Fragilaria pectinalis (O.F.Müll.) Lyngb.

(Fig. 5)

Tentamen Hydrophytologiae Danicae Continens omnia Hydrophyta Cryptogama Daniae, Holsatiae, Faeroae, Islandiae, Groenlandiae hucusque cognita, Systematice Disposita, Descripta et iconibus illustrata, Adjectis Simul Speciebus Norvegicis. Hafniae: 185 (1819). — Basionym: Conferva pectinalis O.F.Müll., Nova Acta Academiae Scientiarum Imperialis Petropolitanae 3: 91, figs 4-7 (1788).

MATERIAL USED. — Population from the River Aa, Belgium.

MORPHOLOGY

LM (Fig. 5 A-AJ)

Frustules in girdle view rectangular, solitary or two cells linked together (Fig. 5 T-U). Valves linear throughout the entire cell cycle with parallel margins and protracted, rostrate, rarely capitate, apices.Shoulders poorly developed, gradually tapering into the apices.Valves often irregularly shaped, building sometimes large populations (Fig.5 V-AI).Valve dimensions (n = 60): valve length 20-35 µm, valve width 3.5-5.0 µm. Axial area narrow linear, gradually widening towards the central area.Central area large, forming an asymmetrical, occasionally inflated, fascia, with several shortened striae on one side lacking ghost striae. Several valves observed with depressed central area (Fig. 5W, AA, AE). Striae parallel throughout, becoming weakly radiate near the apices, 14-15 (occasionally up to 18) in 10 µm, often irregularly spaced. Areolae not discernible in LM.

SEM (Fig. 5 AK-AO)

Large, irregularly shaped mantle plaques present on the mantle edge (Fig. 5 AK). Mantle striae continuing from the valve face till 1/3 of the mantle edge (Fig. 5 AK). Striae uniseriate, composed of small, rounded areolae (Fig. 5 AL-AM), individually covered by external hymenes (Fig. 5 AL). Virgae between the striae weakly raised (Fig. 5 AN). One rimoportula present at one apex, obliquely positioned at the before last stria at the apex (Fig. 5 AM). Apical pore field present at both apices composed of several rows of small pores (Fig. 5 AM-AN). Small spines or granules scattered throughout the entire valve face and mantle (Fig. 5 AN). Internally, rimoportula obliquely positioned at one of the apices (Fig. 5 AO).

ECOLOGY OF FRAGILARIA DEFORMIS AND SYNEDRA VAUCHERIAE VAR. DEFORMIS

The original samples used to study the morphology of these two taxa were not accompanied by ecological notes in what environmental conditions these samples were collected. Based on the accompanying diatom flora, it is however possible to get an indication of the possible ecological conditions in what these two species could be found.

The William Smith sample for Synedra deformis is entirely dominated by the latter species. Up to 80% of all observed diatoms in the sample belong to this species. Less frequent taxa in the slide include Fragilaria vaucheriae, Amphora pediculus (Kütz.) Grunow, Navicula cryptotenella Lange-Bert., N. veneta Kütz., N. lanceolata Ehrenb. and Nitzschia dissipata (Kütz.) Rabenh. All these species are known to prefer circumneutral to alkaline, eutrophic, α- mesosaprobic conditions (Lange-Bertalot et al. 2017). Sample Grunow 907 was dominated in almost equal numbers by three taxa: Synedra vaucheriae var. deformis, Odontidium mesodon (Kütz.) Kütz. and Meridion circulare (Grev.) C.Agardh. The latter two taxa are often found in oligo- to mesotrophic, circumneutral to weakly acid conditions (Lange-Bertalot et al. 2017).