New methods for improving water management - Exploring the role of diatoms in ecosystems

Abstract: Diatoms are photosynthetic microalgae which are well known to react quickly to environmental changes and are widely used as bioindicators within Water Framework Directive. However, diatom identification by light microscopy is time-consuming and error-prone, creating a need for alternative, faster and more reproducible methods for characterising diatom communities. This thesis describes the development and application of two such methods: a molecular approach using metabarcoding and a trait-based approach using combinations of different diatom morphological traits. Both approaches were shown to have potential for use in environmental assessment, as they revealed the response of diatoms to environmental change and enabled better representation of the diatom community. However, results from the two approaches were not directly comparable to results obtained by conventional microscopy and diatom DNA and trait data should not be used with tools calibrated for morphotaxa data. Discrepancies in molecular output data between laboratories revealed a strong need for standardisation and best-practice guidelines. Nutrient indices developed for each novel approach performed well and showed potential for use in assessment of total phosphorus levels in Fennoscandia freshwater. The molecular-based index showed differences in performance for stream and lake samples. The trait-based index performed equally well for both types of site, but current limitation of this index is that selection of meaningful traits must be done for each new gradient. Further work is needed on diatom traits and their response to environmental changes, to obtain reproducible and comparable molecular outputs and to continue the development of new indices calibrated to diatom DNA and trait data.