Ecology of Reservoirs and Lakes

Only one natural lake occurs in the southern Appalachian region -- Mountain Lake in Virginia.

All other lakes in the region are reservoirs created by impounding flowing water. The river system changes dramatically when dammed because of changes in flow, channel geomorphology, chemistry, and biological communities.  Reservoirs are generally located in deep valleys to minimize costs associated with land purchase and maximize hydrostatic head.  In hilly or mountainous terrain, inundation of the multiple valleys formed by the lower reaches of tributaries results in dendritic reservoirs with extensive shorelines.

Dams create new ecosystems that have hybrid characteristics of lakes and rivers (Soballe and others 1992) that appear as longitudinal zones in reservoirs.

The residence time of a reservoir (the time it takes a molecule of water to pass through) is much longer than that of an equivalent length of river, but usually shorter than that of a lake.  Reservoir residence time may be only a quarter that of a similar-sized lake (Burgis and Morris 1987).

The trophic status of lakes and reservoirs typically refers to the availability of nutrients that determine primary productivity (Rodhe 1969). Trophic status varies considerably across the region due to differences in residence time, nutrient loading, water depth, watershed size, and other factors. Oligotrophic generally refers to water bodies that are poor in plant nutrients, and in which 7 to 25 grams of carbon per square meter per year are fixed by phototrophic producers, such as plants, algae, and some bacteria. In eutrophic lakes the production rate is 75 to 250 g C/m²/yr. Mesotrophic lakes have production rates between 25 and 75 g C/m²/yr.  In polluted lakes (hypereutrophic), the production rate may be 350 to 700 g C/m²/yr.  An assessment of reservoirs 500 acres or larger in the southern Appalachian region listed 38 percent as eutrophic, 46 percent as mesotrophic, and 16 percent as oligotrophic (SAMAB 1996).

Reservoirs and lakes have several important biological communities that interact in food webs. A number of aquatic ecosystem processes like nutrient cycling, energy flow, the microbial loop, and lake food webs are critical to the function of reservoirs and lakes.