TWO COMMON NORTH AMERICAN DIATOMS, ACHNANTHIDIUM RIVULARE SP. NOV. AND A. DEFLEXUM (REIMER) KINGSTON: MORPHOLOGY, ECOLOGY AND COMPARISON WITH RELATED SPECIES

Abstract

Two species of Achnanthidium, which are fairly common in benthic samples collected from North American rivers, have posed considerable identification problems for analysts enumerating diatoms for water quality assessment programs in the United States. Both diatoms belong to the apparently diverse and insufficiently studied group of Achnanthidiumspecies with terminal raphe fissures sharply bent to the same side of the valve. The goal of this study was to clarify the taxonomic position of these taxa and document their morphology, ecology and distribution. One of these, Achnanthidium rivulare Potapova & Ponader sp. nov., is especially abundant in soft-water, phosphorus-poor rivers of coastal regions of North America, and has been confused in the past with various species of Achnanthidium and Rossithidium. This species is most similar to Achnanthidium convergens (Kobayasi) Kobayasi known from Japan and A. crassum (Hustedt) Potapova & Ponader comb. nov. Achnanthidium crassum is a poorly known species; therefore, SEM observations of its type material from Sumatra and another population from Hawaii are presented here. A second problematic North American species is A. deflexum (Reimer) Kingston, a diatom very similar in its morphology to A. pyrenaicum (Hustedt) Kobayasi. Study of the type materials and a number of other collections of these species with SEM and morphometric analysis of selected populations allowed us to conclude that A. deflexum, known from North America, and A. pyrenaicum, reported from Europe and Japan, are not conspecific. The shape and size of the frustule and the shape of the external areolae foramina are consistently different between the two species. Both A. deflexum and A. pyrenaicum are alkaliphilous, calciphilous diatoms, found mostly in phosphorus-poor waters.