The diatom Thalassiosira faurii (Gasse) Hasle in the Ziway–Shala lakes (Ethiopia) and implications for paleoclimatic reconstructions: Case study of the Glacial–Holocene transition in East Africa

• A strain of the diatom Thalassiosira faurii was isolated from a tropical lake.
• Morphological variability of T. faurii was investigated from lake samples and cultures.
• T. faurii showed a low tolerance to salinity in a laboratory experiment.
• Empiric and experimental conductivity optima revealed apparent discrepancies.
• The Younger Dryas appeared less dry than in previous reconstructions.

The species Thalassiosira faurii (Gasse) Hasle was found dominating the planktonic diatom community during summer 2012 in Lake Langano which is part of the Ziway–Shala alkaline lake system in Ethiopia. In a previous study, the species had been recorded in high abundance in 13.5–12 cal ka old sediments from the neighbor Lake Abiyata. Using a transfer function for conductivity, peaks in abundance had been interpreted as dry spells at the Pleistocene–Holocene transition since the species was considered strictly halophilous.In this study, a strain of Thalassiosira faurii was isolated from Lake Langano and grown in the laboratory. The observation of its morphology and reproduction, and comparison between modern and fossil populations helped to improve the taxonomic definition of the species. The autecology of the species was also investigated through a salinity tolerance test. The results revealed that the species was much more sensitive to salinity than previously believed. A new conductivity optimum was derived from the experiment and incorporated into the transfer function. The reassessment of lake conductivity variations along the core of Lake Abiyata showed that the intensity of the dry periods preceding the onset of the Holocene had been probably over-estimated.The reconsideration of the species relationship to conductivity may have important consequences for paleoclimatic reconstructions in tropical Africa where Thalassiosira faurii has been reported in abundance. The use of experimentation on local diatom strains allows paleoecologists to solve the problems of low representation of some species in modern dataset and of regional autecological variability within morphotypes.

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