The effects of stream size and flow regime on spatial and temporal variability of stream fish distribution, abundance, and diversity patterns were investigated. Assemblage variability and species richness were each significantly associated with a complex environmental gradient contrasting smaller, hydrologically variable stream localities with larger localities characterized by more stable flow regimes. Assemblages showing the least variability were the most species-rich and occurred in relatively large, stable environments. Theory suggests that species richness can be an important determinant of assemblage variability. Although this appears to be true in our system, we suggest that spatial and temporal heterogeneity in the environment largely determines both assemblage richness and variability, providing a more parsimonious explanation for the diversity–variability correlation. Changes in species richness of local assemblages across time were coordinated across the landscape, and assemblages formed spatially and temporally nested subset patterns. These results suggest an important link between local community dynamics and community-wide occurrence. At the species level, mean local persistence was significantly associated with regional occurrence. Thus, the more widespread a species was, the greater its local persistence. Our results illustrate how the integrity of local stream fish assemblages is dependent on local environmental conditions, regional patterns of species distribution, and landscape continuity.

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Phenotypic plasticity in snail shells is a well-documented phenomenon, specifically in freshwater species. In riverine taxa, shells respond to the unidirectional gradient of flow and depth as well as to predation by crushing predators. Using populations of Lithasia geniculata from the Duck River, Tennessee, USA, we examined environmental correlates of shell shape change and resistance to crushing along a riverine gradient. Shells were more globose, more robust, and more resistant to crushing forces downstream relative to upstream; these characteristics were correlated with river discharge and presence of molluscivorous fish. Size, however, did not have effects on shape nor crushing strength. These data are consistent with those observed in other snail species, and expand on our knowledge of potential fitness benefits and causes of plasticity in freshwater snail shells.

The Ouachita madtom, Noturus lachneri, is a small, uniformly-colored catfish endemic to the upper Saline and Ouachita river drainages in central Arkansas (Robison and Buchanan, 1988), where it is often found in shallow pools associated with clear, high gradient, rock-bottomed streams (Robison and Harp, 1985). Distribution, habitat, diet, and conservation status of N. lachneri were examined by Robison and Harp (1985). However, information on parasites of this endemic species has not been reported. Herein, we report .crn. species richness and mean abundance of hehninth parasites of N. kuhna'. Compared to free-living organisms, parasite assemblages are spatially well-defined and show distinct levels of hierarchical organization (Poulin, 1997). All parasites within a single host constitute an infracommunity, whereas all helminths in a host population define the component community (Holmes and Price, 1986). At the infracommunity scale many replicates are possible because each individual host within a population of hosts constitutes a fully censused assemblage. Therefore, parasite assemblages provide an excellent system for hypothesis testing in community ecology (Poulin, 1997). Parasites often influence critical aspects of the lie-history and ecology of their host species. For example, parasites can affect repro

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