No lukewarm diatom communities - the response of freshwater benthic diatoms to phosphorus as basis for a new Phosphorus Diatom Index (PDISE)

Abstract In the present study, we developed a new Swedish phosphorus diatom index (PDI SE ) 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 sites) collected in recent years. During our work, we found an unexpected bimodal response of the diatom taxa to phosphorus, with the taxa clustering into either an assemblage with a low, or with a high site-specific averaged optimum comprised of the diatom taxa optima. We could not find a characteristic diatom assemblage for sites with intermediate TP-site-specific averaged optima. To our knowledge, this bimodal community response has not been shown earlier. We found that the PDI SE responded better than the currently used TDI and IPS to changes in TP concentrations, and thus should replace the TDI in the Swedish standard method. The modeled sensitivity values (expressed as categories) for TP were different compared to the TDI for most of the taxa included in the index development, indicating that the realized niche for these morphotaxa was different between Sweden and the UK where the TDI was developed originally. The correlation of the PDI SE to TP was 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.