Periphytic Community Response to Chronic Nutrient Enrichment by a Reservoir Discharge

Periphytic communities were investigated using glass slide substrates at four sites downstream from the montane Hyalite Reservoir, Montana, USA. Comparison of the most upstream site with the three lower sites revealed that the discharges to Hyalite Creek stimulated periphytic productivity, increased periphytic proportions of chlorophyll ° in the organic accumulations, and increased diatom species diversity and evenness. As nitrogen concentration was the only stream physiochemical parameter which correlated with periphytic variations, it is probable that ammonia nitrogen discharged from the reservoir was the primary factor influencing periphytic growth. Cocconeis placentula var. lineata dominated generally °70—80% of the diatom communities at the three downstream sites whereas only three individuals of this species were observed at the upper site. Data from two successional series following colonization of the slides suggested that species other than Cocconies had higher initial growth rates. It appears the Cocconies become dominant at the three downstream sites because of higher efficiencies for obtaining or incorporating limiting nitrogen resources: species other than Cocconeis dominated the diatom communities in which nitrogen concentrations were enriched,apparently because of higher potential growth rates which could be realized with the elevated nutrient conditions. A brief review of the literature and new data presented indicate that species diversity first increases and then decreases as a function of resource abundance.