the freezing mark. Thus, as lake waters move toward freezing, the water sinks to
the lake bottom when it reaches 4°C (water is most dense at 4°C). Colder water
remains above, perhaps eventually becoming capped by an ice layer, which
further prevents the winds from stirring the water mass. With spring, the surface
ice begins to melt, and cold surface waters warm until they reach the
temperatures of the bottom waters, again producing a fairly uniform temperature
distribution throughout the lake. When this occurs, winds blowing over the lake
again set up a full circulation system known as spring turnover.
Figure 4.11 Seasonal lake turnoverWETLANDS
Wetlands are areas that are covered with water at some point in a year and that
support aquatic plants. High plant productivity supports a rich diversity of
animal life. The water found in wetlands can be saltwater, freshwater, or
brackish (water that has more salinity than freshwater but not as much as
seawater).
Wetlands are characterized by:
■ A water table that stands at or near the land surface for a long enough
season each year to support aquatic plants ■ Shallow or standing water
with emergent vegetation
■ Vegetation that is water tolerant
■ Soil that is permanently or seasonally saturated by water, resulting in
anaerobic conditions (also known as hydric soils)