WaterSubject

Diatoms from the genus Achnanthidium are abundant in rivers, streams, and springs of the Appalachian Mountains. They inhabit clean and polluted waters, including those affected by acid mine drainage. The identification of Achnanthidium taxa is difficult due to their small cell size and insufficient information in the diatom floras. We studied the taxonomy and ecology of Achnanthidium in Appalachian rivers by analyzing a data set of benthic diatom samples and corresponding water chemistry data collected during several water-quality surveys from 181 sampling sites. Ten species were identified using scanning electron and light microscopy: A. alpestre (Lowe & Kociolek) Lowe & Kociolek, A. atomus (Hustedt) Monnier, Lange-Bertalot, & Ector, A. deflexum (Reimer) Kingston, A. duthii (Sreenivasa) Edlund, A. eutrophilum (Lange-Bertalot) Lange-Bertalot, A. cf. gracillimum (Meister) Lange-Bertalot, A. cf. latecephalum Kobayasi, A. minutissimum (Kützing) Czarnecki (sensu lato), A. reimeri (Camburn) comb. nov., and A. rivulare Potapova & Ponader. The distribution of common taxa in relation to water chemistry was studied by fitting non-parametric regression models (generalized additive models, GAM, and non-parametric multiplicative regression models, NPMR) to species relative abundances. Studied Achnanthidium species differed considerably in their responses to water chemistry. These results suggest that species-level identifications will lead to more accurate bioassessments.

Diatoms from the genus Achnanthidium are common in North American rivers; however their identification remains difficult and their ecology is insufficiently studied. There are several reasons why the identification of Achnanthidium taxa is complicated. First, because of the small cell size of these diatoms, some morphological features can be difficult to observe. Second, there is a considerable overlap among species in quantitative characters traditionally used for species identification. Difficulties in separating this inter-specific variability from intra-specific variability led to grouping of some Achnanthidium taxa into “species complexes”. Third, several species that are common in North America have been described in local floras but not included in floras commonly used for diatom identifications. Finally, the nomenclature of Achnanthidium is complicated because the genus concept has changed several times since its establishment by. According to the current concept of Achnanthidium, this genus encompasses monoraphid freshwater species with small linear-lanceolate to lanceolate elliptic cells less than 30 ?m long and less than 5 ?m wide, a concave raphe valve (RV) and a convex rapheless valve (RLV), uniseriate striae that are spaced wider in the center and denser towards the apices on the RV and that are more evenly and wider spaced on the RLV, and a fine raphe with terminal raphe fissures that can be either straight or turned to one side.

Examining diatom samples collected in the course of several water-quality surveys and stored at the Diatom Herbarium at the Academy of Natural Sciences (ANS) of Philadelphia (ANSP), we found that representatives of Achnanthidium are among the most abundant benthic diatoms in rivers of the Appalachian Mountains of Eastern North America. Achnanthidium species in Appalachia inhabit clean and polluted waters, including those affected by acid mine drainage. This led us to further investigate Achnanthidium species, especially with regards to their potential use as indicators of water quality. The goal of this paper was to describe the diversity of Achnanthidium species in the Appalachian Mountains and to explore their ecology and related distribution patterns.It has been suggested that diatom-based water-quality assessment methods can be based on genus-level identifications because there is considerable similarity in ecological characteristics of species within a genus. This study shows that various Achnanthidium species inhabiting Appalachian waters are different in some aspects of their ecology. Although most representatives have a tendency to occur in low-nutrient environment, the ranges of their distribution along the nutrient gradient vary considerably.

We also found large differences in species distributions in relation to pH and ionic content. These findings suggest that species-level identifications would help to improve the accuracy of bioassessments.

Species-level identifications are, however, often precluded by the absence of comprehensive diatom floras for many areas of the world. Among the ten diatom taxa covered by this paper only two, A. minutissimum and A. eutrophilum, could be found in the diatom flora of Middle Europe, the most commonly used reference for routine identification of diatoms around the world. In fact, A. eutrophilum was listed as a “Sippe” (meaning a “clan” or “tribe” in German) of A. minutissimum in the first edition of this flora, as it was not yet formally described as a species in 1991. Most of the other eight Achnanthidium species (A. rivulare, A. deflexum, A. alpestre, A. reimeri, A. duthii), have been so far found only in the Americas, and are therefore absent in European floras. A. cf. latecephalum and A. cf. gracillimum are quite similar morphologically to Eurasian taxa, but small differences in shape, size and frustule structure suggest that they may be separate species. The need to base water-quality assessment on the detailed studies of regional diatom floras is obvious.

Our study also shows that it is beneficial for bioassessments to use ecological characteristics of diatoms obtained from regional datasets. For example, the distribution of A. rivulare in relation to nutrient and pH in Appalachia was slightly different from its distribution at the large US-scale. It is possible that several morphologically indistinguishable populations can represent different ecotypes or even cryptic species that vary in their responses to the environment. This is probably the case of A. minutissimum, which is usually characterized as a cosmopolitan and ubiquitous diatom, tolerant to various types of stressors (hydrologic disturbance, low pH, heavy metals) and an early colonizer. It may consist of many entities (ecotypes or species) that are very similar morphologically, but differ in their ecology.

Although this study dealt only with Achnanthidium species, similar regional studies on other diatom genera are also likely to reveal a high diversity even in areas so seemingly well studied as North America, not to mention more remote areas of the world. We believe that further detailed studies of diatoms from the floristic and ecological perspectives are critical for their successful use as accurate environmental indicators.

This study was funded by the Patrick Center for Environmental Research, ANSP through the Staff Education and Development (SEDG) grant (KCP) and by the USGS NAWQA program through a cooperative agreement (MGP). We thank the staff of the USGS NAWQA Program, of the USGS Northern Appalachian Research Laboratory, of the Virginia DEQ, and of the Patrick Center for Environmental Research, ANSP, who collected and processed algal and water chemistry samples. We are also grateful to Sarah Hamsher for editing the manuscript, to Dee Breger, Drexel University, James Ferris, University of Pennsylvania, and Paul Hamilton, Canadian Museum of Nature, for the assistance with SEM and to Don Charles, ANSP for the support of this study. Finally, we would like to thank three anonymous reviewers for their helpful comments on the manuscript.