Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 110

Wind stops

A D

Epilimnon

Hypolimnion

Wind

Hypolimnion begins to rock

E

B

Hypolimnion rocks to opposite extreme,
then returns to original position

Wind

C

F

Mixing zones

Seiche continues but slowly dissipates

FIGURE 6.16 Formation of an internal seiche and entrainment associated with wind.
Dashed arrows show water flow. (A) The lake under calm conditions; (B) the wind deepens
the epilimnion on the right; (C) a strong wind mixes some of the epilimnion with the hy-
polimnion; (D) the wind stops and the hypolimnion begins to oscillate; and (E and F) the am-
plitude of the seiche diminishes over time.

surface of the lake, and the hypolimnion moves back toward its original po-
sition. Like a pendulum, the surface of the hypolimnion rocks back farther
than its original position. Thus, an internal seiche is created where the sur-
face of the lake appears still, but the plane that forms the top of the hy-
polimnion continues to oscillate for hours or days after the wind ceases. Aside
from the intrinsic elegance of the seiche as a physical phenomenon, this type
of water movement has an important biological implication.
Even though the hypolimnion is very stable, seiches can lead to a moder-
ate amount of mixing of hypolimnetic and epilimnetic water. The movement
of this water up to the epilimnion is called entrainment . Entrainment causes
nutrient-rich water from the hypolimnion to reach the epilimnion (Fig. 16.16C),
causing stimulation of primary production. Nutrient mixing can be significant
biologically because the mixing rate far exceeds the rate of molecular diffusion
that usually predominates between the hypolimnion and the epilimnion.
Seiches can also influence rooted plants and benthic invertebrates by altering
temperature and nutrient regimes. The lake model exercises discussed by Wet-
zel and Likens (1991) are highly recommended for students who want a
clearer understanding of the processes of stratification and seiches.

SUMMARY

1. A variety of processes form lake basins, including tectonic, glacial, fluvial,
volcanic, and damming processes. Glacial lakes are the most numerous