Using diatoms to assess the biological condition of large rivers in Idaho (U.S.A.).

SUMMARY 1. Diatoms were collected from 49 sites located on 23 rivers and identified to species. We defined 26 attributes of the diatom assemblage on the basis of tolerance and intolerance, autecological guild, community structure, morphological guild and individual condition. 2. We grouped sites into three geographical regions on the basis of altitude, land cover and use. Within each region, we tested for an association between diatom attributes and human disturbance. 3. We selected nine attributes for inclusion in a multimetric index, the river diatom index (RDI): percentage of valves belonging to species sensitive to disturbance, percentage of valves belonging to species very tolerant of disturbance, eutrophic species richness, percentage of valves belonging to nitrogen heterotrophic species, percentage of valves belonging to polysaprobic species, alkaliphilic species richness, percentage of valves belonging to species that require high oxygen, percentage of valves belonging to very motile genera and percentage of deformed valves. 4. The RDI was significantly correlated with measures of human disturbance made at the site (conductivity, percentage of fine sediments and number of human activities) and at the catchment level (percentage of urbanisation and agriculture in the upstream catchment). 5. The percentage of the total variance of RDI related to transect location was very small (1%) compared with among-site differences (73%) and time of sampling (26%). The RDI could reliably detect approximately three categories of biological condition based on annual monitoring and potentially more if sampling were restricted to the same month each year. [ABSTRACT FROM AUTHOR]