Paleolimnology

Paleolimnology is a scientific subdiscipline closely related to both limnology and paleoecology. Palaeolimnological studies are concerned with reconstructing the paleoenvironments of inland waters (lakes and streams; freshwater, brackish, or saline) – and especially changes associated with such events as climatic change, human impacts (e.g., eutrophication, or acidification), and internal ontogenic processes.

Paleolimnological studies are commonly based on meticulous analyses of sediment cores, including the physical, chemical and mineralogical properties of sediments, and diverse biology records (e.g., fossil diatoms, cladocera, ostracodes, molluscs, pollen, pigments, or chironomids). One of the primary sources for recent paleolimnological research is the Journal of Paleolimnology.

Lake ontogeny
Most early paleolimnological studies focused especially on the biological productivity of lakes, and the role of internal lake processes in directing lake development. Although Einar Naumann had speculated that the productivity of lakes should gradually decrease due to leaching of catchment soils, August Thienemann suggested that the reverse process likely occurred. Early midge records seemed to support Thienemann's view.

Hutchinson & Wollack suggested that following an initial oligotrophic stage lakes would achieve and maintain a trophic equilibrium. They also stressed parallels between the early development of lake communities, and the sigmoid growth phase of animal communities - implying that the apparent early developmental processes in lakes were dominated by colonization effects, and lags due to the limited reproductive potential of the colonising organisms.

In a classic paper, Raymond Lindeman outlined a hypothetical developmental sequence, with lakes progressively developing through oligotrophic, mesotrophic, and eutrophic stages, before senescing to a dystrophic stage and filling completely with sediment. A climax forest community would eventually be established on the peaty fill of the former lake basin. These ideas were further elaborated by Ed Deevey, who suggested that lake development was dominated by a process of morphometric eutrophication. As the hypolimnion of lakes gradually filled with sediments, oxygen depletion would promote the release of iron-bound phosphorus to the overlying water. This process of internal fertilization would stimulate biological productivity, further accelerating the in-filling process.

Deevey and Lindemann's ideas were widely, if uncritically, accepted. Although these ideas are still widely held by some limnologists, they were effectively refuted in 1957 by Deevey's student Daniel A. Livingstone. Mel Whiteside also criticized Deevey and Lindemann's proposal, and palaeolimnologists now consider that a host of external  factors are equally or more important as regulators of lake development and productivity. Indeed late-glacial climatic oscillations (e.g., the Younger Dryas) appear to have been accompanied by parallel changes in productivity, illustrating that 1) lake development is not a unidirectional process, and 2) climatic change can have a profound effect on lake communities.

Anthropogenic eutrophication, acidification and climatic change
Interest in paleolimnology eventually shifted from esoteric questions of lake ontogeny to applied investigations of human impact. Torgny Wiederholm and Bill Warwick, for example, used chironomid fossils to assess the impacts of increased nutrient loading (anthropogenic eutrophication) on lake communities. Their studies reveal pronounced changes in the bottom fauna of North American and European lakes, a consequence of severe oxygen depletion in the eutrophied lakes.

From 1980 to 1990 the primary focus of paleolimnologists efforts shifted to understanding the role of human impacts (acid rain) versus natural processes (e.g., soil leaching) as drivers of pH change in northern lakes. The pH-sensitivity of diatom communities had been recognised since at least the 1930s, when Friedrich Hustedt developed a classification for diatoms, based on their apparent pH preferences. Gunnar Nygaard subsequently developed a series of diatom pH indices. By calibrating these indices to pH, Jouko Meriläinen introduced the first diatom-pH transfer function. Using diatom and chrysophyte fossil records, research groups led by Rick Battarbee (UK), Ingemar Renberg (Sweden), Don Charles (US), John Kingston (US), and John Smol (Canada) were able to clearly demonstrate that many northern lakes had rapidly acidified, in parallel with increased industry and emissions. Although lakes also showed a tendency to acidify slightly during the their early (late-glacial) history, the pH of most lakes had remained stable for several thousand years prior to their recent, anthropogenic acidification.

In recent years palaeolimnologists have recognised that climate is a dominant force in aquatic ecosystem processes, and have begun to use lacustrine records to reconstruct paleoclimates. Sensitive records of climate change have been developed from a variety of indicators including, for example, paleotemperature reconstructions derived from chironomid fossils, and palaeosalinity records inferred from diatoms.