Gomphonema supersaprophilum Van de Vijver & E.Reichardt sp. nov. (Figs 1–20 LM, 21–27 SEM)

Valves clavate, elliptic-lanceolate with broadly rounded valve margins, more strongly tapering towards the acutely ending footpole than towards the broadly rounded, typically protracted, rostrate headpole. Larger, more lanceolate valves, naviculoid valve outline (Figs 1, 2). Valve dimensions (n=25): length 24–52 µm, width 7.5–10.0 µm, the largest width almost at or slightly above the valve middle. Axial area linear, narrow, only 1/6 of the total valve width. Irregular pattern of very shallow depressions present in the axial area, not discernible in LM (Figs 21–23). Central area asymmetrical with shortened striae. On the primary side, one stria hardly shortened, terminated by a small, rounded isolated pore, clearly different in size and shape from the areolae (Figs 21–23). On the secondary side, stria either missing (Figs 1, 9, 13) or shortened. Raphe clearly lateral with undulating outer branches. Central raphe endings inflated, almost straight to weakly deflected (Figs 21–23). Terminal raphe fissures elongated onto the valve mantle, hooked (Figs 21, 22, 24). Striae uniseriate, 10–12 in 10 µm. Striae composed of slit-like, elongated, to weakly commashaped areolae (Figs 23–24), continuing onto the mantle. Mantle areolae likewise slit-like, terminating before the mantle edge, leaving a moderately broad hyaline zone at the edge (Fig. 22). Apical pore field at the footpole rather smaller, bisected by the terminal raphe fissure, composed of several densely packed rows of small, rounded pores. Internally, areolae located in shallow grooves, presenting slit-like foramina (Figs 25–27). Silica struts very small, not forming bars separating the areolae (Fig. 26). Central raphe endings hooked and recurved (Figs 25–26). Terminal raphe endings terminating onto distinct helictoglossae (Figs 25, 27). Internal foramen of the isolated pore slit-like, long (Fig. 27). Small pseudosepta present on both footpole and headpole (Fig. 25).

Type:— BELGIUM. Devebeek, Ingelmunster, West-Vlaanderen, Belgium, sample APM23 _73 (coll. date 24.viii.2023), leg. Vlaamse Milieu Maatschappij (holotype slide BR-4831= Fig. 6, isotype slide 436, University of Antwerp, Belgium).

Registration:— http://phycobank.org/104391

Etymology:— The specific epithet “ supersaprophilum ” refers to the superficial resemblance to G. saprophilum in combination with the much larger valve dimensions.

Ecology & associated diatom flora:— Sample APM23-73 was collected from submerged Phalaris arundinacea stems in the Devebeek, a small lowland river in Flanders, the northern part of Belgium, not far from the small town of Ingelmunster. The diatom flora in the river is characterised by high frequencies of Craticula subminuscula (Manguin 1942: 139) C.E.Wetzel & Ector (in Wetzel et al. 2015: 229), Gomphonema parvulum (Kützing 1844: 83) Kützing (1849: 65), Melosira varians C. Agardh (1827: 628), Navicula trivialis Lange-Bertalot (1980: 31), N. veneta Kützing (1844: 95), and several Nitzschia species such as N. amphibia Grunow (1862: 574), N. palea (Kützing 1844: 63) W. Smith (1856: 89) and N. pseudofonticola Hustedt (1942: 209). According to the ecological data in Lange-Bertalot et al. (2017), this diatom community is typical for alkaline, eutrophic, electrolyte-rich, up to polysaprobic waters, indicating a high level of pollution.