Contemporary and historical pollen recruitment to a small lowland English lake
The source of the pollen preserved within fossil assemblages is one of the most important factors to be considered when attempting to reconstruct fonner vegetation landscapes. Due to its mode of dispersal, at anyone time a pollen assemblage is a spatially aggregated record or pollen from local, extra-local and regional vegetation. Studies of pollen recruitment to lakes with surface inflows have shown that fluvial transport plays a dominant role in the recruitment of pollen to these sites. Pollen assemblage fonnation in lakes is further complicated by lirnnological processes which may affect the stratigraphic 'integrity' of the sediment record. A range of techniques and methodologies have been employed to investigate the influence of recruitment and lirnnological processes on the assemblage characteristics and pollen catchment area of an intennediate sized lake basin with surface inflows. The impact of land-use change OIi sediment yields and sediment sources has also been investigated through the use of mineral magnetic and sediment analytical techniques. The relative importance of aerial and streamborne recruitment has been established by , momtoring of pollen input to the lake from both sources over a 17 month period. Results , confmn the importance of fluvially transported pollen at this site and indicate that between 70-90% Qf the total pollen reaching the lake is derived via inflow streams at the present day. Downstream changes in the composition of streambome pollen caught in Tauber traps submerged within the main inflow stream suggests that at least 35% of the pollen recruited via the inflow is currently derived from 'local' riparian vegetation communities. Spatial and temporal patterns of intra-lake pollen deposition have been assessed by analysis and comparison of 24 surface lake mud samples and the pollen content of two parallel sediment cores. Central surface mud samples exhibit less 'noise' than those from marginal zones. Higher variability in littoral surface sediments, as measured by mean, standard deviation and coefficient of variation values of individual pollen types, is thought to reflect high deposition from lake marginal vegetation as well as limnological factors such as water turbulence. A greater diversity of pollen types were found in streamborne assemblages but in other respects percentages of the major pollen types in air, stream and surface lake samples were very similar. Comparison of the pollen, magnetic and sedimentological characteristics of parallel cores taken from the central lake area show close downcore strati graphic correspondence over the topmost 80cm. The shallowness of the lake does not appear to have produced any noticeable discontinuities or non-unifonn patterns of pollen and sediment deposition in central zones. . The effective pollen catchment area of the lake has been established by determining the 'sensitivity' of radiometrically dated lake pollen profiles to vegetation changes at different spatial scales within the landscape, established from documentary records of local and regional land-use and woodland history. From these data it is estimated that c. 70% of the pollen recruited to the lake originates from vegetation growing within c. 2.0 km of the lake. This conclusion contrasts with models of pollen recruitment proposed for similar sized lakes without inflow streams where a significant regional component (>60%) is predicted. Results suggests that the single largest contribution to total influx comes from stream marginal vegetation. A lakewide change in sediment type is recorded after Enclosure in 1789, as the focus of arable activity shifted from outside to inside the lake's drainage catchment. Mineral magnetic properties of lake sediment and catchment soils and sediments also indicate a shift in the source of allochthonous inputs reaching the lake at this time, from subsoil to topsoil derived material. Estimated inorganic sediment yields over the last 50 years are Iow when compared to similar sites with high lake/catchment ratios.