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Thin needle-shaped species of NitzschiaHassall were found to form a key component of the diatom plankton in the deep Kenyan waters of Lake Victoria. An attempt was made to classify the six most common Nitzschiaspecies of this growth form type on the basis of morphology, although they exhibit only small differences. Nitzschia lacustrisHustedt, previously described from Lake Victoria, was confirmed and recorded as an essential element of the phytoplankton. Three species were close to N. bacataHustedt, N. graciliformisLange-Bertalot & Simonsen and N. nyassensisO. Müller, but could not be definitively identified due to specific morphological deviations from the original descriptions, such as the overall size range, striation density and valve outline. Nitzschia bacataand N. nyassensishave repeatedly been recorded from the East African Great Lakes. Two more taxa are proposed as new to science, N. kavirondoensissp. nov. and N. rusingaesp. nov., since neither could be matched to descriptions of the existing nitzschielloid taxa around N. acicularis(Kützing) W. Smith. Their morphological features were documented by light and scanning electron microscopy and differentiated from similar taxa recorded from Africa and other parts of the world.

Populations of the delicate planktonie freshwater centric diatom genus Urosoleniafrom several lentic freshwater environments in Africa, Asia, Australia and South America were studied with both LM and SEM. The classical morphological features, e.g. frustule size, shape of the valve, length and positioning of the process and number of girdle bands per unit length, were re-evaluated and additional previously overlooked valve structures were identified as useful for species differentiation. Although fine resolution LM revealed the most important elements of the spine-like process, the SEM showed additional ultrastructural features on the frustule surface and terminal ends as well as structures on the valve margins not resolved in detail by LM. A few new morphological terms are proposed mainly for terminal ends of the process (ligula, seta), the process shaft (poroids, protrusions, ribs) and the valve margin. Using these LM and SEM observations, the genus description is emended and two species previously described earlier under Rhizosoleniaare transferred into the recently established genus Urosolenia (U. victoriae(Schroeder) Kling & Rott comb. nov. from Lake Victoria Africa and U. brauni(Hustedt) Rott & Kling comb. nov. from Jurucui Brasil). Three new Urosoleniaspecies are described, two from Australia (U. brevispinosaKling, McGregor & Rott sp. nov., U. parvaKling, Rott & McGregor sp. nov.), and one from Sri Lanka (U. diademataRott & Kling sp. nov.). Type localities including ecological data of the environments are listed and a key for identification of the named species is given.

Phytoplankton assemblages reflect the trophic state of lakes. As observed in Piburger See rapid oxygen depletion under the icecover can occur even in a moderately eutrophicated lake. In order to avert fish-kill by complete oxygen depletion a hypolimnetic outlet was introduced to enhance the export of oxygen poor and nutrient rich water. This paper deals with the effects of this restoration measure on phytoplankton species composition and biovolume. However the delicate problem to separate normal long-term variations of phytoplankton from changes due to the continuous hypolimnetic water removal is to be considered.

The influence of light on phytoplankton photosynthesis in the mesotrophic and meromictic Piburger See is investigated by the temporal analysis of the components of the underwater light climate. The annual time course of global radiation is affected by topographical screening of the horizon, which accentuates the difference in energy income between summer and winter. From December to April additional light attenuation by the winter cover causes a limitation of photosynthesis. During the ice-free seasons the vertical light gradient (of the most penetrating component) controls the structure of the photosynthesis depth profile. In spring, however, the maximum phytoplankton density in mid-depth layers causes irregular vertical distribution of photosynthesis. Phytoplankton is highly productive in near-surface layers in summer, as is shown by the maximum photosynthetic capacity found, i.e. 7.9 mg C · mg chlor-1 · h-1.