MORPHOLOGY, VALVE ULTRASTRUCTURE AND STRATIGRAPHICAL VARIABILITY OF DISCOSTELLA TAXA IN A TROPICAL CRATER LAKE, NORTHERN TANZANIA

Abstract

Morphological and ultrastructural variability of Discostella Houk et Klee (Bacillariophyceae) valves from a small crater lake in northern Tanzania have been examined under scanning electron microscope (SEM) and light microscope (LM). Three types of Discostella are described; Discostella woltereckii, as well as a small variety of D. woltereckii denoted Discostella woltereckii var. minor Öberg, Risberg et Stabell and an unidentified species, Discostella sp. The diatom record during the last ~100 years is dominated by D. woltereckii and D. woltereckii var. minor. The lithology and diatom stratigraphy of a 36 cm sediment surface core show a period of low lake level with extensive aquatic vegetation from around AD 1600. Sometime around AD 1900 there is a substantial rise in the lake water level indicated by an increase in euplanktonic Aulacoseira ambigua and facultative planktonic Discostella spp.