Invertebrate data from rivers in the Tennessee Valley were used to: (1) evaluate the utility of 18 characteristics of invertebrate assemblages (attributes) to assess the biological condition of streams and (2) develop a comprehensive benthic invertebrate index that reflects important aspects stream biology and responds to the effects of human society in detectable ways. We used data from the Tennessee Valley Authority's (TVA) fixed—station monitoring program established in 1986 to evaluate water resources in the Tennessee Valley. The streams covered sites with various types and levels of human disturbance. TVA biologists sampled riffles and pools using quantitative methods. To evaluate attributes we determined (1) the ability of each attribute to distinguish among sites (analyses of variance), (2) the associations among attributes (correlation coefficients), (3) the concordance of assessments of sites using each attribute with two independent site assessments (water quality and analysis of fish assemblages based on the index of biotic integrity), and (4) the variability of each attribute across habitats (riffles and pools) and ecoregions. Thirteen attributes (total taxa richness and taxa richnesses of intolerant snails and mussels, mayflies, caddisflies, and stoneflies; relative abundances of Corbicula, oligochaetes, omnivores, filterers, grazers, and predators; dominance; total abundance) were valuable in discriminating sites, exhibited concordance with other methods of assessment, and were relatively uncorrelated among themselves. Five attributes (sediment—surface taxa richness; relative abundances of shredders, gatherers, detritivores, and chironomids) were not acceptable for inclusion in the index because they either were not in concordance with other methods of assessment or were strongly correlated with other attributes. Based on the results of this evaluation, 13 attributes were included in a benthic index of biotic integrity (B—IBI). The B—IBI was evaluated using a more extensive fixed—station data set and was tested using an independent data set where samples were taken above and below the release of an industrial effluent to the North Fork Holston River. The fixed—station sites were correctly ranked, although rankings were not always consistent for pool and riffle habitats. B—IBI also distinguished the industrial impact. The B—IBI has potential as an assessment tool for streams, although more testing and evaluation of both the attributes and the index are warranted.
Whirling disease, caused by the parasite Myxobolus cerebralis, has infected rainbow trout (Oncorhynchus mykiss) and other salmonid fish in the western United States, often with devastating results to native populations but without a discernible spatial pattern. The parasite develops in a complex 2-host system in which the aquatic oligochaete Tubifex tubifex is an obligate host. Because substantial differences in whirling disease severity in different areas of North America did not seem explainable by environmental factors or features of the parasite or its fish host, we sought to determine whether ecological or genetic variation within oligochaete host populations may be responsible. We found large differences in compatibility between the parasite and various laboratory strains of T. tubifex that were established from geographic regions with different whirling disease histories. Moreover, 2 closely related species of tubificids, Limnodrilus hoffmeisteri and Ilyodrilus templetoni, which occur naturally in mixed species assemblages with T. tubifex, were incompatible with M. cerebralis. Virulence of the parasite was directly correlated with the numbers of triactinomyxon spores that developed within each strain of T. tubifex. Thus, the level of virulence was directly related to the compatibility between the host strain and the parasite. Genetic analyses revealed relationships that were in agreement with the level of parasite production. Differences in compatibilities between oligochaetes and M. cerebralis may contribute to the spatial variance in the severity of the disease among salmonid populations.
Myxobolus cerebralis, the parasite that causes salmonid whirling disease, has had detrimental effects on several salmonid populations in the Intermountain West, including the rainbow trout in the Madison River, Montana, USA. The goal of this study was to examine relationships among characteristics of the environment, Tubifex tubifex (the alternate host) populations, and rainbow trout whirling disease risk in the Madison River. Environmental characteristics were measured in side channels of the Madison River, and differences were described with a principal components analysis. The density of T. tubifex, the prevalence of infection in T. tubifex, and the density of infected T. tubifex were determined for the side channels using benthic core samples and examination of live tubificids for infection. The site-specific contribution to whirling disease risk in the side channels was determined using in situ exposures of sentinel rainbow trout. Regression analyses were used to determine correlations among these characteristics. Side channels differed in site-specific contribution to rainbow trout whirling disease risk, which was positively correlated to the density of infected T. tubifex. Side channels with fine sediments and lower water temperatures made greater site-specific contribution to whirling disease risk and had higher densities of infected T. tubifex than side channels with coarser sediments and higher temperatures. The ability to characterize areas of high whirling disease risk is essential for improving our understanding of the dynamics of M. cerebralis such that appropriate management strategies can be implemented. In addition, this study provides a model of how the disease ecology of complex aquatic parasites can be examined when the influential processes operate on different spatial scales.
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