Fish assemblage and environmental differences upstream and downstream of a cave: a potential reset mechanism

The traditional view of stream systems, exemplified by the river continuum concept, shows predictable shifts in species composition and ecosystem function based on changes in nutrients and energy. Usually headwaters have fewer species, lower nutrients and greater habitat variability than downstream sites. The modern view of stream systems is much more complex with disturbances altering expected environmental conditions and assemblages. Reset mechanisms function to change the expected downstream biotic assemblages so that they resemble what would be expected at an upstream location. The presence of a reset mechanism may be indicated by a variety of factors including decreases in biomass and diversity. To assess the potential impacts of a cave, which can impact a variety of hydrologic factors including temperature, fish assemblages and environmental conditions were sampled around a cave. Differences were seen both in assemblage and environmental conditions between sites located upstream and downstream of a cave; variation in assemblage and environment also occurred at a 10 m scale. Overall, species diversity and richness were lower downstream of the cave, resembling a headwater-like site. There was a shift from warmer water species upstream to an assemblage defined by high abundances of mottled sculpin and the presence of brown trout. These differences in assemblage were linked with environmental conditions, including temperature, that were likely influenced by the presence of the cave. Results of this study indicate that, in this case, a cave has impacted fish assemblages and the surrounding stream ecosystem and may be functioning as a reset mechanism, resulting in decreased species diversity and changes in assemblage composition over a short distance.