People often visualize a lake as a uniform mass of water, almost like a full bathtub that is evenly mixed from top to bottom, side to side and front to back. In fact, lakes are extremely heterogeneous, or patchy. The physical, chemical, and biological characteristics of lakes are extremely variable. Lakes vary physically in terms of light levels, temperature, and water currents. Lakes vary chemically in terms of nutrients, major ions, and contaminants. Lakes vary biologically in terms of structure and function as well as static versus dynamic variables, such as biomass, population numbers, and growth rates. There is a great deal of spatial heterogeneity in all these variables, as well as temporal variability on the scales of minutes, hours, diel (day/night), seasons, decades, and geological time. Though lakes vary in many dimensions they are actually highly structured, similar to a forest ecosystem where, for example, a variety of physical variables (light, temperature, moisture) vary from the soil up through the canopy.