Lake Variability

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 not uniform. 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 the total weight of living organisms / biomass, population numbers, and growth rates. There is a great deal of difference in all of these variables, as well as seasonal variability on the scales of minutes, hours, diel (a 24 hour period of time - day/night), seasons, decades, and geological time. Though lakes vary in many different ways they are actually highly structured, similar to a forest ecosystem (all of the interacting organisms in a defined space)  where, for example, a variety of physical variables (light, temperature, moisture) vary from the soil up through the canopy.

Lakes and Light

Perhaps the most fundamental set of properties of lakes relates to the interactions of light, temperature and wind mixing. The absorption and decreasing of light by the water column ( a section of the lake from surface to bottom) are major factors controlling temperature and potential photosynthesis. Photosynthesis provides the food that supports much of the food web. It also provides much of the dissolved oxygen in the water. Solar radiation is the major source of heat to the water column and is a major factor determining wind patterns in the lake basin and water movements.

Light intensity at the lake surface varies seasonally and with cloud cover and decreases with depth down the water column. The deeper into the water column that light can penetrate, the deeper photosynthesis can occur. Photosynthetic organisms include algae suspended in the water (phytoplankton – microscopic floating plants), algae attached to surfaces (periphyton – the green slime that attaches shoreline and bottom vegetation and the brown stuff attached to rocks), and vascular aquatic plants (macrophytes – plants like milfoil and cattails).

The rate at which light decreases with depth depends upon the amount of light-absorbing dissolved substances (mostly organic carbon compounds washed in from decomposing vegetation in the watershed) and the amount of absorption and scattering caused by suspended materials (soil particles from the watershed, algae and detritus- dead or decaying organic matter).

The percentage of the surface light absorbed or scattered in a 1 meter long vertical column of water, is called the vertical extinction coefficient – how much light decreases as it passes through the water column.

The maximum depth at which algae and macrophytes can grow is determined by light levels. Limnologists estimate this depth to be the point at which the amount of light available is reduced to 0.5%–1% of the amount of light available at the lake surface. This is called the euphotic zone.

Since photosynthesis depends fundamentally on light, significant changes in light penetration in a lake will produce a variety of direct and indirect biological and chemical effects. Significant changes in lake transparency are most often the result of human activities, usually in association with land use activities in the watershed