New record of Geissleria declivis (Hust.) Lange-Bert. (Bacillariophyceae) in Europe, the first in Poland

As a result of the research carried out in Lakes Duszatyńskie and the Olchowaty stream, a new European site with Geissleria declivis (Hust.) Lange-Bert. was found — a new species for Poland. The highest abundance of G. declivis has been observed on sandy substrate and small stones in the littoral zone of the Upper Lake during the spring season. The species occurred together with Achnanthidium minutissimum (Kütz.) Czarn. var. minutissimum, A. catenatum (Bily & Marvan), Fragilaria leptostauron (Ehrenb.) Hust. var. leptostauron and F. pinnata Ehrenb. var. pinnata. Our observations revealed that G. declivis develops in lentic water bodies with medium organic matter content and neutral pH. The species seems to prefer mesotrophic waters with low to medium electrolyte content and oligosaprobous. The species morphology and identity is documented in a series of light microscopic (LM) images and for the first time under a scanning electron microscope (SEM).