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1728 D. B. OSMUNDSON ET AL. Ecological Applications Vol. 12, No. 6 TABLE 3. Factor loadings for the first factor from principal- components analysis using several attributes of the stream- bed. Attribute Loading Substrate <2 mm -0.835 Dso 0.857 Void volume 0.818 % Embedded -0.832 Total DFS 0.876 Midcolumn velocity 0.465 Detritus 0.500 Notes: All data were loge transformed for analysis. Attri- butes, in order, are percentage of substrate particles <2 mm, median particle size of the surface layer (Dw), volume of interstitial void space, percentage of the surface area con- sisting of fines (% Embedded), absolute depth of free space (DFS), midcolumn water velocity, and detritus dry mass. indicating low amounts of fine sediment and lower scores indicating higher amounts. The size of the load- ing coefficients reinforce the strength of the interre- lationships among these physical parameters. Water-quality parameters.-Nutrient analyses indi- cated high variability among samples, but NO,, NO,, total inorganic N, total N, and total P increased sig- nificantly (ANCOVA, P < 0.00001 for all) with dis- tance downstream (not, shown). NH3 and orthophos- phorus did not display significant increases down- stream (ANCOVA, P > 0.15), but strata 10 and 11 had among the lowest concentrations. General observations during the study period indicated turbidity was highly variable over time. Our data, representing only a few snapshots in time, are therefore not particularly instruc- tive. From the three days in which it was measured, the mean significantly increased with distance down- stream (ANCOVA, F,,64 = 19.60, P = 0.00004), but strongly displayed this pattern only during the spring 1995 survey. Biological parameters.-Chlorophyll a, our index of active periphyton biomass, declined downstream. This decline was significant in riffles (ANCOVA, F1,s91 = 136.8, P < 0.00001) and runs (ANCOVA, F1.423 = 133.3,P < 0.00001), with the decline in runs especially pronounced (Fig. 3a). Although chlorophyll a was greater in riffles than in runs (ANOVA, F,,701 = 68.34, P < 0.00001), differences occurred only downstream of stratum 9. In strata 9-11, chlorophyll a was essen- tially the same in runs and riffles. Invertebrate biomass also declined downstream in both riffles (ANCOVA, F,,391 = 201.5, P < 0.00001) and runs (ANCOVA, F,,186 = 337.5, P < 0,00001) and was also higher in riffles than in runs (ANOVA, F1,726 = 269.0, P < 0.00001). However, unlike chlorophyll a, invertebrate biomass was much higher in riffles than in runs of strata 9-11 (Fig. 3b). Detritus showed a less precipitous de- cline with distance downstream (Fig. 3c), but the de- clines were still significant for riffles (ANCOVA, F,, 391 = 69.57, P < 0.00001) and runs (ANCOVA, F1,as6 = 57.05, P < 0.00001). Riffle substrates generally con- tained a greater biomass of detritus than did run sub- strates (ANOVA, F,,711 = 126.9, P < 0.00001). Catch rates of all fish declined with distance down- stream when expressed as either number caught (Fig. 3d; ANCOVA, Fl, 196 = 191.2, P < 0.00001) or as biomass caught (not shown) (ANCOVA, F,, 196 = 147.9, P < 0.00001). Subsets of the total catch rate (numbers caught), presumably more relevant to pikeminnow food availability, displayed similar trends: the catch rate of forage-sized (100-300 mm) individuals (excluding spined species) declined downstream (ANCOVA, F,, 196 = 74.88, P < 0.00001); catch rates of the three dom- inate soft-rayed, fusiform, native fish species: roundtail chub (Fig. 3d), flannelmouth suckers (Fig. 3e), and bluehead suckers (Fig. 3f) also declined downstream (ANCOVA, all 17,,391 > 60.0, P < 0.00001). For all species (all sizes), catch rates were highest in strata 7- 9 and 11; for forage-sized fish, catch rates were similar among strata 7-11 (Fig. 3d). For total fish, native fish, and forage-sized fish, catch rates were consistently much lower downstream of stratum 7. Catch rates of all sizes of the two dominant native fish species, flannelmouth sucker and particularly blue- head sucker, were "significantly higher in riffles than in runs on a river-wide basis (Fig. 3e and f ). For bluehead sucker in strata 10 and 11, catch rates in the two habitat types were nearly identical, but rates were significantly higher in riffles than in runs in strata 5-9 (P < 0.05). Although bluehead suckers made up a majority of for- age-sized fish in downstream strata 2-6, very few were captured from run habitats. On a river-wide basis, catch rates of bluehead sucker from riffles were higher than from runs (ANOVA, F1,363 = 84.09, P < 0.00001). Flannel-mouth suckers were similarly distributed, with greater catch rates from riffles than from runs (ANO- VA, F1.311 = 20.25, P < 0.00001); however, no within- stratum differences were significant (P > 0.05). For roundtail chub, catch rates in the two habitat types were similar (ANOVA, 171, 113 = 1.39, P = 0.24), and only a few fish were found downstream of stratum 7 (Fig. 3d). Relative body condition (K) of both sucker species and roundtail chub decreased downstream (regression, for all: N = 405-3203; P < 0.0003), with the lowest condition exhibited by bluehead sucker in the three most downstream strata (Fig. 3g). Densities of adult Colorado pikeminnow ?550 mm TL also decreased in a downstream direction (Fig. 3h). Densities were highest in strata 8 and 9, declined ex- ponentially between strata 8 and 5, and were consis- tently low in strata 1-5. Because of instream barriers at the top of stratum 9, pikeminnow did not occur in strata 10 and 11. Interrelationships Physical habitat-periphyton/macroinvertebrate bio- mass relationship.-Highly significant correlations were found between both chlorophyll a and inverte-