Description: The Republic of North Macedonia (RNM) is a small country in the Balkan Peninsula. Despite its small size, the country is characterized by a high diversity of aquatic habitats such as springs, streams, rivers, natural lakes (glacial and tectonic origin), halomorphic soils, bogs, fens, wetlands, and reservoirs. To date, 1768 taxa have been recorded and illustrated. I will introduce the amazing ancient Lake Ohrid, with approximately 900 taxa. The high diversity of this lake will be placed in light of recent studies of speciation (Wilke et al. 2020, Jovanoska et al. submitted).

Species within some genera present challenges to classification, such as Cyclotella vs. Pantocsekiella; Tertiarius vs. Lindavia, Lindavia vs. Edtheriotia. What are the considerations for treating these taxa? I will introduce the Cyclotella cavitata/fottii/hustedtii complex from Lake Ohrid. This complex comprises more than 25 different morphologies, appearing over the last 1.5 million years. I suggest that these forms represent the evolutionary phenomenon of punctuated equilibrium.

I will also present examples of morphological variability in Staurosirella and Staurosira from alpine ponds, glacial lakes and Lake Ohrid. As in other parts of the world, a challenging genus for identification is Fragilaria with 37 taxa (20 of them lacking formal names). RNM includes high diversity in the genera Rhoicosphenia, Frustulia, Geissleria, Karayevia, and Diploneis. Challenges also are present for Neidium, Navicula, Pinnularia, and Caloneis. Yet, Gomphonema is the most speciose genus RNM, with more than 160 species.

Target audience: This presentation will be of interest to those who wish to learn more about taxonomy, biogeography, ancient lakes, and evolution of diatoms. Students working on freshwater diatom diversity will find kinship in their challenges to identify species/genera. I will reassure you that the same challenges in diatom identification are present in every lab. You are not alone.