Four new European Achnanthidium species (Bacillariophyceae)

Four new Achnanthidium species belonging to the species complex around A. minutissimum (Kutz.) Czarn. are described from different localities in Europe. Achnanthidium hoffmannii Van de VijVer, eCtor, a. Mertens et jarlMan nov. spec. was found in several oligotrophic, circumneutral rivers in Sweden. The species is characterized by the presence of irregular, slit-like areolae near the valve margin, clearly visible in LM. Achnanthidium acerosum Van de VijVer, lange-Bert. et jarlMan nov. spec., a narrow, linearlanceolate species with protracted, narrowly capitate apices, is likewise described from Sweden, but only found in one oligotrophic river in southern Sweden. Achnanthidium ertzii Van de VijVer et lange-Bert. nov. spec. was observed in a moss vegetation in a Laurisilva forest on the Portuguese island of Madeira. The species has an elongate, linear-elliptical valve outline with clearly capitate apices. The last species, A. tepidaricola Van de VijVer et M. de Haan nov. spec., found in a greenhouse of the National Botanic Garden of Belgium, has almost parallel margins and broadly rounded, protracted, capitate apices. All four species are sufficiently different from similar existing species.