Brachysira, Kutzing, 1836

Kennedy, Bryan & Allott, Norman, 2017, A review of the genus Brachysira in Ireland with the description of Brachysira praegeri and Brachysira conamarae, new raphid diatoms (Bacillariophyceae) from high status waterbodies, Phytotaxa 326 (1), pp. 1-27 : 3-6

Brachysira distribution

A total of 12 species of Brachysira were reported from more than half of the lakes (n=50) and almost one-fifth (n=39) of river stations surveyed. For waterbodies where Brachysira was encountered, the average and maximum taxa number was higher for lakes (4, 10) than rivers (2, 6). The largest populations were found in catchments with acid bedrock and peatlands, but the genus was absent from waterbodies having an annual average pH below 5.1 or minimum pH below 4.6. The dominance of Brachysira in naturally acidic waterbodies declines with increasing alkalinity and it is replaced by other taxa like Achnanthidium minutissimum (Kützing) Czarnecki (1994: 157) . However, selected Brachysira taxa were found in certain calcareous lakes in the midlands and the Burren ( Fig. 1 ). Brachysira was replaced in epilithic communities by Eunotia and Pinnularia spp. in sites with lower pH values.

Two new Brachysira species are proposed including B. praegeri which reached maximum abundance in two reference low alkalinity lakes in the west. B. praegeri was also recorded in lotic waters but always at densities <1.0% and was restricted to the most pristine oligotrophic and naturally acidic watercourses. The other new species, B. conamarae is a rare diatom, found in the epilithon from two reference lakes in the Connemara region.

Several cosmopolitan taxa were rare in epilithic samples and only encountered as isolated valves in the west of Ireland, including the acidobiontic Brachysira serians (Brébisson in Kützing) Round & D.G. Mann (1981: 227), Brachysira styriaca (Grunow) Ross (in Hartley 1986: 607) and Brachysira zellensis (Grunow) Round & D.G. Mann (1981: 227) . The genus was mostly absent from catchments draining calcareous bedrock, but with some important exceptions including Brachysira vitrea (Grunow) Ross (in Hartley 1986: 607) and Brachysira liliana Lange-Bertalot & Moser (1994: 41) which were found predominately in lakes in the central lowlands and the Burren region ( Fig. 1 ). B. neglectissima was more widespread and reached a maximum relative abundance in Lough Bunny. Reduced densities were also encountered in some oligotrophic calcareous lakes in the midlands and the west ( Fig. 2 ). These alkaliphilous species were almost exclusively restricted to lotic habitats with the exception of a single river station which had a diverse Brachysira community. Their exceptional occurrence may be owing to populations flushed from a small lake upstream of the sampling location, or from epilithic communities established in lower velocity areas of the cascadestep-pool habitat.

The ecology and morphology of many of these taxa conform to the typical range reported for their type specimens and in published accounts. Some differences in the morphology of B. neglectissima relative to the type description are apparent, while the morphology of Brachysira intermedia (Østrup) Lange-Bertalot in Lange-Bertalot & Moser (1994: 34) is discussed in further detail in the species accounts provided below. B. garrensis , B. brebissonii and B. intermedia had slightly lower optima for pH ( Table 3) and very large abundances of B. garrensis (approximately 50% abundance) were observed in samples from two impounded coastal lakes.

The ubiquitous B. microcephala complex was the most commonly encountered and abundant in both low alkalinity lakes and rivers ( Fig. 2 ). It was also the most morphologically heterogeneous group, with many morphotypes co-occurring in the same samples. The polymorphic valve outline fits the general concept for this taxon reported elsewhere ( Lange-Bertalot & Metzeltin 1996, Wolfe & Kling 2001, Shalyer & Siver 2004a, Hamilton 2010). The categories BMIC-1, BMIC-3 and BMIC-4 were similar in dimension and although differences in their outline and the shape of their poles usually enabled differentiation among them, a continuum between some forms was apparent. Form BMIC-2 had a maximum frustule length exceeding that reported for B. microcephala and had a close affinity with Brachysira procera Lange-Bertalot & Moser (1994: 55) and some of the comparable unidentified taxa illustrated in the same monograph (Tafel 46, Fig. 19–25 ). Its stria density was higher and never approached the published range reported for B. procera ( Table 2). Because of their similar morphology, smaller valves of this form were difficult to separate from non-protracted frustules of BMIC-1. However, the greater maximum valve length, non-protracted poles, slightly coarser stria density and elongate central area were distinctive in larger valves. The environmental optima calculated for B. microcephala morphotypes were similar ( Table 3). The broadly capitate BMIC-4 had a higher optima for pH but populations were encountered less frequently relative to the other more widespread forms and it never reached a high abundance.

Figures

FIGURE 1. Ireland. (a) Map showing the location of the sampling sites and the distribution of the primary acidic bedrock geology. The regions and lakes indicated are referred to in the text. (b) Brachysira species as % of all diatom species at sites. (c) number of Brachysira species at sites. Light grey refers to river sites and dark grey refers to lakes in (b) and (c).

FIGURE 2. The distribution of the more abundant Brachysira taxa in Irish lakes and rivers.

FIGURES 3–24. Brachysira praegeri sp. nov. Figs 3–18. LM. Fig. 8. Holotype specimen. Figs 7–10. Range of valves from the holotype population(TCD0018137) L.Nambrackmore.Figs 3–6, 11–17.Valves showing morphological range andsize variation in other populations. Fig. 18. A teratological valve giving the abnormal impression of rostrate ends. Figs 19–24. SEM. Fig. 19. External view. Fig. 20. Internal view. Figs 21–24. Variation in valve outline and occurrence of papillae in the central area. Scale bar in Fig. 18 = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.

FIGURES 25–33. Brachysira conamarae sp. nov. Figs 25–26. LM. Fig. 25. Holotype specimen (TCD0018138). Figs 27–33. SEM. Fig. 27. External view of an elongate valve with slightly more prominent axial area. Figs 28–30. Variation in valve morphology apparent for the holotype population with varying pattern of occurrence of papillae over the valve face. Fig. 31. Oblique valve in tilt illustrating ultrastructural features. Fig. 32. Detail of the centre of the frustule with papillae over the entire valve face. Fig. 33. Detail of the valve end and T-shape distal raphe endings. Scale bar = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.