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Gomphonema caperatum sp. nov., G. obstipum sp. nov. and similar taxa from rivers of North America

Two new species of Gomphonema with linear-lanceolate valves, narrow cells (width ≤6 µm), wide axial areas, short uniseriate striae, and one stigmoid per valve are described. Gomphonema caperatum sp. 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 obstipum sp. 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 Gomphonema from North American rivers that are morphologically similar, such as Gomphonema amerhombicum Reichardt, Gomphonema stoermeri Kociolek & 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 incognitum in North America. This study underscores the insufficiency of our knowledge of diatom diversity in North America and the need for more detailed taxonomic investigations.