From continua to patches : longitudinal patterns in the Middle Fork of the John Day River, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6969z3268

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  • The Middle Fork John Day River is composed of low gradient, alluvial valley segments separated by constrained, canyon-like reaches, and has a history of multiple land-uses. These factors can alter the physical and biological structure of streams, and disrupt the longitudinal river continuum. I examined habitat, fish, macroinvertebrates, and primary production at 20 sites along a 1st-5th order gradient of this eastern Oregon river during summer low flow in 1996-1998. Using non-parametric, multivariate ordination, I examined longitudinal patterns of physical and biological characteristics by rotating ordinations to a common set of variables. Three a priori, qualitative null hypotheses of ordination patterns were established: 1) an ideal longitudinal continuum, 2) a highly variable distribution and 3) discrete patches. Effects of spatial perspective on longitudinal patterns were considered by comparing site- and reach-scale patterns. The potential influence of abiotic factors and trophic interactions were examined using correlations with ordination coordinates and regressions with individual parameters. Despite variations in valley constraint and land-use, both physical and biological elements occurred along longitudinal gradients. The most consistent longitudinal gradients were observed for physical characteristics, and there was no evidence of subsurface water upwelling associated with discontinuities in valley constraint. Fish and macroinvertebrate communities were more strongly correlated with landscape scale, geomorphic gradients than with site- scale habitat variability. Algal biomass and productivity were highly variable. Most individual physical and biological variables (e.g. substrates or insect taxa) reflected discontinuous or patchy gradients from a site-scale perspective, and more continuous gradients from a reach-scale perspective. There was little evidence of trophic interactions affecting community structure in a consistent pattern. The spatial scope and breadth of measures in my study revealed how perceptions of stream gradients absolutely depend upon the spatial extent of our observations. This study illustrates that continuous and patchy aspects of both physical and biological parameters occur together in stream systems and are not mutually exclusive.
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