Cells about as long as broad, moderately constricted. Sinus linear, closed for the most part. Semicells in frontal view trapeziform with broadly rounded angles, in lateral view subglobose, in apical view elliptic with a slight median inflation. Cell wall smooth, provided with widely spaced, fine pores. Chloroplast with a single pyrenoid.

Dimensions: length 11–14 μm, breadth 11.5–13 μm, thickness 7.5–9 μm, isthmus 6.8–7.3 μm.

Type:— THE NETHERLANDS. Drenthe: shallow pool in Hijkerveld near Hijken, 52.920° N, 6.487° E, among submerged Sphagnum on sandy soil, pH 4.3 conductivity 52 μS cm-1. Van Westen, 4 Februari 2017 (holotype L! Hugo de Vries Lab 2018.03, preserved as a fixed natural sample).

Etymology:—The above-described species is named in honour of Dr Frans Kouwets, recognized specialist in small, taxonomically problematic Cosmarium species.

Differential diagnosis:— Cosmarium kouwetsii belongs to a group of small-sized, smooth-walled Cosmaria the taxonomy of which often is a source of confusion by want of well-marked, differentiating characteristics. Also in relation to our newly described species a number of taxa may be found resembling it to a lesser or greater degree. In particular may be considered Cosmarium sp. and C. subadoxum Grönblad in Kouwets (1987) from the French Auvergne, C. succisum forma jaoi Woodhead et Tweed in Gerrath & Denny (1988) from Sierra Leone, C. geometricum var. suecicum Borge in Taft (1945) from Lake Erie in northern USA, and in Messikommer (1963) from western Switzerland.

Apart from any possibly incorrect identification, none of those taxa fully agrees with our C. kouwetsii. Most consistent difference is in the relative breadth of the cell isthmus and the absence of a central inflation. Where all pictures provided in the above-mentioned references show deeply constricted cells, breadth of the isthmus being at best one third of cell breadth, C. kouwetsii is characterized by but moderately constricted cells with an isthmus that is at least half of the cell breadth.

Cosmarium kouwetsii was only found in rather acid moorland pools among Sphagnum vegetation. pH 3–5, EC 10– 60 μS cm-1. Accompanying desmid species: Actinotaenium cucurbita (Ralfs) Teiling, Cosmarium pyramidatum Ralfs, Euastrum binale var. gutwinskii (Schmidle) Homfeld, Micrasterias truncata Ralfs, and Spondylosium pulchellum W. Archer.