Two new needle-shaped Nitzschia taxa from a deep East African crater lake

Abstract

Two new diatoms, Nitzschia fabiennejansseniana Cocquyt & Ryken sp. nov. and Nitzschia pseudoaequalis Cocquyt sp. nov., are described from Lake Challa, a deep crater lake in East Africa, based on light and electron microscopical observations. These Nitzschia species are morphologically similar to other needle-shaped, planktonic Nitzschia present in freshwater lakes of tropical Africa. The differences between the newly described species and related Nitzschia taxa are a combination of the shape of the poles, the number of striae and fibula in 10 µm, and the presence of a central gap between the fibulae. Nitzschia fabiennejansseniana is common in the plankton of Lake Challa towards the end of the main dry and windy season during the northern hemisphere summer, when deep water-column mixing causes upwelling of nutrient-rich hypolimnetic water. Together with Afrocymbella barkeri, this taxon is the dominant diatom in the pelagic phytoplankton of Lake Challa today, and has been continuously over at least the last several hundreds of years, as recorded in its sediments. The temporal dynamics of both taxa in Lake Challa play a major role in the interpretation of past climate change in equatorial East Africa. The other new taxon, N. pseudoaequalis, was common in some epilithic samples taken from the littoral zone of Lake Challa. It was also sporadically observed in the pelagic phytoplankton near the end of the mixing season.

Keywords:

• Africa
• diatoms
• Lake Challa
• new species
• Nitzschia
• taxonomy

Acknowledgements

The authors wish to thank Caxton Mukhwana Oluseno for the monthly sampling of Lake Challa phytoplankton, and Jorunn Dieleman for her help with the epilithic sampling. Many thanks are due to Myriam de Haan for technical assistance in SEM, to Wannes De Crop for physical and chemical data on Lake Challa, to Dirk Verschuren for instructive discussions, and to two anonymous reviewers for their constructive remarks on the text.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The present study is part of the Brain-be project BR/121/A2 PAMEXEA (Patterns and mechanisms of climate extremes in East Africa) sponsored by the Belgium Science Policy (BELSPO).