Abstract
We developed and compared the performance of freshwater benthic diatom indices calculated from (i) traditional morphological species identification, (ii) Amplicon Sequence Variants (ASVs) obtained via DNA metabarcoding, and (iii) morphological traits to indicate eutrophication in rivers and lakes in Fennoscandia. Based on the results, we provided recommendations for the future routine use of diatom bioassessment tools in environmental monitoring and assessment. Our results show that ASVs are the most promising candidates to be used in environmental assessment. Indices based on ASVs correlated better with TotP concentrations than morphological taxa data, whereas the trait indices correlated least. We could see by studying the taxonomic assignments of the ASVs that traditional morphotaxa were divided up into several ASVs with different ecological profiles, which explained part of the better index performance and also encourages further studies on diatom diversity and ecological preferences. In general, ASV- and morphotaxon-specific optima differed slightly between streams and lakes, but were significantly correlated with each other. This means that it should be possible to develop a common index that is applicable in both streams and lakes, but boundary values with respect to TotP might need to be set separately for them. More knowledge on diatom traits is required to enable their use for environmental assessment.
Highlights
Freshwater benthic diatoms have been used in aquatic environmental assessment for >100 years (Kolkwitz and Marsson, 1908)
We developed and compared the performance of freshwater benthic diatom indices calculated from (i) tradi tional morphological species identification, (ii) Amplicon Sequence Variants (ASVs) obtained via DNA meta barcoding, and (iii) morphological traits to indicate eutrophication in rivers and lakes in Fennoscandia
The sites in our study were well distributed along a nutrient gradient (Fig. 2), which was the main structuring environmental factor
Summary
Freshwater benthic diatoms have been used in aquatic environmental assessment for >100 years (Kolkwitz and Marsson, 1908). Diatom indices have been developed for manifold pressures, including, for example, acidification or salinization (Smol and Stoermer, 2010). Diatom indices which currently are in use for freshwater monitoring are based on inventories of the taxa that occur at the sampling sites. Light microscopy is used to identify diatoms by morpho logical characters, and to quantify the relative abundance of different taxa (Smol and Stoermer, 2010). This process is, timeconsuming and correct species identification is difficult (Alers-García et al, 2021; Hicks et al, 2006; Mann, 1999). Taxonomic consistency is a problem both within and between countries (Charles et al, 2021), and challenges related to diatom identification contribute more to the variability in reported diatom assemblages than differences in methods used for diatom sampling, sample preparation or the way of counting (Charles et al, 2021)
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