Pollen deposition in lakes as measured by sediment traps

Sediment traps submerged in lakes were used to measure the numbers of pollen grains falling through the water toward the lake bottom. The traps are open, wide-mouth gallon bottles enclosed in wire mesh cages and suspended upright approximately 2 m above the lake bottom between an anchor and a submerged float. The numbers of grains captured in traps having openings 25-100 sq cm in area were directly proportional to the mouth area, demonstrating that these sizes trap with equal efficiency. Deposition rates measured at different stations within each of the three lakes studied were similar; however, different rates (29,000 and 53,000 grains per sq cm) were measured during the same year in two lakes only 11 km apart in Vermont. In a lake in Massachusetts the rate was 13,000 per sq cm one year and 28,000 the next. Differences in deposition rates may be related to differences in the pollen input to the lake from the air or from inflowing streams, or to differences in the amount of resuspension, mixing, and subsequent redeposition of pollen from the surface layer of sediment. The latter could explain the uniform intensity of the pollen rain measured by traps within each lake and the apparent independence of pollen percentage values from the absolute numbers deposited.