Differential sedimentation of pollen grains in lakes

Water circulation affects the sedimentation of pollen grains in lakes, sorting them according to morphological type. Pollen grains with rapid rates of sinking fall downward through the water and are deposited evenly onto the sediment throughout the lake. Those with slower sinking speeds in water, due to small size or low density, are kept in suspension in the turbulent waters of the epilimnion. They are carried across the lake in wind-driven water currents and are deposited preferentially onto littoral sediment. Preferential deposition distorts the original ratios in which pollen enters the lake from the air, causing variations in the pollen percentages in sediment from different parts of the basin. The process is thus of concern to paleoecologists, who seek to use pollen percentages as an index of regional vegetation.