Natural VS. Artificial Lakes

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

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

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.

Deep vs. Shallow Lakes

While natural lakes tend to be categorized based their geomorphology (e.g. glacial lakes vs. riverine-created oxbow lakes), here we compare deep versus shallow waterbodies.

The depth of a lake has a profound effect on its ecology. If a lake is deep enough, typically a mean depth of 8 to 10 feet or greater, it can thermally stratify, which means the surface waters are a lot warmer than the deep waters. If the bottom waters are “sealed off” from the atmosphere, they can’t mix with the surface waters due to temperature differences in the summer (called stratification). In turn, the bottom waters can become depleted of dissolved oxygen. This can have a potentially negative impact on deep water fishery habitat and the lake’s nutrient (generally nitrogen and phosphorus) loads. Many of our clients utilize sub-surface aeration systems to keep the bottom waters in deeper lakes oxygenated over the summer month to enhance the fishery habitat and minimize the phosphorus that “leaks” from the sediments in the absence of dissolved oxygen.

Lake Stratification

Graphic adapted from Nebraska DEQ slide found on slideshare.net

In sharp contrast to deep lakes, shallow lakes (typically mean depths less than 8 feet) can remain well-mixed and oxygenated from surface to bottom over the summer months. Thus, the depletion of dissolved oxygen is typically not a problem in many shallow lakes. However, shallower water depths result in a larger portion of the lake bottom receiving a sufficient amount of sunlight to stimulate aquatic macrophyte (aquatic plants and mat algae that initiate growth along the sediments) growth. Thus, shallow lakes, or shallow areas of deeper lakes, can experience nuisance densities of aquatic macrophytes that can negatively impact their ecological value and recreational use. Such conditions tend to be fairly common in the Northeast region of the United States since there are far more shallow lakes than deep ones.

In addition to sunlight reaching the bottom, shallow lakes – as well as deeper lakes – can also experience planktonic (freely floating in the water column) algal blooms. Essentially, the more nutrients, such as nitrogen and, in particular, phosphorus, in the water column, the more algae that can grow in the open waters. Some algae, such as blue-green algae (also called cyanobacteria) can produce nasty and unpleasant surface scums when nutrient concentrations are high. Thus, in shallow lakes there is interplay between rooted aquatic vegetation and mat algae (the macrophytes) vs. the free floating planktonic algae. The more nutrients in the water column, the more planktonic algae present; the lower the nutrient concentrations, the clearer the water and the more aquatic macrophytes. Of the two shallow lake conditions, most lake management programs lean toward the clear water condition with more macrophytes since it is easier to manage for recreational use. However, whether you have a shallow or deep lake, the foundation of any effective, long-term lake management program is to minimize its nutrient load.

Nutrient Loading in Shallow Lakes

Graphic adapted from www.ecologyandsociety.org

Check back for our coming discussion of the difference between natural lakes and artificial impoundments. For more information, please feel free to contact me at flubnow@princetonhydro.com.