Results
Fish species diversity was highest at Wadley (H=2.88) and lowest at Horseshoe Bend (H=2.49), although values were generally similar across sites (range among sites was 0.39; Table 1). Centrarchids made up the largest proportion of collected fish across all sites with their highest contribution being in the tailrace and at Horseshoe Bend (Figure 2). Cyprinids/leuciscids contributed the second largest proportion of fish collected from all sites except the tailrace (where they were third highest), where the second largest proportion was from percids (Figure 2). The proportion of the assemblage composed of percids was also relatively high at Wadley, with much lower values at Lee’s Bridge and Horseshoe Bend (Figure 2). Catastomids made up larger proportions of the collected families at Lee’s Bridge, Wadley, and Horseshoe Bend and a much lower proportion in the tailrace (Figure 2). At Lee’s Bridge, 3 of the 10 most abundant species were centrarchids and at the downstream sites it was between 3 of 10 in the tailrace and 4 of 10 at Horseshoe Bend (Table 2).
NMDS yielded stable, two-dimensional ordinations (Figure 3a, final stress = 0.09) in 40 iterations. Ordination axis MDS-1 correlated most positively with Snail Bullhead (T= 0.98), Bandfin Shiner (T= 0.80), Black Madtom (T =0.80), and Rough Shiner (T= 0.79; Figure 4) and most negatively with White Bass (T=- 0.96), Yellow Perch (T =-0.92), River Redhorse (T =-0.92), and Bullhead Minnow (T =-0.92). Axis MDS-2 correlated most positively with Blueback Herring (T =0.63), Skipjack Herring (T =0.63), Golden Shiner (T =0.63), and Black Bullhead (T =0.63); and most negatively with Grass Carp (T =-0.62), Pretty Shiner (T =-0.62), Snail Bullhead (T =-0.34), and Bandfin Shiner (T= -0.31; Figure 4). At the family level, MDS-1 correlated most positively with ictalurids and percids and most negatively with clupeids, catastomids, and moronids (Table 3).
Ordinations showed clear separation between sites as well as seasonal variation in the assemblages within sites (Figure 3a). Separation across sites in MDS-1 showed longitudinal variation in fish assemblage downstream of Harris Dam; in addition, as distance downstream of the tailrace increased, MDS-1 of the fish assemblage approached that of the upstream reference site, identifying a potential recovery gradient (Figure 3a). Differences between seasons were driven by generally negative summer and positive spring values for MDS-2 (Figure 3a). MDS-2 appeared to vary seasonally, being higher in the spring (March-May) and lowest in the summer (June-August; Figure 3a).
Multiresponse Permutation Procedure and Indicator Species Analysis
MRPP indicated a significant difference among site-specific fish assemblages (p = 0.001, A =0.298, δ=0.290; Figure 3b). Indicator species analysis identified 1 - 5 significant indicator species depending on site (Table 4). Lee’s Bridge had the highest number of indicator species which included five separate families (Table 4). In the tailrace three indicator species were identified including one percid (an Alabama listed Priority 2 species; Mirarchi et al. 2004), one ictalurid, and one centrarchid. Wadley had three indicator species including one percid and two campostomids (Table 4). The single indicator species identified at Horseshoe Bend was Black Redhorse, a campostomid (Table 4). Average between- and within-group dissimilarities grouped Horseshoe Bend and Wadley as the site block with the least dissimilarity and Lee’s Bridge as the most dissimilar (Figure 5). The Tailrace fell within a block containing the downstream sites although separated from Wadley and Horseshoe Bend (Figure 5).