Responses of Diatom Communities to Heavy Metals in Streams: The Influence of Longitudinal Variation

Abstract

We investigated longitudinal variation (e.g., from upstream to downstream) in diatom community composition in an unpolluted stream (the Cache la Poudre River, Colorado, USA) and demonstrated how natural variation in community structure and morphological growth forms influence responses of diatoms in metal-polluted streams. Upstream communities in the Cache la Poudre River were physiognomically simple and dominated by small, adnate species (Achnanthes minutissima and Fragilaria vaucheriae), which decreased in relative abundance downstream. Responses of diatom communities to Zn in six metal-polluted streams were influenced by elevation and other variables correlated with the stream’s longitudinal gradient (pH, conductivity, alkalinity). Although clear community responses were observed at sites where Zn concentration exceeded 200 μg/L, effects of metals at moderately polluted sites were not significant. These field results were compared to responses of periphyton communities in experimental streams dosed with heavy metals. We hypothesized that communities collected from high elevation sites, which were dominated by early successional species (A. minutissima), would be more tolerant to metals than downstream communities. To test this hypothesis, diatom communities collected from a high elevation site (2340 m) and a low elevation site (1536 m) on the Cache la Poudre River were exposed to a mixture of Cd, Cu, and Zn in experimental streams. Small, adnate species (Achnanthes minutissima and Fragilaria vaucheriae) were tolerant to metals and increased in abundance in treated streams. In contrast, late successional species, which were found only at the low elevation site (Diatoma vulgare and Melosira varians), were highly sensitive to metals and were eliminated from treated streams after 24 h. These results indicate that natural variation in diatom communities may complicate interpretation of routine biomonitoring studies. We suggest that it may be useful to study periphyton community responses to pollutants in relation to ecological characteristics and morphological growth forms. Conducting research within this framework may improve the predictive ability of periphyton models and provide a theoretical basis for understanding the relationships between community patterns and the evolutionary processes that determine responses to disturbance.