These regions range in size of just a few square meters to thousands of square kilometers. Scattered throughout the earth, many of the first lakes evolved during the Pleistocene Ice Age. Many ponds are seasonal, just lasting a couple of months, such as sessile pools, while lakes last many years. There is not that much diversity in species since ponds and lakes are often isolated from one another and from other water sources like rivers and oceans. Lakes and ponds are divided into three different
"zones" which are usually determined by depth and and its distance
from the shoreline.
The top most zone near the shores of the lake or pond is the littoral zone. This shallow zone is the warmest since it is the area that light hits. It contains flora such as rooted and floating aquatic plants. It also contains a very diverse community, which can include several species of algae (like diatoms), grazing snails, clams, insects, crustaceans, fishes, and amphibians. In the case of the insects, such as dragonflies and midges, only the egg and larvae stages are found in this zone. The fauna includes such species as turtles, snakes, and ducks. These feed on the vegetation and other animals in the littoral zone.
Next to the littoral
zone is the limnetic zone, which is basically the open water away from
the shore. This zone like the littoral zone is also well-lighted, which is why
it is 
dominated
by plankton, such as phytoplankton and zooplankton. Plankton are small
organisms that can feed and reproduce on their own and serve as food for small
chains. Without plankton in the water, there would not be any living organisms
in the world, including humans. A variety of freshwater fish also occupy this
zone.
The small plankton do not live for a long time. When they die, they fall into the deep-water part of the lake/pond, the profundal zone. This zone is much colder and denser than the other two zones. This zone is colder since not too much light penetrates all the way through the limnetic zone into the profundal zone. Fauna in this zone are heterotrophs, meaning that they eat dead organisms, and use oxygen for cellular respiration.
Temperature varies in
ponds and lakes during the different seasons. During the summer, the
temperature can range from 4 C near the bottom to 22 C at the top. During the
winter, the temperature at the bottom would be 4 C while the top would be 0 C
(ice). In between the two layers, there is a narrow zone called thermocline
where the temperature of the water changes rapidly. During the spring and fall
seasons, there is a mixing of the top and bottom layers, usually due to winds,
which cause a uniform temperature of around 4 C. This mixing also allows the
oxygen level to be the same through the lake. Of course this is only a
generalization since there are many lakes and ponds that do not freeze during
the winter, thus the top layer would be a little warmer.
Streams
and Rivers
These
regions are bodies of flowing water moving in one direction. They can be found
everywhere, and their exact origin
is not really known.They start off at headwaters, such as springs, snowmelt or
even lakes, then travel all the way to a mouth, which is usually a channel or
the ocean. The characteristics of a river or stream changes from the
source to the mouth. The temperature is cooler at the source than it is at the
mouth. The water is also clearer, has high oxygen levels and freshwater fish
such as trout along with heterotrophs can be found 
there.
Towards the middle part of the stream/river, the
width increases, and is less shaded. Also the greatest species diversity
along streams and
rivers, including
numerous aquatic green plants and algae, can be found around this area. 
Towards
the mouth of the river/stream, the water becomes murky from all the sediments
that it has collected from the upper parts. This decreases the
amount of light that can penetrate through the water. Since there is
less light, there is less diversity of flora, and because of the lower oxygen
levels, fish that do not need as much oxygen, such as catfish and carp,
can be found.
Wetlands
Wetlands are
areas covered with standing water that supports aquatic plants. Marshes,
swamps, and bogs are all considered wetlands. The plant species that are 
found
in wetlands have been adapted to the very moist and humid conditions are
called hydrophytes. They include pond lilies, cattails, sedges, tamarack, and
black spruce. The flora also includes such species as cypress and gum that are
found in the marshes. In terms of species diversity, the wetlands are the most
abundant of all ecosystems. Many species of amphibians, reptiles, birds, such
as ducks and waders, and furbearers can be found in the wetlands. By the way,
wetlands do not belong to freshwater ecosystems. There are other wetlands,
such as salt marshes, that have a high concentration of salt. These marshes
house different species of animals, such as shrimp and shellfish, and plants,
such as grass.
