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.