Annual Variation in Fish Assemblages of Watersheds with Stable and Changing Land Use
Schweizer, Peter E., Matlack, Glenn R., The American Midland Naturalist
Freshwater fish assemblages are sensitive to changes in land use, but it is unclear how rapidly assemblages respond to such change or how closely they track physical changes in the stream environment. We monitored fish assemblages at the outflow of three watersheds on the outer coastal plain of Mississippi over a period of 8 y. The watersheds differed in patterns of land use, being predominantly forested, rapidly urbanizing, and long-urbanized, respectively. Watersheds were distinct in fish species composition, differing primarily in abundance of Etheostoma lynceum, E. stigmaeum, Luxilus chrysocephalus and Lythrurus roseipinnis. Species richness was consistently greatest at the predominantly forested site and least at the urbanized site. Fish assemblages were distinguishable in terms of substrate preference, water-speed, silt tolerance, and trophic habit. The stream draining the urbanizing watershed showed an increase in richness of species of high silt tolerance and a decrease in richness of those preferring a gravel substrate over the sampling period. Contrary to expectation, annual variation in composition was modest relative to differences among sites. Annual variation in richness and numbers was unrelated to measures of streamflow or seasonal precipitation. Changes in stream character related to land use change appear to have impacted the fish assemblages of these streams, but the transition to an urban fish assemblage in the urbanizing stream occurred before the study period, well before the obvious signs of physical degradation appeared at the site.
Human land use strongly influences the quantity and character of rainwater runoff, affecting the hydrology and chemistry of streams draining developed land. Physical effects accompanying land development include increased suspended sediment, deposition of silt, altered flow regime, increased nutrient loading and loss of riparian vegetation (Klein, 1979; Welsch, 1991; Stevens and Cummins, 1999; Sonoda et al., 2001; Sutherland et al., 2002). Such changes in the aquatic environment are easily detected by fish, whether directly through changes in water quality (Reash and Berra, 1987; Aadland, 1993; Schleiger, 2000) or changes in habitat (Angermeier and Schlosser, 1989; Poff and Ward, 1990; Sutherland et al., 2002), or indirectly through the aquatic food web (Power et al., 1985; Power, 1992; Tong, 2001). Streams in urbanized areas frequently have depauperate fish assemblages relative to those of undeveloped landscapes, dominated by species with high silt-tolerance, low demand for dissolved oxygen, and little specificity in their ecological and trophic requirements (Scott et al., 1986; Steedman, 1988; Albanese and Matlack, 1999; Schleiger, 2000).
The contribution of landuse change to loss of aquatic species and degradation of stream quality has been well documented (e.g., Garie and McIntosh, 1986; Booth and Jackson, 1997; Finkenbine et al., 2000; Bledsoe and Watson, 2001). It is unclear, however, what ecological mechanism controls the transition from a natural stream assemblage to an urbanized assemblage. It is reasonable to assume the fish community changes rapidly in response to chemical and physical changes occurring in the impacted stream. Individuals of many species have been shown to respond directly to amount of suspended silt, turbidity, dissolved oxygen, and canopy cover (Karr, 1981; Scott and Hall, 1997; Magoulick, 2000; Davis et al., 2003). Alteration in fish assemblages is frequently linked to presence of pollutants such as herbicides and petroleum products (Marsh and Minckley, 1982; Reash and Berra, 1987) presumably acting through their toxic effect on individual fish. Short-term physiological responses may translate into community structure through selective mortality (Tramer, 1977; Ross et al., 1985) or emigration from unsuitable sites (Scott et al., 1986).
In other studies, changes in the fish assemblage do not occur immediately following urbanization, but may take many years with gradual replacement of species (Fitzgerald et al. …