Numerical analysis of small Arctic diatoms (Bacillariophyceae) representing the Staurosira and Staurosirella species complexes

Small benthic fragilarioid diatoms (Bacillariophyceae) are dominant taxa in circumpolar Arctic lakes and ponds. These small forms are difficult to identify under light microscope (LM) because of valve size, the limited number of visible characters, and considerable morphological variability within populations. For this reason, ultrastructural characters visible only under a scanning electron microscope (SEM) such as areolae and spine structure are usually recommended for species- and even genus-level identification. This is, however, often impractical. In this study we show that selected characters of Staurosirella pinnata (Ehrenberg) Williams & Round and Staurosira venter (Ehrenberg) Cleve & Moller measurable under LM correlate with diagnostic characters that can only be evaluated under SEM. We measured 32 morphological characters from SEM observations of 510 valves representing Staurosirella pinnata sensu Hustedt, Staurosira venter sensu Hustedt, and two morphologically distinct forms of S. venter sensu lato from lakes in Canada, Fennoscandia, and Siberia. Specimens from the circumpolar Arctic region identified as Staurosirella pinnata can be distinguished from Staurosira venter sensu lato using valve width and areola length (at margin), both visible under LM. Specimens with areolae lengths (at margin) larger than 0.4 mu m are Staurosirella pinnata. For specimens with areolae lengths (at margin) smaller than 0.4 mu m, those with valves wider than 2.9 mu m are Staurosira venter sensu lato, whereas those with valves smaller than 2.9 pin are Staurosirella Pinnata. Application of these numerical criteria allows for correct identification of 94% of specimens to either Staurosira venter (including morphotypes) or Staurosirella pinnata.