Hypothesized resource relationships among African planktonic diatoms

Several hypotheses are advanced for resource relationships among planktonic diatoms in African freshwater lakes that are consistent with the light and nutrient conditions of the lakes and the extant and fossil distributions of the diatom species in them. The hypotheses are all testable and are potentially powerful tools for interpreting past climatic conditions. A ranking is proposed along a Si : P gradient: at the high end are the planktonic Synedra spp. with the highest Si requirements and lowest P requirements (high Si: P), the planktonic Nitzschia spp. are intermediate, and the Stephanodiscus spp. are at the low end with the lowest Si requirements and highest P requirements (low Si : P). Melosira species may be ranked along a light: P gradient. We suggest that Melosira distans and Melosira arnbigua grow under high light and have low P requirements, Melosira agassizii and Melosira granulata are intermediate, and Melosira nyassensis has the lowest light and highest P requirements. There also appears to be a relationship between pore size and the light regime for growth among the Melosira species; thus, M. distans and M. ambigua have the smallest pores and highest light requirements, M. nyassensis has the largest pores and lowest light requirements. Melosira granulata is intermediate and seems to be very variable in pore size, depending on the light environment. One diatom, Nitzschia fonticola, lives in and on colonies of Macrocystis and is considered to be an obligate nitrogen heterotroph. Planktonic diatoms from Africa have long been of interest to diatom& (Miiller 1905; Hustedt 1949) and paleoecologists (see Richardson et al. 1978). Many fossil diatom records for sediment cores and exposed diatomite beds from Africa have been used to interpret climatic changes over the last 20,000 years or more. These cores are dominated by a few diatom species, often for thousands of years (Fig. 1). In Lake Albert (=Lake Mobutu Sese Seko), Stephanodiscus species have dominated the plankton diatom assemblage for about 12,000 years. In lakes Naivasha, Kivu, and Tanganyika there were several abrupt shifts in dominance, especially about 5,000 B.P., to new species that then persisted for long periods. The challenge is to identify the important features of the ecology of these species in order to provide the most precise reconstruction possible of past conditions in Africa. We present here several hypotheses concerning resource relationships of planktonic diatoms in African lakes and rivers. These