Figure 13.12: The Great Lakes are the largest lakes in the world. They are found along the
border of the United States of America and Canada. ( 1 )
the last Ice Age.
Limnologyis the study of all bodies of freshwater and the organisms that live there. The
ecosystem of a lake is divided into three distinct sections (Figure13.13):
- The littoral zone, which is the sloped area closest to the edge of the water.
- The open-water zone (also called the photic or limnetic zone), where sunlight is abun-
dant. - The deep-water zone (also called the aphotic or profundal zone), where little or no
sunlight can reach.
Much life is found in the littoral zone, because sunlight allows the growth of plants on the
lake bed. These plants in turn, provide food and shelter to animals like snails, insects, and
fish. Other plants and fish such as bass and trout live in the open-water zone. The deep-
water zone does not allow for plants to grow, so fewer organisms live there. In this zone,
most organisms are scavengers like crabs and catfish, which feed on dead organisms that fall
to the bottom. Fungi and bacteria aid in the decomposition of those dead organisms, too.
Though different creatures live in the oceans, ocean waters also have these same divisions
based on sunlight with similar types of creatures that live in each of the zones.
Lakes are not always permanent features of a landscape. Someintermittent lakescome
and go with the seasons, as water levels rise and fall. Over a longer time period, lakes can
disappear when they are filled in with sediments, if the springs or streams that fill them
diminish, or if their outlets grow due to erosion. When the climate of an area alters, lakes
can either expand or shrink, and lakes may disappear altogether if precipitation significantly
diminishes.