Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 111

worldwide, but some of the largest, deepest, and oldest lakes are formed
tectonically. Fluvial lakes can be very important to riverine ecology.
2. Lake basin morphology is described with various parameters,
including mean depth, area, maximum depth, volume, shoreline
development ( D L ), and watershed area relative to lake surface area.
Shallow lakes with large watersheds and highly dissected shorelines
are generally the most productive.
3. Waves are greatest where the wind has the longest length of lake
(fetch) to act on.
4. Wind causes Langmuir circulation patterns, which lead to streaks of
floating material on the water surface but also mix the lake to depth.
5. Stratification can alter the water circulation in lakes and thus alter
biogeochemical, ecosystem, and community properties. Mixing can
occur often (polymictic), once a year (monomictic), twice a year
(dimictic), or rarely (amictic or meromictic), depending on climate and
type of stratification.
6. Thermal stratification occurs when warm surface water sits above
denser, cooler waters. The warm surface layer of a thermally stratified
lake is the epilimnion, the zone of steep temperature transition is the
metalimnion, and the deepest stable zone is the hypolimnion.
7. High concentrations of dissolved substances can also lead to stratified
layers in lakes. Such chemically driven stability can exceed
temperature-driven stability because density differences can be greater
than are possible with natural temperature differences.
8. A sustained wind that suddenly stops can cause oscillation of the lake
surface (an external seiche) or the hypolimnion (internal seiche). This
rocking can lead to breakdown of stratification. Mixing of deeper
waters into the surface is called entrainment.
QUESTIONS FOR THOUGHT
1. Why is it sometimes difficult to assign a single geological explanation
for a lake's origin?
2. Why is a lake with a high value for D L likely to have smaller waves
than a lake of comparable surface area with a D L close to 1?
3. Why do more lakes occur farther from the equator?
4. In which order (from greatest to least) should lakes be ranked with
respect to the ratio of maximum depth divided by the mean depth:
tectonic, glacial, and fluvial?
5. Langmuir circulation cells concentrate particles slightly more dense
than water below the surface of the water: Where will these particles
be concentrated and why?
6. Under what conditions would thermal stratification lead to anoxia in
the hypolimnion?
7. Explain why some rivers flowing into lakes flow down into the
hypolimnion, some flow across the surface, and others flow into the
metalimnion.