Visualizing Environmental Science

Aquatic Ecosystems 143

Littoral zone

Profundal

zone

Limnetic zone

Kathleen Revis/NG Image Collection

A lake is a standing-water ecosystem surrounded by

land. The littoral zone is the shallow-water area around

the lake’s edge. The limnetic zone is the open, sunlit

water away from the shore. The profundal zone, under

the limnetic zone, is below where light penetrates.

The zonation in Bear Lake, in Rocky Mountain National

Park, Colorado, is not apparent to a visitor.

WHAT A SCIENTIST SEES

Zonation in a Large Lake

the shore. The limnetic zone extends down as far as

sunlight penetrates to permit photosynthesis. The

main organisms of the limnetic zone are microscopic

plankton. Larger fishes also spend most of their time in

the limnetic zone, although they may visit the littoral

zone to feed and reproduce. The deepest zone, the pro-

fundal zone, is beneath the limnetic zone of a large lake;

smaller lakes and ponds typically lack a profundal zone.

Because light does not penetrate effectively to this

depth, plants and algae do not live there. Detritus drifts

into the profundal zone from the littoral and limnetic

zones; bacteria decompose this detritus. This marked

zonation is accentuated by thermal stratification, in

which the temperature changes sharply with depth.

Temperate lakes undergo fall and spring turnovers,

when changing surface temperatures break down the

thermal stratification and the water layers mix. In fall,

as surface water cools, its density increases, and eventu-

ally it displaces the less dense, warmer, mineral-rich wa-

ter beneath. The warmer water then rises to the surface

where it, in turn, cools and sinks. This process of cooling

and sinking continues until the lake reaches a uniform

temperature throughout. In the spring, surface ice melts

and surface water again sinks to the bottom, resulting in

a mixing of the layers. In summer, thermal stratification

occurs once again. The mixing of deeper, nutrient-rich

water with surface, nutrient-poor water during the fall

and spring turnovers brings essential nutrient minerals

to the surface and oxygenated water to the bottom.

Human effects on lakes and ponds include

eutrophication, which is nutrient enrichment of a body

of water with inorganic plant and algal nutrients like