Comparison of continuous phytoplankton records with the diatom stratigraphy in the recent sediments of Blelham Tarn

The stratigraphy of diatom remains in the sediment of Blelham Tarn, in the English Lake District, has been compared with population data collected by J. W. G. Lund over about 35 years. There is sufficient correlation for the production of an algal-based time scale for recently accumulated sediments which can be difficult to date precisely. Although there can be considerable variation in the pattern of deposition at different sites in the lake they can all be correlated with the written record. In paleolimnology it has always been assumed that the diatom assemblages found in sediments reflect the past diatom populations to some unknown degree. Improved understanding of diatom ecology now allows us to study and interpret the changes in the sedimentary profiles in more detail and it is therefore necessary to know just how closely the stratigraphic remains of once living populations correspond to the actual events; i.e. to what extent increases and decreases in population are recorded in the sediments. Can minor peaks of diatoms in sediments be identified and assumed to represent some real increase in a past population, or is the pattern of sedimentation such that several years are always averaged out by the mixing of sediment at the time of initial deposition? Recently attention has been focused on the comparison of algal records with sediment stratigraphy. Battarbee (1979) discussed the importance (and the problems) of occasional algal collections and records as a means of dating recent sediments. Livingstone and Cambray (1978) have shown that algal remains in Rostherne Mere, near Manchester, are sufficiently discretely preserved for several changes, of known dates, to be identifiable in the sediments. These sediments are laminated, due to calcite deposition, and the laminations, visible on X-radiographs, indicate the lack of disturbance of this material. Simola (1977, 1979) has shown that even the seasonal diatom succession within laminations of recent sediments is distinguishable in the surface record of Lovojarvi in Finland. He ascribed the preservation of this succession to the conditions of oxygen deficit and the cyclical nature of deposition in a small deep lake having a long period of ice cover during winter. These discrete sequences have both been deposited under specific conditions, leading to the formation of calcite and deoxygenation in Rostherne and deoxygenation and prolonged ice cover in Lovojarvi. It has now been possible to examine the recent sediments of one lake where laminated sediments are not produced, Blelham Tarn, in northwest England; this is a small, moderately eutrophic lake, stratified for about 6 months of the year, with a maximum depth of 14.5 m. The existence of a continuous series of records of the phytoplankton populations of this lake between 1945 and 1977 (Lund 1979) provides a rare opportunity to compare diatom stratigraphy in recent sediments with the populations that it represents and to see whether the pattern of annual changes in phytoplankton can be discerned in the stratigraphic record of these sediments despite any modifications due to wind turbulence, littoral resuspension, redeposition (especially after autumnal overturn), losses due to floods, or mixing by benthic fauna (Davis 1974). Recognition of the population increases in productive years in both living and relic assemblages also provides a more closely dated stratigraphy over this period than has previously been possi-