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Six species of large-celled Pinnularia Ehrenberg from lakes of eastern North America, P. dariana (A.W.F. Schmidt) Mills, P. alabamae Krammer, P. bihastata (A. Mann) R.M. Patrick, P. torta (A. Mann) R.M. Patrick, P. rexlowei sp. nov. and P. spinifera sp. nov. are investigated with light and scanning electron microscopy. All six species are shown to possess marginal spines, a trait sporadically encountered in Pinnularia and generally rare in raphid diatoms. Besides having spines, the six species differ in their frustular morphology, including appearance and position of spines. We summarize data on their ecology and distribution using a set of 702 sediment core samples from 424 lakes from eastern USA and Canada. Pinnularia rexlowei is only known from Quaternary lacustrine deposits in the State of Connecticut and from the surface and subfossil sediments of two lakes in northeastern USA. The other five species are relatively common, and at least one species of spine-bearing Pinnularia was found in 34% of studied lakes. Pinnularia dariana and P. torta reach as far north as Labrador, while P. alabamae, P. bihastata and P. spinifera were only encountered south of Canada. Pinnularia alabamae and P. spinifera are characteristic for lakes with relatively soft and slightly acidic water, while P. bihastata, P. dariana and P. torta are found in a wider range of physico-chemical conditions, including alkaline and eutrophic lakes. [ABSTRACT FROM AUTHOR]

Stauroneis terryi D.B. Ward ex T.C. Palmer is a diatom so far reported from only five locations in the states of Connecticut, Massachusetts and Maine, USA, where it was collected in 1890—1911. There were no records of this large and rather conspicuous species in any collections made after 1911 despite the considerable number of diatom surveys and paleolimnological studies conducted in the northeastern USA. We studied material from the type locality of this species to document its morphology with SEM, and screened diatom slides representing recent and subfossil samples collected from 169 lakes in northeastern North America from 1979 to 2017 for the presence of this species. Using both whole-slide scanning and traditional microscopy, we detected S. terryi in 15 lakes in the northeastern USA north of 41° N latitude and in five lakes of Labrador and adjacent Quebec, Canada. In the USA, S. terryi populations have declined since pre-industrial times, although the species is still present in a few lakes with relatively undisturbed watersheds. Our observations demonstrate the value of museum collections for generating data on diatom ecology and biogeography and the need for the development of high-throughput methods of extracting information from diatom slides and images. [ABSTRACT FROM AUTHOR]

Consistent identification of diatoms is a prerequisite for studying their ecology, biogeography, and successful application as environmental indicators. However, taxonomic consistency among observers has been difficult to achieve because taxonomic information is scattered across numerous literature sources, presenting challenges to the diatomist. Firstly, literature is often inaccessible because of cost or its location in journals that are not widely circulated. Secondly, taxonomic revisions of diatoms are taking place faster than floras can be updated. Finally, taxonomic information is often contradictory across literature sources. These issues can be addressed by developing a content creation community dedicated to making taxonomic, ecological, and image-based data freely available for diatom researchers. Diatoms.org represents such a content curation community, providing open, online access to a vast amount of recent and historical information on North American diatom taxonomy and ecology. The content curation community aggregates existing taxonomic information, creates new content, and provides feedback in the form of corrections and notices of literature with nomenclatural changes. The website not only addresses the needs of experienced diatom scientists for consistent identification but is also designed to meet users at their level of expertise, including engaging the lay public in the importance of diatom science. The website now contains over 1000 species pages contributed by over 100 content contributors, from students to established scientists. The project began with the intent to provide accurate information on diatom identification, ecology, and distribution using an approach that incorporates engaging design, user feedback, and advanced data access technology. In retrospect, the project that began as an 'extended electronic book' has emerged not only as a means to support taxonomists, but for practitioners to communicate and collaborate, expanding the size of and benefits to the content curation community. In this paper, we outline the development of diatoms.org, document key elements of the project, examine ongoing challenges and consider the unexpected emergent properties, including the value of diatoms.org as a source of data. Ultimately, if the field of diatom taxonomy, ecology, and biodiversity is to be relevant, a new generation of taxonomists needs to be trained and employed using new tools. We propose that diatoms.org is in a key position to serve as a hub of training and continuity for the study of diatom biodiversity and aquatic conditions. [ABSTRACT FROM AUTHOR]

Two new species ofGomphonemawith linear-lanceolate valves, narrow cells (width ≤6 µm), wide axial areas, short uniseriate striae, and one stigmoid per valve are described.Gomphonema caperatumsp. nov. has narrow, almost linear valves with short marginal striae, apically elongated slit-like areolae, and transapically oriented linear markings on the external valve surface, which are only observable under scanning electron microscopy. This is a common and often abundant species in rivers across Eastern North America, especially in the Appalachian region, but is also found on the West Coast.Gomphonema obstipumsp. nov. has lanceolate valves with bent apices, variously shaped apically elongated areolae, and shallow circular depressions on the external valve surface. This species has only been found in two Virginian rivers. We compare the new taxa with several species ofGomphonemafrom North American rivers that are morphologically similar, such asGomphonema amerhombicumReichardt,Gomphonema stoermeriKociolek & Kingston, and other species with linear-lanceolate valves and wide axial areas. In addition, we report here for the first time the occurrence ofGomphonema incognitumin North America. This study underscores the insufficiency of our knowledge of diatom diversity in North America and the need for more detailed taxonomic investigations. [ABSTRACT FROM PUBLISHER]

Variation of frustular morphology within the Achnanthidium minutissimum (Kütz.) Czarn. species complex was studied in type populations of 12 described taxa and in 30 recent North American river samples. The SEM observations in this study and other publications showed that ultrastructural characters on their own do not discriminate among taxa within the A. minutissimum complex. Therefore, an attempt was made to use other characters, such as valve shape and striation pattern, to delineate morphological groups. The sliding-landmarks method was used to obtain valve-shape descriptors. These shape variables were combined with conventional morphological characters in multivariate analyses. It was shown that some historically recognized taxa are morphologically distinct, while others are difficult to differentiate. Morphological grouping of “old” taxa most similar to A. minutissimum did not correspond to their taxonomic hierarchy in contemporary diatom floras. Morphometric analysis of a data set of 728 specimens from North American rivers revealed six morphological groups, although it was impossible to draw clear boundaries among them. These morphological groups differed significantly in their ecological characteristics and could be recommended as indicators of water quality. Application of the discriminant function analysis based on shape variables and striation pattern showed that North American specimens could be more consistently classified into the six groups identified in our analysis than into historically recognized taxa. [ABSTRACT FROM AUTHOR]

Variation of frustular morphology within the Achnanthidium minutissimum (Kütz.) Czarn. species complex was studied in type populations of 12 described taxa and in 30 recent North American river samples. The SEM observations in this study and other publications showed that ultrastructural characters on their own do not discriminate among taxa within the A. minutissimum complex. Therefore, an attempt was made to use other characters, such as valve shape and striation pattern, to delineate morphological groups. The sliding-landmarks method was used to obtain valve-shape descriptors. These shape variables were combined with conventional morphological characters in multivariate analyses. It was shown that some historically recognized taxa are morphologically distinct, while others are difficult to differentiate. Morphological grouping of “old” taxa most similar to A. minutissimum did not correspond to their taxonomic hierarchy in contemporary diatom floras. Morphometric analysis of a data set of 728 specimens from North American rivers revealed six morphological groups, although it was impossible to draw clear boundaries among them. These morphological groups differed significantly in their ecological characteristics and could be recommended as indicators of water quality. Application of the discriminant function analysis based on shape variables and striation pattern showed that North American specimens could be more consistently classified into the six groups identified in our analysis than into historically recognized taxa. [ABSTRACT FROM AUTHOR]