Biotic patterns in stream communities often are attributable to the combined influences of broad-scale environmental factors, regional species pool, watershed-specific processes, and local conditions (Frissell et al., 1986; Poff, 1997; Fausch et al., 2002). Human activities at the landscape level can affect these filters and thus have dramatic effects on stream community structure and function. In particular, watershed land use and land cover (LC) can alter local conditions by directly affecting water physicochemistry, hydrology, and instream habitat, which, separately or in combination, can influence biotic composition and ecological integrity (Lenat and Crawford, 1994; Clements et al., 2000;Paul and Meyer, 2001; Allan, 2004; Schoonover et al., 2006).
Increased levels of agriculture and urbanization in watersheds can lead to several significant effects in many stream features. In general, agricultural and urbanized land has been implicated in increased streamwater pollutants, decreased riparian cover, elevated water temperatures, altered hydrology, increased storm flows and sedimentation, and overall reduced habitat quantity and quality (Paul and Meyer, 2001; Allan, 2004). All of these impacts have been shown to decrease biotic integrity, such as reducing species richness, increasing physiological stress, and causing assemblage shifts (Scott et al., 1986; Weaver and Garman, 1994; Schleiger, 2000; Walters et al., 2003; Helms et al., 2005; Roy et al., 2005a).
Of the multiple direct abiotic consequences watershed land use has on receiving streams, hydrological alteration is one of the more obvious and pervasive (Booth and Jackson, 1997;Groffman et al., 2003; Wang and Lyons, 2003; Walsh et al., 2005). As a result of high levels of watershed imperviousness, streams draining urban and developing watersheds often display flashy hydrographs with multiple peak flows and reduced base flows (Ferguson and Suckling, 1990; Rose and Peters, 2001; Schoonover et al., 2006). Storm flows often increase in magnitude and frequency in agricultural settings because of the use of drainage ditches, loss of wetlands, and soil compaction (Peterson and Kwak, 1999; Allan, 2004). Such hydrological alteration can have far-reaching effects on instream conditions. Increased peak flows can accelerate geomorphic changes in stream channels, leading to increased sedimentation, scour, and channelization, the combination of which may reduce biotic habitat quality and quantity (Wolman, 1967; Hammer, 1972; Bledsoe and Watson, 2001). In addition, stormwater runoff in urbanized watersheds often elevates concentrations of chemical pollutants, including nutrients, metals, pesticides, and pharmaceuticals (Paul and Meyer, 2001; Kolpin et al., 2002; Wang and Lyons, 2003), and water temperature. Changes in temperature may cause thermal pulses and altered thermal regimes in receiving waters, which can increase mortality of sensitive species and skew assemblages towards tolerant species (Galli, 1991; Wang et al., 2000; Krause et al., 2004).
Conceptually, watershed LC can directly alter hydrological regimes which in turn can lead to degradations in physicochemical and geomorphic conditions. Watershed LC also can directly influence physicochemical (e.g., point-source pollution) and geomorphic conditions (e.g., livestock trampling). Altered hydrological regimes can then directly or indirectly influence biota through the alterations in physicochemical and geomorphic conditions.
Previously, we described the relationships between urbanization and fish assemblage structure in streams of western Georgia (Helms et al., 2005). There we reported that declines in biotic integrity and assemblage shifts were associated with watershed LC as well as broad environmental features in high flow seasons. We suspected that differences in hydrographs across these watersheds were important in explaining fish assemblages. This study was designed to investigate the association of LC and altered hydrology on stream fish assemblages in more detail. Specifically, we examined the direct relationships among (1) watershed LC and physical instream factors (hydrology, habitat, and water physicochemistry) and (2) physical instream factors with fish assemblages. Our objective was to determine relative explanatory power of hydrology, physicochemistry, and habitat variables associated with LC change on variation in fish assemblages.