Figure 13.13: The three primary zones of a lake are the littoral, open-water, and deep-water
zones. ( 12 )
Wetlands
The word wetlandis well-named. It refers to land that holds a great deal of water for
significant periods of time, and that contains specialized plants able to grow in these wet
conditions. Wetlands are created where bodies of water and bodies of land meet. They
can be large flat areas or relatively small and steep areas. Wetlands tend to create unique
ecosystems that rely on both the land and the water for survival. Wetlands are important
regions of biological diversity, yet they can also be fragile systems that are sensitive to the
amounts and quality of water.
Types of Wetlands
A marsh is a type of wetland usually around lakes, ponds, streams, or the ocean where
grasses and reeds are common but trees are not (Figure13.14). Animals present in marshes
usually include frogs, turtles, muskrats, and many varieties of birds. The water in a marsh
is generally shallow and may be either freshwater or saltwater.
A swamp is a wetland characterized by lush trees and vines in a low-lying area beside slow-
moving rivers (Figure13.15). Like marshes, they are frequently or always inundated with
water. Sincethewaterinaswampmovesslowly, oxygeninthewaterisoftenscarce, soplants
and animals must be adapted for these low-oxygen conditions. Swamps can be freshwater,
saltwater, or a mixture of both.
An estuary is an area where saltwater from the sea mixes with freshwater from a stream or
river (Figure13.16). These semi-enclosed areas are home to plants and animals that can
tolerate the sharp changes in salt content that the constant motion and mixing of waters
creates. Estuaries contain brackish water, which has more salt than freshwater but less than
sea water. Because estuaries contain areas of water with many different levels of dissolved
salt, they tend to have many different habitats for plants and animals. As a result, estuaries