Water-level changes result from a variety of geological, biological, and/or climatic processes. Many of these changes occur over long periods; others may be rapid or result from catastrophic events. In aquatic environments, diatoms are highly sensitive indicator organisms and their microfossils, deposited in lake sediments, can be used to infer environmental changes (Smol, 2008). Unambiguous diatom signals can be reconstructed from lakes isolated from marine or brackish waters (e.g. Fritz et al., this volume; Horton & Sawai, this volume). However, in freshwater systems lake-level changes are often recorded as increases in planktonic (free-floating) diatoms – although as discussed below, interpretation of this signal should be supported by additional evidence.
In the Laurentian Great Lakes region of North America (e.g. Finkelstein & Davis, 2006; Wolfe et al., 2000; Wolin, 1996) and the North Sea and Baltic regions of Europe (e.g. Digerfeldt, 1998), freshwater lake levels are commonly affected by geological processes of isostatic rebound, subsidence, and outlet incision following glaciation, and lake isolation by coastal-sediment transport (e.g. Karrow & Calkin, 1985; Larsen & Schaetzl, 2001; Lewis et al., 2008).
Biological processes, such as vegetation succession, can alter drainage patterns and groundwater flow, which in turn affect water levels. As vegetation develops following glaciation, surface runoff patterns change and this can moderate water levels on a seasonal or short-term basis. Natural deposition processes of internal (autochthonous) plant and animal remains, and mineral and organic inputs from the catchment (allochthonous) result in a shallower lake over time (Wetzel, 2001).