ALGAL COMMUNITIES NEAR CAPE MACLEAR, SOUTHERN LAKE MALAWI, AFRICA

Algal communities were compared among benthic and net plankton samples from Cape Maclear, Lake Malawi. In the cool mixing season (from May to August), rocks were overgrown by Cladophora or Calothrix, accompanied by the diatoms Rhopalodia, Cymbella, and Navicula. These diatoms, together with Epithemia and Cocconeis, were epiphytic on Cladophora and macrophytes. In sandy areas, the common diatoms were Rhopalodia, Fragilaria, Epithemia, Navicula, Surirella, and Melosira. In all phytoplankton samples, taken with a 10-mu-m mesh net, cyanophyte cells were the most common (70 to 80%), especially those of Oscillatoria. Biomass, however, was dominated by Peridinium from November to April and by Anabaena and Oscillatoria from September to April when the mixolimnion was stratified. Among the chlorophytes, Oedogonium was the most common, especially from May into December when Pleodorina became more common. Diatoms dominated the biomass in the mixing season (May to September): Stephanodiscus in May, followed by Melosira nyassensis and lanceolate Nitzschia species from mid-June through August. For the rest of the year the epilimnion was stratified and these Nitzschia species were virtually the only diatoms present. Benthic and planktonic communities share few taxa: benthic taxa never made up more than 2% of cells in offshore tows. This conclusion contrasts with previous reports, especially regarding Surirella. Consequently, an abundance of benthic taxa in sediment cores may be interpreted as lower lake level if sediment redistribution can be excluded. The seasonality of the planktonic diatoms is compatible with current ecological hypotheses, and therefore increases their value as paleolimnological indicators.