Lake morphometry, sediment mixing and the selection of sites for fine resolution palaeoecological studies

Palaeoecologists are increasingly using close interval or continuous sampling of lake sediments to construct fine resolution records of environmental change. The degree of temporal resolution sought in these studies is typically decades or years. The use of lake sediments for such fine resolution reconstructions assumes that temporal mixing of the sediments is negligible. In this paper we examine the physical and biological processes that cause sediment mixing in small lakes. We use physical models and empirical data to develop a set of heuristics relating lake depth and area to sediment mixing by slumping, waves, currents and bioturbation. We also develop a heuristic that allows information on lake depth and area to be used to determine if the sediments of a lake will likely be massive or annually laminated. In general, the degree of sediment mixing likely to occur in a small lake is directly related to the depth and surface area of the lake. However, each of the agents of mixing has a different critical relationship with lake depth and surface area. We present a series of equations and a summarizing figure that, based on lake surface area and maximum depth, allows the type of thermal stratification (polymictic, dimictic or meromictic) and the degree of sediment mixing present in the lake to be inferred.