The Third International Symposium on Paleolimnology was held at the beginning of September 1981 in Finland. After registration at the University of 10ensuu and the opening session and welcome reception in the Town Hall, the participants moved to a hotel at Koli, a hill famed as a landmark and for its panoramic view. Here the lecture session, lasting four days, took place. Momentarily, the mosaic of the lakeland became visible between the clouds showing the hill slopes covered by mature spruce forest and scattered old birch trees, a reminder of the last slash-and-burn phase some 130 years ago. Altogether 120 paleolimnologists participated in the meetings and most also attended one of the two post-symposium excursions to the Arctic Circle and to South Finland. The lectures covered a wide range of paleolimnological subjects. The main topics, however, included current aspects in the dating of sediments and the history of eutrophication. The symposium was advised by the international organizing committee under the presidency of Professor Edward S. Deevey. Other members present at Koli were Dr. Maj-Britt Florin, Dr. Heikki Ignatius, Prof. Heinz Lamer, Prof. Gerbert Martinson and Dr. Magdalena Ralska-lasiewiczowa. They also gave direction to a discussion on future plans for international activity in paleolimnology. Later in the autumn, after the end of the symposium, the partici pants sent special thanks to the secretaries, Mrs. Eija Merilainen, Mrs. Tuula Nylander and Mrs. Anita Toukomaa, for solving individual problems during the symposium with smooth and rapid efficiency."

During the last few years, lake acidification has become a topic of international interest because of the detrimental effects of this problem on lake ecosystems. The stratigraphic record of diatoms and other paleoindicators in lake sediments has been used increasingly to document past limnological conditions. Because many diatom taxa are sensitive indicators of lakewater pH, this approach is especially useful in assessing impacts of atmospheric pollutants and watershed land use on lake pH. The idea that a book be prepared on this subject originated at a workshop entitled 'Reconstructing pH from sedimentary diatom remains - recent insights and update on methods' organized by R.B. Davis and held at the XXII Congress of the International Association of Limnology at Lyon, France in August 1983. At this workshop, presentations were made by many of the authors in this volume. The rapid progress in this field was evident when a second workshop, this time organized by R.W. Battarbee and R.J. Flower, was held one year later at the VII International Symposium on Diatoms at Paris, France. Many individuals helped with the preparation of this volume, most notable were our referees, who thoughtfully reviewed the manuscripts and suggested many improvements. I would also like to thank my colleagues at Queen's University who helped in innumerable ways over the last two years. A special word of gratitude is due to Dr. M.M. Boucherle, Dr. S.R. Brown, Ms. T.C. Oliver, and Mr. P.R. Leavitt. J.

During the last few years, lake acidification has become a topic of international interest because of the detrimental effects of this problem on lake ecosystems. The stratigraphic record of diatoms and other paleoindicators in lake sediments has been used increasingly to document past limnological conditions. Because many diatom taxa are sensitive indicators of lakewater pH, this approach is especially useful in assessing impacts of atmospheric pollutants and watershed land use on lake pH. The idea that a book be prepared on this subject originated at a workshop entitled 'Reconstructing pH from sedimentary diatom remains - recent insights and update on methods' organized by R.B. Davis and held at the XXII Congress of the International Association of Limnology at Lyon, France in August 1983. At this workshop, presentations were made by many of the authors in this volume. The rapid progress in this field was evident when a second workshop, this time organized by R.W. Battarbee and R.J. Flower, was held one year later at the VII International Symposium on Diatoms at Paris, France. Many individuals helped with the preparation of this volume, most notable were our referees, who thoughtfully reviewed the manuscripts and suggested many improvements. I would also like to thank my colleagues at Queen's University who helped in innumerable ways over the last two years. A special word of gratitude is due to Dr. M.M. Boucherle, Dr. S.R. Brown, Ms. T.C. Oliver, and Mr. P.R. Leavitt. J.