Recruitment of contemporary pollen grains to the sediments of Blelham Tarn, Cumbria.

Investigation of contemporary pollen recruitment to lakes can help to elucidate the fossil record in lake sediments. In the English Lake District, pollen recruitment was studied at Blelham Tarn (area 10.2 hectares), into which two experimental tubes have been introduced. Each isolates a water column (45 m diameter X 11-12 m deep). Estimates were made of pollen deposited per unit area per unit time from the air (in traps on land and floating inside the tubes), and within the lake (at the mud surface and in traps submerged both inside and outside the tubes to catch seston). Samples from inflows were also analysed for pollen. Results indicated that: (i) Airborne pollen deposition varied in intensity, (a) seasonally, and (b) between years, with the magnitude of annual pollen production. (ii) Floating traps underestimated (by c. X 2) the actual intensity of pollen deposition on the water. (iii) Most pollen disseminated was deposited near the parent plants. Hence terrestrial surfaces form a reservoir from which pollen is recruited by surface runoff. (iv) Catches in the submerged traps varied seasonally with the relative recruitment of (a) fresh airborne pollen, and (b) pollen resuspended from the sediment surface by turbulence. (v) Delivery of streamborne pollen to the lake varied non-seasonally with rainfall/stream discharge. (vi) Ratios for pollen: seston trapped varied seasonally with plankton production, (vii) Pollen taxa were not dispersed with equal efficiency in air: poorly-dispersed taxa were recruited mainly to the lake by runoff. The effects are considered of recruitment processes and of limnological variables upon the following: (a) the spatial distribution of pollen over the mud surface; (b) the relationship between the pollen composition of lake sediment and vegetation composition around Blelham Tarn. Factors which apparently determine the recruitment of pollen to lake sediment today are discussed generally, and the possible effects of these factors upon the fossil record (pollen percentages and absolute pollen deposition rates) are considered.