and Other Aquatic Animals