Cavinula jaernefeltii Mann & Stickle

Cavinula jaernefeltii (Hustedt) Mann & Stickle (Figs 82–92, 146–151)

Basionym: Navicula jaernefeltii Hustedt 1936 (in Schmidt 1874 –1959)

Valves elliptical, rounded in smaller specimens. Hustedt’s original description (1942), valve length 12–20 µm, width 7–11 µm, with 25–30 striae in 10 µm. Populations in this study: length 8–17 µm, width 5–9 µm, with 20–24 striae in 10 µm. Striae parallel in center, more radiate towards poles. Striae uniseriate, small areolae barely discernable in LM observations. At center one or more short striae. Internally, areolae between thickened costae, covered with hymen. Axial area narrow, linear, with small central area. Raphe straight, externally positioned within depression; distal and proximal raphe ends enlarged, not curved. Distal ends terminate on valve face. Internally, raphe with raised sternum.

Type: — Wuokatti, Finland. Simonsen 1987, fig. 308:1–5 (lectotype). Here presented in Fig. 84 from slide BRM N12 /84.

Observations:— Cavinula jaernefeltii is distinguished from other small Cavinula taxa by the valve shape, the shape of the axial area, the smaller areolae, and by the raphe formation, especially the position of the terminal raphe fissures away from the apex mantle on the valve face. The striae of C. jaernefeltii also appear to be finer, less punctate and less radiate than similar small forms of Cavinula hilliardii, typically making distinctions of this form from other Cavinula taxa relatively straightforward. However, there is a large range in valve forms from elliptical to linear-elliptical, and water chemistry optima, which may suggest more than one taxon is present within this group. Also compare with C. intractata (Hustedt) Lange-Bertalot, a taxon with narrowly rounded to cuneate ends and more radiate striae. Specimens identified as C. mollicula from Finland by Lange-Bertalot & Metzeltin (1996, fig. 24: 10–12) are also comparable but more similar to C. cocconeiformis f. elliptica sensu stricto which has a higher stria density, different raphe formation, and more consistent elliptical valve outline.

Distribution:— A total number of 77 records for C. jaernefeltii were reported in the CANA database from across North America, 69 of which originate from different localities in Canada and 8 samples were collected in Massachusetts, United States. During our LM observations, the presence of this taxon in North America was verified in the following localities: Ashigami Lake, Lake Abitibi, Moot Lake, Wawashkesh Lake in the province of Ontario; two localities in the Québec province: Madawaska and Maskinonge; Hilltout Lake in British Columbia; Baffin Island in Nunavut and another station in the Yukon Territory. During the observation of slides from Massachusetts, C. jaernefeltii was not verified. In the ANSP collection, C. jaerenfeltii was identified in different localities across South Carolina, Virginia, California, Colorado, Delaware, Indiana and Wisconsin. However, during our observation of a number of diatom slides from these localities, we did not confirm the presence of C. jaernefeltii. The southern distribution of C. jaernefeltii is restricted to Canada and the northern United States. C. jaernefeltii appears in oligotrophic waters which are mildly alkaline to acidic.A single locality in the Arctic with mildly alkaline waters (pH 8.3) had a population which represented 54% of the community (Antoniades et al. 2008).

Distribution Records:— Simonsen (1987, pg. 198, lectotype, fig. 308: 1–6, Finland), Antoniades et al. (2009: 61, SEM type material: 65–68, Arctic Archipelago), Metzeltin & Witkowski (1996, fig. 2: 42–44, Bear Island, Svalbard), Watanabe (2005, fig. II B -18: 5–9, Japan), Foged (1974, fig. 17: 13, Iceland), Potapova (2014, fig. 210, Bering Island, Kamchatka Peninsula, Russia).