The development of biological communities is an important research issue regarding biological quality elements. However, it is still uncertain how different organism groups and their metrics are affected by different environmental factors. A study of high-quality sites of medium-sized lowland streams typical for Latvia (ecoregion Baltic province) was carried out with an emphasis on hydrological and hydromorphological characteristics. The investigations were carried out within the STAR project using the project designated standard methods. We found that the level of saprobity according to benthic macroinvertebrates was mainly linked with substrate type and stream depth. The trophic diatom index for diatoms on a soft substratum had a negative correlation with altitude and a positive correlation with distance from the source and also stream depth. There were no significant correlations between macrophyte trophic indices and environmental factors associated with stream hydrological conditions. The European fish index was influenced mainly by stream width and distance from the source, which agrees with the results of other researchers. In general, a direct or indirect role of hydrological and hydromorphological factors was evident in the forming of communities characterizing stream ecological quality.

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