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The persistence and structure of a small but morphologically diverse fish assemblage of a California stream was examined to see if it had the characteristics of a deterministically regulated or stochastically regulated community. We evaluated persistence by monitoring the fish populations for 5 yr and examined resource use by measuring summer microhabitats and diets of each species. We also compared the microhabitats and diets of the juveniles and adults of each species to investigate the hypothesis that in stream fish assemblages at low diversity, juvenile fishes essentially function ecologically as separate species. Species composition in each of four stream sections was persistent over the 5—yr period, which included extreme flood conditions. Differences in species composition among sections reflected differences in their physical characteristics. The most abundant species were segregated by habitat, microhabitat, and/or diet. In addition, in all but one species, young—of—year were ecologically segregated from the adults; this increased the functional complexity of the assemblage. The observed fish assemblage thus appears to have the characteristics of a highly structured community, as expected under Connell's “niche diversification hypothesis.” The structure observed is probably characteristic of fish assemblages in cold—water streams throughout North America that contain the same or similar species.

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Best management practices (BMPs) have been developed to address soil loss and the resulting sedimentation of streams, but information is lacking regarding their benefits to stream biota. We compared instream physical habitat and invertebrate and fish assemblages from farms with BMP to those from farms with conventional agricultural practices within the Whitewater River watershed of southeastern Minnesota, USA, in 1996 and 1997. Invertebrate assemblages were assessed using the US EPA's rapid bioassessment protocol (RBP), and fish assemblages were assessed with two indices of biotic integrity (IBIs). Sites were classified by upland land use (BMP or conventional practices) and riparian management (grass, grazed, or wooded buffer). Physical habitat characteristics differed across buffer types, but not upland land use, using an analysis of covariance, with buffer width and stream as covariates. Percent fines and embeddedness were negatively correlated with buffer width. Stream sites along grass buffers generally had significantly lower percent fines, embeddedness, and exposed streambank soil, but higher percent cover and overhanging vegetation when compared with sites that had grazed or wooded buffers. RBP and IBI scores were not significantly different across upland land use or riparian buffer type but did show several correlations with instream physical habitat variables. RBP and IBI scores were both negatively correlated with percent fines and embeddedness and positively correlated with width-to-depth ratio. The lack of difference in RBP or IBI scores across buffer types suggests that biotic indicators may not respond to local changes, that other factors not measured may be important, or that greater improvements in watershed condition are necessary for changes in biota to be apparent. Grass buffers may be a viable alternative for riparian management, especially if sedimentation and streambank stability are primary concerns.

[1] Natural pieces of wood provide a variety of ecosystem functions in streams including habitat, organic matter retention, increased hyporheic exchange and transient storage, and enhanced hydraulic and geomorphic heterogeneity. Wood mobilization is a critical process in determining the residence time of wood. We documented the characteristics and locations of 865 natural wood pieces (>0.05 m in diameter for a portion >1 m in length) in nine streams along the north shore of Lake Superior in Minnesota. We determined the locations of the pieces again after an overbank stormflow event to determine the factors that influenced mobilization of stationary wood pieces in natural streams. Seven of 11 potential predictor variables were identified with multiple logistic regression as significant to mobilization: burial, effective depth, ratio of piece length to effective stream width (length ratio), bracing, rootwad presence, downstream force ratio, and draft ratio. The final model (P < 0.001, r 2 = 0.39) indicated that wood mobilization under natural conditions is a complex function of both mechanical factors (burial, length ratio, bracing, rootwad presence, draft ratio) and hydraulic factors (effective depth, downstream force ratio). If stable pieces are a goal for stream management then features such as partial burial, low effective depth, high length relative to channel width, bracing against other objects (e.g., stream banks, trees, rocks, or larger wood pieces), and rootwads are desirable. Using the model equation from this study, stewards of natural resources can better manage in-stream wood for the benefit of stream ecosystems.

We assessed the relationship between riparian management and stream quality along five southeastern Minnesota streams in 1995 and 1996. Specifically, we examined the effect of rotationally and continuously grazed pastures and different types of riparian buffer strips on water chemistry, physical habitat, benthic macroinvertebrates, and fish as indicators of stream quality. We collected data at 17 sites under different combinations of grazing and riparian management, using a longitudinal design on three streams and a paired watershed design on two others. Continuous and rotational grazing were compared along one longitudinal study stream and at the paired watershed. Riparian buffer management, fenced trees (wood buffer), fenced grass, and unfenced rotationally grazed areas were the focus along the two remaining longitudinal streams. Principal components analysis (PCA) of water chemistry, physical habitat, and biotic data indicated a local management effect. The ordinations separated continuous grazing from sites with rotational grazing and sites with wood buffers from those with grass buffers or rotationally grazed areas. Fecal coliform and turbidity were consistently higher at continuously grazed than rotationally grazed sites. Percent fines in the streambed were significantly higher at sites with wood buffers than grass and rotationally grazed areas, and canopy cover was similar at sites with wood and grass buffers. Benthic macroinvertebrate metrics were significant but were not consistent across grazing and riparian buffer management types. Fish density and abundance were related to riparian buffer type, rather than grazing practices. Our study has potentially important implications for stream restoration programs in the midwestern United States. Our comparisons suggest further consideration and study of a combination of grass and wood riparian buffer strips as midwestern stream management options, rather than universally installing wood buffers in every instance. RID="" ID="" The Unit is jointly sponsored by the US Geological Survey, Biological Resources Division; the Minnesota Department of Natural Resources; the University of Minnesota; and the Wildlife Management Institute.

We tested the hypothesis that fish decrease shredder abundance in leaf packs, thereby reducing leaf breakdown rates. Our goal was to test for the occurrence of a trophic cascade in a detritus-based food web. Willow leaves (Salix spp.) were fastened into leaf packs and placed into cages (13×13×13 cm) in Valley Creek, Minnesota, USA. Fish were excluded from leaf packs that were placed in cages with mesh on all sides, whereas open control cages allowed fish access to leaf packs. We collected leaf packs from two replicate cages 0, 14, 31, 55, and 112 days after placement in each of three riffles (n=6 per collection). Total abundance of invertebrates and shredders inhabiting leaf packs was significantly higher in exclosures than controls (P<0.01) and increased with exposure time in the stream (P<0.01). Three of the four common shredder taxa had significantly higher biomass in exclosures than controls (P<0.015). Biomass of Hesperophylax (Trichoptera) larvae was significantly higher in controls during the final collections (P<0.03), probably because these large, case-building larvae were less vulnerable to fish predation. Leaf breakdown rates differed significantly between exclosures and controls (P=0.003), but the direction of effects varied among riffles. When shredder density was analyzed separately for each riffle, we found that shredder density may explain differences in leaf breakdown rates between exclosures and controls. The differential responses of shredder taxa to predators may explain variability in fish effects on leaf breakdown. In conclusion, leaf packs did not provide invertebrates refuge from fish predation and fish reduced the densities of most shredders. Fish can indirectly affect leaf breakdown rates, but different responses to predation among taxa within the shredder guild can cause interactions that contradict trophic cascade predictions.

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