Brachysira microcephala (Grunow) Compère, 1986

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 : 17-21

Brachysira microcephala (Grunow) Compère ( Figs 136–252 )

Varying taxonomic concepts contributed to the historical confusion and ensuing divergence of opinion around the mostcorrect specific epithet applicable to this taxon (Wolf & Kling 2001, Shayler & Siver 2004a). The name Brachysira neoexilis Lange-Bertalot in Lange Bertalot & Moser 1994 is still retained in some floras (e.g. Hoffmann et al. 2013). A large variation in its morphology may contribute to the lack of consistency in the identifications and the species is often documented with reference to its capitate or non-capitate morphotypes ( Lange-Bertalot & Metzeltin 1996, Fallu et al. 2000, Shayler & Siver 2004a).Although Wolf & Kling (2001) discount this as merely intra-specific variability an equivalent approach is followed here to help assess and illustrate the variability for four dominant forms.

BMIC-1: Morphotype with rounded or rostrate poles ( Figs 136–172 )

This category conforms to the non-capitate and rostrate valve outline illustrated in Compère (1988) and has the equivalent higher range in stria density compared to other forms of B. microcephala . Valve dimensions (n=35): Length 12.0–27.0 μm, width 3.5–5.0 μm and striae density 33–38 in 10 μm. This morphotype is well dispersed in the periphery in peatland catchments ( Fig. 2 ) and was found equally abundant in rivers and lakes from a total of 55 samples (max=32.9%).

BMIC-2: Non-protracted morphotype aff. B. procera ( Figs 173–205 )

This morphotype has an affinity with B. procera with its non-protracted outline, larger maximum valve length and a propensity for widening of the central area apically in larger specimens. There was a statistically significant linear relationship (r 2 =0.86, p<0.001) between valve length and distance between the proximal raphe endings, with a continuum in the latter character from the largest to the smallest valves examined. The higher stria density and a lower minimum valve length do not fit the existing concept for B. procera . Valve dimensions (n=88): Length 15.5–44.5 μm, width 4.0–6.0 μm and striae density 32–36 in 10 μm. This morphotype was widespread and occurred in 34 samples in association with BMIC-1. It occurred in both lotic and lentic habitats (max=14.9%) but was more prevalent and reached a higher density in low alkalinity oligotrophic lakes ( Fig. 2 ).

BMIC-3: Lanceolate to linear-lanceolate outline with protracted poles ( Figs 206–239 )

Variability in the form of the protracted ends in this morphotype was obvious while a continuum of smaller valves with BMIC-4 was also apparent. Occasional heteropolar specimens with clearly pointed and capitate ends on the same valve were observed. A less numerous form with more produced or elongate capitate apices ( Figs 224–229, 238–239 ) was occasionally encountered in several lake samples. Valve dimensions (n=30): Length 16.0–33.5 μm, width 4.0–5.0 μm and striae density 33–37.5 in 10 μm. This category was abundant (max=31.3%) and widespread in lakes and rivers (n=39 samples) in the periphery.

BMIC-4: Elliptic-lanceolate outline with clearly capitate poles ( Figs 240–252 )

Populations of this clearly capitate elliptic-lanceolate form were found in lakes at the extreme end of the alkalinity gradient. Smaller valves with a high maximum stria density ( Fig 224–246, 248–249, 251 ) were found in low alkalinity lakes and probably represent no more than a size reduction of BMIC-3. A further small population of larger specimens ( Figs 240–242, 247, 252 ) with a lower stria density was found in a highly calcareous marl precipitating lake. This is an unusual habitat for B. microcephala and was sufficient to result in higher optima for pH for this morphotype ( Table 3). Its trophic preference was equivalent to the other forms. Valve dimensions (n=15): Length 16.0–25.5 μm, width 3.5–5.0 μm and striae density 33–39 in 10 μm. This less common form (n=11, max=4.4%) was restricted to the west coast.

Figures

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

FIGURES 136–172. Brachysira microcephala morphotype BMIC-1. Figs 136–165. LM. Figs 166–172. SEM. Scale bar = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.

FIGURES 173–205. Brachysira microcephala morphotype BMIC-2. Figs 173–197. LM. Figs. 198–205. SEM. Scale bar = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.

FIGURES 206–239. Brachysira microcephala morphotype BMIC-3. Figs 206–229. LM. Figs 230–239. SEM. Scale bar = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.

FIGURES 240–252. Brachysira microcephala morphotype BMIC-4. Figs 240–246. LM. Figs 247–252. SEM. Scale bar = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.