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Six species of large-celled PinnulariaEhrenberg from lakes of eastern North America, P. dariana(A.W.F. Schmidt) Mills, P. alabamaeKrammer, P. bihastata(A. Mann) R.M. Patrick, P. torta(A. Mann) R.M. Patrick, P. rexloweisp. nov. and P. spiniferasp. nov. are investigated with light and scanning electron microscopy. All six species are shown to possess marginal spines, a trait sporadically encountered in Pinnulariaand 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 rexloweiis 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 Pinnulariawas found in 34% of studied lakes. Pinnularia darianaand P. tortareach as far north as Labrador, while P. alabamae, P. bihastataand P. spiniferawere only encountered south of Canada. Pinnularia alabamaeand P. spiniferaare characteristic for lakes with relatively soft and slightly acidic water, while P. bihastata, P. darianaand P. tortaare found in a wider range of physico-chemical conditions, including alkaline and eutrophic lakes.

A new species of AulacoseiraThwaites is described from sediment deposits of Surprise Lake, an isolated lake located in the New Jersey Highlands. The species is characterized by (1) a low mantle, (2) unevenly distributed mantle areolae of variable size and shape, and (3) a valve face covered by rounded areolae. The new species is most similar to Aulacoseira biseriata(Grunow in Van Heurk) Houk, R. Klee & H. Tanaka. Both taxa possess frustules with mantles that are low, with irregular pervalvar rows or scattered areolae. However, the two species differ in the shape and pattern of the areolae, the shape and structure of the mantle wall, the shape and depth of the ringleist, and other morphological features. In this study, we present light and scanning electron microscopic images of the new species and compare it to the most similar taxa.

Stauroneis terryiD.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. terryiin 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. terryipopulations 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.

Cyclotella lacuskarlukiManguin ex Kociolek & Reviers is a small diatom known only from the original drawing that provides little information on the morphology of this taxon. As Manguin’s original materials are lost, recently collected samples from Lake Karluk, Alaska, the type locality of the species, were studied with light and scanning electron microscopy to determine its identity. As a result of this investigation, C. lacuskarlukiis transferred here to the genus Discostella. Discostella lacuskarlukiis 2.5–7 µm in diameter, has a shallow mantle, a stellar pattern of alveoli in the valve centre that may be reduced or completely absent, bifurcating costae and 4–5 marginal fultoportulae opening externally by short thickened tubes with round openings. We observed this species in numerous lakes across Alaska, in Lake Tahoe, Nevada, in several lakes across Canada, and in tundra and tree-line lakes of Northeastern Siberia. Although direct comparison with other Discostellaspecies is difficult because the small size of these diatoms requires high-resolution imagery to reveal ultrastructural details, it appears that D. lacuskarlukiis conspecific with D. tatricaProcházková et al. recently described from lakes of Central Europe and also reported from the European part of Russia. It is also conspecific with D. nipponica(Skvotzov) Tuji & D. M. Williams known from Japan. The apparent wide distributional range of this species indicates that it has been overlooked and commonly reported as D. pseudostelligeraor D. stelligerain arctic, boreal and alpine lakes.

The genus KrsticiellaLevkov was previously known only from the basins of Lakes Ohrid and Prespa, and from Lake Baikal. Examination of type materials of diatoms originally described by Niels Foged from the North Slope of Alaska revealed that Navicula lenisFoged and N. meadeensisFoged belong to Krsticiella, to which they are formally transferred here. One of these species, K. lenis, is also found in thermokarst tundra lakes of the Kolyma Lowland in Northeastern Siberia. Several illustrations in previously published floras of Pliocene and Pleistocene diatoms of Eurasia indicate wider past distributions of Krsticiellaspecies. The current restriction of this genus to non-glaciated regions of the Arctic and large ancient lakes in temperate zone indicates that it is a Cenozoic relict.

Two new species of Gomphonemawith 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 of Gomphonemafrom North American rivers that are morphologically similar, such as Gomphonema 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 of Gomphonema 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.

Stauroneis pachycephalawas described in 1881 from the Baakens River, Port Elizabeth, South Africa. Recently, it was found during surveys of the MacKenzie River (Victoria, Australia), the Florida Everglades (USA) and coastal marshes of Louisiana (USA). The morphology, ecology and geographic distribution of this species are described in this article. This naviculoid species is characterised by lanceolate valves with a gibbous centre, a sigmoid raphe, an axial area narrowing toward the valve ends, and capitate valve apices. The central area is a distinct stauros that is slightly widened near the valve margin. The raphe is straight and filiform, and the terminal raphe fissures are strongly deflected in opposite directions. Striae are fine and radiate in the middle of the valve, becoming parallel and eventually convergent toward the valve ends. The external surface of the valves and copulae is smooth and lacks ornamentation. We also examined the type material of S. pachycephala. Our observations show this species has morphological characteristics that fit within the genus Envekadea. Therefore, the transfer of S. pachycephalato Envekadeais proposed and a lectotype is designated.

Surirella cruciataA. Schmidt and S. terryiTerry are rare and poorly known diatom species originally described from North America and previously illustrated only by drawings. The goal of this study was to examine and document the morphology and distribution of these species. First photographs designating the lectotype of S. terryiand the neo- and epitypes of S. cruciataare presented. Our revision of the distribution records shows that both species are found mostly along the east coast of the USA with a main occurrence in the New England states, while records from outside North America were not confirmed. Surirella terryiand S. cruciatasometimes co-occur, although S. terryiis a strictly freshwater species, while S. cruciatatolerates wider range of salinities from strictly fresh to brackish waters.

Achnanthes decipiensReimer was described from a benthic sample collected from a small stream in South Carolina, USA. Examination of the type material with light and electron microscopy showed that this species belongs to the genus NupelaVyverman & Compère to which it is transferred. Nupela decipiens(Reimer) Potapova is similar to N. praecipua(Reichardt) Reichardt by the size and shape of the valve and the striation pattern, but differs from it by the large central area on the raphe valve. So far, N. decipienshas been reported from a few slightly acid soft-water streams in eastern North America.

A new diatom species, Eucocconeis americana, is described from eastern North America. The new species differs from previously reported representatives of EucocconeisCleve by relatively flat valves and the characteristic shape of the central areas in both raphe and rapheless valves. We present details on the frustule structure and size variation, and compare the new species with other representatives of Eucocconeis. This new species was found in rivers and streams from the eastern USA characterized by low conductivity and pH.

Several species of Nupelawere found in samples collected in 2009 from streams and aerophytic habitats of North-central Pennsylvania. Some of these species appeared either new to science or insufficiently known and, although described earlier, not yet transferred to the genus Nupela. This triggered an investigation of type materials of several Nupelaspecies described in the past, especially those with type localities in the Appalachian region. As a result of this study, two new Nupelaspecies, N. elegantulaand N. frezeliiare described, while Navicula pennsylvanica, Navicula poconoensisand Achnanthes subrostrataare transferred to Nupela. Examination of type material of Achnanthes subrostratavar. appalachianashowed that this taxon is conspecific with Nupela lapidosa.