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Molecular-assisted alpha taxonomy, which combines molecular species delimitation withpost-hocmorphological examinations, has proved to be an effective tool for the classification of morphologically similar species. We employed this approach to examine the diversity of the genusFrustuliain northern Europe. First, we used two molecular markers to delimit species and then characterized their morphology using conventional and geometric morphometrics. Next, we employed machine-learning methods to identify valves in benthic diatom communities in order to infer the distribution and pH preference of individual species. Unlike previous studies using automated identification of diatom species, we examined the performance of the semi-supervised classifier. Supervised methods, which have been used before, only employ labelled valves to train the classification algorithm. The semi-supervised approach is, in addition, able to benefit from unlabelled valves in the natural populations. It is usually superior in cases in which there are few labelled data available. Finally, we compared the classification accuracy of the algorithms and five volunteer specialists. We found five molecular lineages, theF. crassinervia-saxonicaspecies complex,F. gaertnerae,F. septentrionalis,F. krammeri, andF.cf.maoriana. The most valuable characteristics for species identification were length, width, striation pattern, and allometric shape changes described by the first axis in the geometric morphometric analysis. We found that a semi-supervised approach that does not rely solely on the morphology of isolated cells, but also accounts for variation among valves from natural populations, has superior performance. Based on valves from natural populations, we observed marked differences in species abundances and pH tolerances that have a bearing on their geographical distributions. [ABSTRACT FROM PUBLISHER]

During our investigations of freshwater sites along the Atlantic Coastal Plain of North America we had the opportunity to critically study five interesting diatoms with light (LM) and scanning electron microscopy (SEM), each of which is described here as a new species. Two of the organisms are in the genus Eunotia, Eunotia lewisii Siver & Hamilton sp. nov. and Eunotia quadra Siver & Hamilton sp. nov.; two in Frustulia, Frustulia crispula Siver, Pelczar, & Hamilton sp. nov. and Frustulia inculta Siver, Pelczar, & Hamilton sp. nov.; and the last is Neidium trainori Siver & Hamilton sp. nov. All five species are reported from acidic, poorly buffered, dilute waterbodies high in dissolved humic substances. Two of the taxa are found in suites of waterbodies in close geographic proximity, two in somewhat disjunct regions, and the fifth is only known from the type locality. The relationships of each taxon to similar species are discussed. [ABSTRACT FROM AUTHOR]