ASSEMBLAGE ORGANIZATION IN STREAM FISHES - EFFECTS OF ENVIRONMENTAL VARIATION AND INTERSPECIFIC INTERACTIONS

We assessed the relative importance of environmental variation, inters pecific competition for space, and predator abundance on assemblage st ructure and microhabitat use in a stream fish assemblage inhabiting Co weeta Creek, North Carolina, USA. Our study encompassed a 10-yr time s pan (1983-1992) and included some of the highest and lowest flows in t he last 58 years. We collected 16 seasonal samples which included data on: (1) habitat availability (total and microhabitat) and microhabita t diversity, (2) assemblage structure (i.e., the number and abundances of species comprising a subset of the community), and (3) microhabita t use and overlap. We classified habitat availability data on the basi s of year, season, and hydrologic period. Hydrologic period (i.e., pre -drought [PR], drought [D], and post-drought [PO]) represented the tem poral location of a sample with respect to a four-year drought that oc curred during the study. Hydrologic period explained a greater amount of variance in habitat availability data than either season or year. T otal habitat availability was significantly greater during PO than in PR or D, although microhabitat diversity did not differ among either s easons or hydrologic periods. There were significantly fewer high-flow events (i.e., greater than or equal to 2.1 m(3)/s) during D than in e ither PR or PO periods. We observed a total of 16 species during our i nvestigation, and the total number of species was significantly higher in D than in PR samples. Correlation analyses between the number of s pecies present (total and abundant species) and environmental data yie lded limited results, although the total number of species was inverse ly correlated with total habitat availability. A cluster analysis grou ped assemblage structure samples by hydrologic period rather than seas on or year, supporting the contention that variation in annual flow ha d a strong impact on this assemblage. The drought had little effect on the numerical abundance of benthic species in this assemblage; howeve r, a majority of water-column species increased in abundance. The incr eased abundances of water-column species may have been related to the decrease in high-flow events observed during the drought. Such high-fl ow events are known to cause mortality in stream fishes. Microhabitat use data showed that species belonged to one of three microhabitat gui lds: benthic, lower water column, and mid water column. In general, sp ecies within the same guild did not exhibit statistically distinguisha ble patterns of microhabitat use, and most significant differences occ urred between members of different guilds. However, lower water-column guild species frequently were not separable from all members of eithe r benthic or mid-water-column species. Variations in the abundance of potential competitors or predators did not produce strong shifts in mi crohabitat use by assemblage members. Predators were present in the si te in only 9 of 16 seasonal samples and never were abundant (maximum n umber observed per day was 2). In conclusion, our results demonstrate that variability in both mean and peak flows had a much stronger effec t on the structure and use of spatial resources within this assemblage then either interspecific competition for space or predation. Consequ ently, we suspect that the patterns in both assemblage structure and r esource use displayed by fishes in Coweeta Creek arose from the intera ction between environmental variation and species-specific evolutionar y constraints on behavior, morphology, and physiology.