The following is an exerpt from, and used with the permission, of:  The Associations of Summer Villages of Alberta - Lake Stewardship Reference Guide - 2006 Edition. Please refer to the links page for a full PDF document.

What is a Lake?

A lake is a body of standing water entirely surrounded by land, with no sustained directional flow detectable to the naked eye. Within its watershed, a lake is often the largest collection point for surface water from the surrounding drainage area. In general, a lake has sufficient depth that light does not penetrate all the way to the bottom in the deepest parts of the lake, and often separates into three distinct layers of water during the summer.

Alberta’s lakes were formed 10,000 to 20,000 years ago when the retreating glaciers formed lake basins by gouging holes in bedrock or loose (glacial) till, or by leaving buried chunks of ice whose melting shaped and filled lake basins with water.

More recently, humans have created lakes and reservoirs by damming rivers and streams.

 Lake Zones

A typical lake has distinct zones of biological communities linked to the physical structure of the lake.

The Littoral Zone

The littoral zone is the shallow near-shore area where sunlight penetrates all the way to the sediment, allowing large aquatic plants (macrophytes) to grow. This is a highly productive area within a lake. The plants in this zone provide food and habitat for fish and other organisms, and protect shores from wave action that may cause erosion.

The Limnetic Zone

The limnetic (or pelagic) zone is the well-mixed surface water layer in offshore areas, beyond the influence of the shoreline. Within this open water area you have the photic (or euphotic) zone of the lake, which is the layer from the surface down to the depth where light levels become too low for photosynthesis to occur. The profundal (or aphotic) zone is also located within the open water area of the limnetic zone, and is that area deep within a lake where light levels are too low for photosynthesis to occur.

The limnetic zone is a very productive region of the lake and is dominated by free-floating microscopic plants and animals (e.g., planktonic algae, cyanobacteria, phytoplankton and zooplankton) suspended in the water.

The Benthic Zone

 The benthic zone is composed of the bottom sediment of the entire lake basin. In shallower areas it is abundant with organisms that are collectively referred to as benthos. In the deepest regions of the lake, the sediment supports a large population of bacteria that break down organic matter and release inorganic nutrients back into the lake. This nutrient rich area is where oxygen is in limited supply, and where available oxygen is quickly consumed.

Water Clarity

 Water clarity is an important area of concern for people living at, or visiting lakes. They want to know how “green” the lake is and how the water quality compares to other lakes. The abundance of phytoplankton in the lake is one measure of general lake productivity. High numbers of phytoplankton indicate high lake productivity. Phytoplankton can be found in the euphotic zone. The depth of this zone increases throughout summer as the sun’s rays penetrate further into the water, allowing a greater area for photosynthesis and therefore phytoplankton growth. A device called a Secchi disk can be used to measure the depth of the euphotic zone and give an indication of lake productivity. For information on using a Secchi disk, contact the Alberta Lake Management Society (ALMS), or visit their web-site at: http://www.alms. ca. Lake productivity is discussed further under the Eutrophication heading of this section.

The Water Cycle: Water Comes and Water Goes

 Water continually cycles through our environment. Water evaporates from soils, vegetation, lakes and other bodies of water; accumulates as water vapour in clouds; returns to the Earth, oceans and other bodies of water as rain and snow; and runs off as river flow, or through the soil and aquifers as groundwater flow, back into lakes and other bodies of water.

Residents are often concerned with the water level at the lake, as it can affect their enjoyment of the lake, the amount of available shore area, the potential for flooding and erosion, and their perceptions of water quality in general.

Water levels within a lake are a function of the amount of water received from the watershed through precipitation and inflows (inputs), and from what is lost through evaporation and outflows (outputs). Most of the water is lost through evaporation from the surface of the lake. In north central Alberta, the average annual evaporation is about 640mm, or just over two feet of lake depth. Normally this is offset by precipitation, groundwater, and other inputs.

Water levels are also affected by changes in climate. In the absence of inputs (e.g., during a drought), water levels can recede noticeably within a short time simply through evaporation. Phytoplankton (algae and cyanobacteria) and zooplankton in lake water.

Source: studenten/bilder/23.jpg 

Association of Summer Villages of Alberta – Lake Stewardship Reference Guide 75

Water also cycles within the lake. Under normal climatic conditions, water entering the lake will eventually replace water leaving the lake. The average time required to completely replace the total volume of water within a lake is called the residence time (or renewal time). Residence time may be short in lakes with large watersheds, as the volume of inflow is high. In contrast, lakes with small watersheds may have long residence times, due to smaller inflows requiring greater time to replenish the lakes’ volume. Most lakes in Alberta have a water residence time greater than 50 years. This is important, because surface runoff from within the watershed carries sediment, nutrients and pollutants into the lake.

What goes into a lake generally stays in the lake.

Eutrophication: The Life Cycle and Aging Process of a Lake

Like all living things, lakes age with time. The process by which lakes gradually age and become more biologically productive is called eutrophication. This is a natural process by which a lake fills in over geologic time, with erosional materials carried in by streams and overland flows, materials deposited directly from the atmosphere, and materials produced within the lake itself.

The aging, or filling-in process, begins from the time that a lake is created. Wind and water move soils from the watershed into a lake, which then settle on the bottom. Large plants and phytoplankton grow seasonally, die off, settle and decompose on the lake’s bottom. With time, these processes cause a lake to become increasingly shallow. The natural succession is from lake to pond, pond to marsh, marsh to wet meadow, and wet meadow to dry land. This natural process can take thousands of years.

Human activities, however, can dramatically change lakes, for better or worse, in a much shorter time. This accelerated transition is called cultural eutrophication. Land use changes can result in significant changes in nutrient runoff. Nutrients from agricultural areas, stormwater runoff, urban development, fertilized yards and gardens, failing septic systems, land clearing, shoreline modification, municipal and industrial wastewater, runoff from construction projects, and recreational activities all contribute to accelerated enrichment (i.e. increased plant and phytoplankton growth) and thus, eutrophication, of the lake.

Association of Summer Villages of Alberta – Lake Stewardship Reference Guide 75

Photo by Bill Benford