The distribution of pollen in the surface sediments of Lake Malawi, Africa, and the transport of pollen in large lakes

Fifty-four surface sediment samples from Lake Malawi and twelve from its catchment were analyzed to determine the patterns of pollen and spore distribution and transport to and within the lake. Riverine input of pollen and spores is important, particularly around river deltas, and the spore to pollen ratio decreases with distance from river inflows. High pollen concentrations in the middle of the lake probably result from a lack of elastic sediment input. The primary transport vector differs among pollen types. Pollen of Gramineae and montane forest taxa (e.g. Podocarpus and Olea) seem to be primarily wind transported, whereas pollen of evergreen forest taxa (e.g. Macaranga, Myrica and Mimusops), woodland taxa (e.g. Brachystegia, Uapaca, and Acacia), Typha and Cyperaceae seem to be transported primarily by water. As a result of variation in transport vectors, no single pollen source area can be defined for the pollen spectra in the lake sediments: it is likely that vegetation types in the catchment are represented to different degrees in different parts of the lake and vegetational variations in the catchment are reflected in the lake only at the coarsest scale. This conclusion is supported by detrended correspondence analysis. Because of the lack of a definable geographical relationship between pollen source areas and distribution patterns of pollen in the surface sediments in the lake, multivariate statistical techniques calibrated to local, habitat-level pollen samples are not valid in large lakes and marine systems.