Paleolimnology is a relatively recent branch of biology. It is the study of fossils in lake sediments. Typically, sampling is conducted by taking a core (a long cylinder of sediment) from a series of lakes in a geographic region, and then slicing each column of sediments into several discrete chunks. Each chunk is normally dated (using Pb radiodating) and then examined for the presence of fossils. The fossils present in most sediment cores are algal (particularly diatoms), zooplanktonic (the carapace) or osseus (fish scales, bones, etc.).

The reason why this kind of research is possible is that hard structures, like a fish scale, will settle to the bottom of a lake and become part of the sediment. Since most lakes in the temperate zone thermally stratify, sediments remain unperturbed from year to year. Thus, when one examines a sediment core, annual layering analagous to tree rings is clearly apparant.

Paleolimnology has become somewhat of a sexy field right now in biology, given that the data collected can be used to construct a portrait of the biological communities of a large number of systems over a very long period of time (100+ years). Using modern statistical methods, researchers can go beyond simply describing historical trends and attempt to infer the important processes that may have changed a system from the changes in the species present or absent over time.

A good example of this latter research is the work being conducted on the effects of acid rain. Using paleolimnological approahces, researchers are able to determine at which point industrial pollution began to impact previously unperturbed sites and can even create historical profiles of water pH.