Morphological and ecological diversity within the ‘Pinnularia gibba’ species group in the eastern United States

Diatoms resembling Pinnularia gibba Ehrenberg constitute a complex of species that are difficult to identify because they intergrade morphologically. We assessed the morphological diversity of this group in the eastern United States using 458 sediment core samples from a national-scale lake survey and applying the ‘voucher flora’ method. In contrast to the traditional approach to diatom identification – which previously yielded just six taxa from the P. gibba group in these samples – we found 38 distinct morphological entities. Some of these entities could be further grouped because they fit within the boundaries of previously described taxa, while others represented novel taxa. Based on this revision, we establish six new species and two new varieties: P. geilerae sp. nov., P. undulatoides sp. nov., P. vanderburghiae sp. nov., P. manoyloviae sp. nov., P. aquilonariformis sp. nov., P. hurleiae sp. nov., P. hurleiae var. minuta var. nov., and P. clericii var. minor var. nov. Eight additional morphological entities may represent previously undescribed taxa, but due to their limited representation in our dataset, they could not be formally described. Autecological analysis revealed a substantial variation in ecological profiles of the studied Pinnularia taxa, highlighting the benefits of using fine-grained taxonomy in bioassessments.