Bijdragen tot de kennis der Nederlandse Desmidiaceeënflora, 3. N.W.-Overijssel (2)

Subsequent to a previous publication (COESEL, 1974), an additional number of noteworthy records of desmids from the NW corner of the province of Overijssel is reported. The complex of Holocenic lakes and peatbogs of that area increasingly proves to be the richest site of desmids in the Netherlands at the present time. It is especially the very shallow, mesotrophic pools in quivering bogs which have repeatedly yielded records of species new for the Dutch desmid flora (such as Cosmarium hornavanense var. dubovianum, C. quasillus, and C. subbroomei). It is to be regretted, however, that there are clear indications of a depauperisation of the tychoplanktonic desmid flora owing to a deterioration of the waterquality. Some species in particular, such as Cosmarium insigne (compare COESEL, 1974) and Staurastrum gladiosum var. delicatulum, which were till recently quite common in old peat pits in the area under discussion, exhibit clear indications of a decrease in population density.