Distribution of benthic diatoms in U.S. rivers in relation to conductivity and ionic composition MARINA POTAPOVA AND DONALD F. CHARLES Patrick Center for Environmental Research, The Academy of Natural Sciences, Philadelphia, PA, U.S.A. SUMMARY 1. We quantified the relationships between diatom relative abundance and water conductivity and ionic composition, using a dataset of 3239 benthic diatom samples collected from 1109 river sites throughout the U.S.A. [U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program dataset]. This dataset provided a unique opportunity to explore the autecology of freshwater diatoms over a broad range of environmental conditions. 2. Conductivity ranged from 10 to 14 500 lS cm )1 , but most of the rivers had moderate conductivity (interquartile range 180–618 lS cm )1 ). Calcium and bicarbonate were the dom- inant ions. Ionic composition, however, varied greatly because of the influence of natural and anthropogenic factors. 3. Canonical correspondence analysis (CCA) and Monte Carlo permutation tests showed that conductivity and abundances of major ions (HCO  3 + CO 2 3 , Cl ) , SO 2 4 , Ca 2+ , Mg 2+ , Na + ,K + ) all explained a statistically significant amount of the variation in assemblage composition of benthic diatoms. Concentrations of HCO  3 + CO 2 3 and Ca 2+ were the most significant sources of environmental variance. 4. The CCA showed that the gradient of ionic composition explaining most variation in diatom assemblage structure ranged from waters dominated by Ca 2+ and HCO  3 + CO 2 3 to waters with higher proportions of Na + ,K + , and Cl ) . The CCA also revealed that the distributions of some diatoms correlated strongly with proportions of individual cations and anions, and with the ratio of monovalent to divalent cations. 5. We present species indicator values (optima) for conductivity, major ions and proportions of those ions. We also identify diatom taxa characteristic of specific major-ion chemistries. These species optima may be useful in future interpretations of diatom ecology and as indicator values in water-quality assessment. Keywords: benthic diatoms, conductivity, ionic composition, rivers, U.S.A. Introduction Diatoms are the most common and diverse group of algae in many rivers and streams, and thus are important components of these ecosystems (Round, 1981). Although it is well known that salinity and concentrations of major ions have a strong influence on distributions of individual diatom taxa (Cholnoky, 1968), the relative importance of these factors has rarely been studied at large regional scales, and particularly not for the United States. Nor have ecological optima of taxa been quantified at these scales using large numbers of samples. This paper provides such information based on diatom data for samples collected by the USGS NAWQA Program. This information improves our understanding of how diatoms are distributed in U.S. rivers with respect to conductivity and major ions, and provides specific autecological data so that diatoms can be used more effectively in making assessments of ecological change. Correspondence: M. Potapova, Patrick Center for Environmental Research, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA 19103, U.S.A. E-mail: potapova@acnatsci.org Freshwater Biology (2003) 48, 1311–1328 Ó 2003 Blackwell Publishing Ltd 1311