Algal remains in recent lake sediments

The preservation of algal remains and their stratigraphy in recent lake sediments is examined in relation to documented fluctuations in the composition and abundance of the phytoplankton in the same lakes. Identifiable remains of non-siliceous algae were commonly recovered, but the degree of preservation varied specifically and between lakes. Smaller species were under-represented owing to rapid decomposition and selective consumption by herbivores. Larger species were relatively better preserved, especially in the sediments of the more productive lakes where long annual periods of anoxia, or the presence of an algal mat, may inhibit bacterial decomposition. The stratigraphy of many algal remains from one site, Rostherne Mere, accurately reflected the documented fluctuations in the phytoplankton. This correlation enabled the establishment of an 'algal chronology' which provided independent verification of radionuclide dating. Viable akinetes of blue-green algae were recovered from sediments up to 70 years old. Cores from the same site in the mere showed no significant qualitative differences although there were quantitative areal variations. Estimates of standing crop and the concentration of algal remains in either entrapped seston or the sediments were typically within the same order of magnitude. Evidence is presented to refute the suggestion that the mere has recently become enriched by gull excreta. The diatom stratigraphy of two Cumbrian lakes - Grasmere and Elterwater - corresponded to recorded alterations in the phytoplankton, associated with recent changes in sewage treatment. Sediments rich in algal remains are compared to similar deposits in other countries and the possibility that some fossil fuels originated from algal oozes is discussed.