In addition to deep versus shallow, waterbodies can also be compared and contrasted as naturally occurring or as the result of an artificial impoundment or reservoir. While there are a wide variety of natural lakes -from the glacial lakes of northern regions, to oxbow lakes adjacent to rivers, to coastal lakes that can be connected to the ocean – most of these natural systems have a number of common characteristics. Some of these include variable nutrient and sediment loading (from low to high, depending on the nature of the watershed) and low to moderate watershed-to-lake area ratios. In addition, natural waterbodies tend to have distinct and sometimes extensive littoral zone fringe habitat along the shoreline. Littoral habitat is the interface between the land and the open waters of a lake. Typically, rooted aquatic macrophytes (plants and mat algae) are found in the littoral zone, along with a number of aquatic organisms that use this habitat for food and/or cover. Thus, the littoral zone of lake is frequently the most productive areas of this ecosystem.
Graphic adapted from www.cues.cfans_umin.edu
In contrast, large artificial impoundments, frequently called reservoirs, are waterbodies typically created by placing a dam across a stream or river (see below). This often results in the triangular shape of a reservoir; the deepest portion is located just behind the dam. Unlike many natural lakes that have a number of small inlet or inflow streams, a reservoir typically has one main inflow, which is essentially the river or stream that was originally dammed. Traveling upgradient from the dam towards the main inlet, water depth will decline. Additionally, many reservoirs are a type of hybrid of natural lakes and rivers. The upgradient/inflow part of the reservoir functions more like a riverine system, while the main body of the reservoir near the dam functions more like a lake (see below).
Graphic adapted from Reservoir Limnology: Ecological Perspectives, edited by K.W. Thornton, B.L. Kimmel and F.E. Payne, 1990
Since reservoirs are essentially dammed rivers, they tend to have very large watershed-to -lake area ratios, which means they tend to experience substantially higher nutrient and sediment loads compared to natural lakes. Thus, the level of productivity (algae growth) in the open waters of a reservoir is substantially higher than those of a natural lake. This means reservoirs have the tendency to experience larger and more frequent algal blooms. High rates of sediment loads also means rates of sedimentation will be higher in reservoirs compared to natural lakes. Finally, since the water level of reservoirs are highly dependent on inflow from the main riverine source, as well as water withdrawals in the case in drinking water supplies, the establishment of a littoral zone in reservoirs tends to be very limited.
In summary, a reservoir of comparable size to a natural lake will typically have a higher level of algal productivity, higher rates of sedimentation, and a smaller amount of biological diversity (with the general absence of a littoral zone). Thus, water quality problems can be larger and more frequent in reservoirs when compared to many natural lakes. Since many reservoirs are vital sources of potable water for millions of people throughout the United States, the general management activities for a reservoir tends to be higher relative to many natural lakes.
Join us next time, when we will discuss lake and pond productivity, the role the watershed plays in productivity, and how this impacts their recreational, potable and ecological value.