PHYSICAL AND CHEMICAL-SCALES OF PATTERN IN FRESH-WATER LAKES AND RESERVOIRS

A series of examples are assembled to illustrate the links between physical and chemical scales of pattern in the open-water environment. The chemical scales of pattern are characterised as large-scale, meso-scale and small-scale using a modification of the three-dimensional diagrams introduced by Stommel (1963). The example of large-scale pattern is taken from a survey of Loch Lomond in Scotland. This loch is oligotrophic, but land-use differences in the catchment give rise to a persistent north-south gradient in productivity. The example of meso-scale pattern is taken from a study of phytoplankton patchiness in Esthwaite Water, Cumbria. Dense blooms of cyanobacteria frequently appear in this lake and sometimes give rise to local shifts in pH and specific conductance. The example of small-scale pattern is also taken from Esthwaite Water and is illustrated with a high-resolution remote sensing image of thermal plumes in the littoral zone. The exchange of water between the littoral and the open water is generally slow, but thermal instabilities can give rise to strong 'episodic' mixing events.