Iconographia Diatomologica, Volume 27: Fossil Diatom Diversity in Lake Ohrid over the Last 1.36 Ma

About this book

Lake Ohrid, on the border of North Macedonia and eastern Albania, is home to one of the richest diatom communities of any freshwater ecosystem. Alongside generalist species, the lake hosts a remarkable number of endemic and relict species. This extraordinary diversity was likely shaped by speciation events and the accumulation of species from the local and regional pools during the lake's million-year history. The relative stability, geographical isolation, and high resilience to environmental and climate changes probably contributed to this richness.

To uncover the processes that have led to this high diatom richness in Lake Ohrid, several drilling campaigns have been conducted over the past two decades. Most significant was the sediment profile DEEP-5045-1, which covers the continuous limnological history of the last 1.36 million years. Investigations of these sediments revealed that diatom species diversity emerged and increased shortly after the lake was formed, reaching the highest level of diversity after 0.7 million years. However, the taxonomic identity of the diatom species in Lake Ohrid's fossil sediments still needs to be more thoroughly investigated and illustrated.

Here, the authors have conducted microscopic observations of diatom species at roughly 2000-year intervals across the entire 1.36-million-year-old DEEP-5045-1 core to provide a detailed inventory of the fossil diatom communities. They have identified around 500 taxa throughout the sequence, both endemic and non-endemic, of which 54 are new to science. Planktonic diatoms dominate the assemblage and show high intraspecific morphological variability. This is particularly evident in the endemic species such as Cyclotella praecavitata s cyclotella fottii Hustedt. A total of 36 different morphological units were observed, which are described here as subspecies. The concept of subspecies is used to describe variable taxa that persisted over significant geological periods and their evolutionary relationship. In addition, numerous other taxa from various genera were observed and described as new species, including Stephanodiscus Ehrenberg (5); Cyclotella (Kiitzing) Brebisson (1); Pantocsekiella K.T.Kiss & Acs (1); Navicula Bory (3); Placoneis Mereschkowsky (2); Eolimna Lange-Bertalot & W.Schiller (1); Fallacia Stickle & D.G.Mann (1); Aneumastus D.G. Mann & Stickle (1); Encyonema Ktitzing (1); Cymbella C.Agardh (1) and Cymbopleura (Krammer) Krammer (1). Most of them appear in the lake's earliest phases and are likely endemic to the lake. Moreover, many of the endemic or relict taxa living in the lake today were also found in larger populations in the fossil sediments. These include relict species such as Amphora transylvanica Pantocsek; Diploneis mauleri (Brun) Cleve; Navicula hastata Jurilj; Cymbopleura budayana (Pantocsek) Krammer, and endemic species like Aneumastus macedonicus Levkov; Diploneis neglecta Jovanovska & Levkov; Gomphonema mihoi Levkov; Placoneis significans Lange-Bertalot; Campylodiscus marginatus Jurilj, among others.

These results provide a comprehensive overview of the diatom community in Lake Ohrid over time and provide a basis for understanding the evolutionary and environmental forces that have shaped the current diversity in this ancient freshwater ecosystem.

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