In the present study, we developed a new Swedish phosphorus diatom index (PDISE) to improve the poor fit of existing indices to match the needs of water managers to detect and mitigate eutrophication. We took advantage of a large amount of data (820 Swedish stream sites) collected in recent years. During our work, we found an unexpected bimodal response of the diatom assemblages to phosphorus. The taxa clustered either into an assemblage with a low or with a high site-specific averaged TP optimum (a calculated value comprised of the diatom taxa-specific optima). We could not find a characteristic diatom assemblage for sites with intermediate site-specific averaged TP optima. To our knowledge, this bimodal community response has not been shown earlier. The PDISEcorrelated more strongly than the currently used TDI to changes in TP concentrations. Thus, the PDISEshould replace the TDI in the Swedish standard method. The modeled TP optima (expressed as categories) were different compared to the TDI for most of the taxa included in the index, indicating that the realized niche for these morphotaxa was different between Sweden and the UK where the TDI was developed originally. With ar2of 0.68, the correlation of the PDISEto TP is among the highest reported for other diatom nutrient indices globally; thus, we believe that it might be worth to test it for other bioregions with similar geography and climate.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10661-023-11378-4.