Freshwater ecosystems can harbor a great variety of animals, but some animals prefer lots of moving water while others like small ponds or marshes. The type of habitat and the kind of freshwater ecosystem animals that are found there depend mainly on the amount of water in the system and the speed at which it flows. Bubbling brooks and fast-flowing rivers are favored by certain species, lakes and slow-flowing rivers by others and swamps by yet more. The freshwater biome supported by each type of habitat is always diverse with many of animal species that interact in complicated ways.
TL;DR (Too Long; Didn't Read)
A freshwater ecosystem can range from having lots of fast-flowing water to small amounts of stagnant water, and the kinds of animals found in the system change accordingly. Fish, mammals, reptiles, birds and insects are the most conspicuous types of animals native to freshwater habitats but there are many small animals such as crustaceans and molluscs that live there as well. Some fish need lots of oxygen in the water and live in fast-flowing streams and rivers. Water-loving mammals such as beavers like smaller streams and marshy habitats. Reptiles and insects like marshes and swamps but tend to avoid large lakes. Freshwater shrimp and mussels like slow-flowing bodies of water or lakes. While freshwater ecosystems always have lots of animals, each type has its own special collection of species that feel comfortable there.
The Four Main Types of Ecosystem Containing Fresh Water
The two main variables determining the type of freshwater ecosystem are the quantity of water and whether the system is lentic (still water) or lotic (flowing water). As a result, the four types of fresh water ecosystems are characterized by lots of still water as in lakes, small amounts of still water as in ponds, large amounts of flowing water as in rivers and small amounts of flowing water as in streams. Around these main types are different kinds of wetlands found at the borders of the four types, where one type merges into another or where water accumulates or stops flowing. Each type of freshwater ecosystem supports its own collection of animals.
Fish Are Found in All Four Ecosystems
Fish are found wherever there is open water, although the species depends on the type of ecosystem. Fish such as salmon and trout need clean water with lots of oxygen, so fast flowing steams are ideal. Some trout can also accept clean lakes, but small and muddy ponds are better suited to species such as carp and catfish. Freshwater fish such as pike and sturgeon grow very large, so they need big lakes or large, slow-flowing rivers while little minnow-sized fish like the safety of shallow water with swampy water lilies or reeds.
Mammals and Reptiles Mainly Inhabit the Edges
While some mammals such as beavers and otters live mainly in the water, most mammals and reptiles have to surface or climb onto dry land to breathe, feed and reproduce. This means these animals are mainly found in ecosystems that have small bodies of water or along the shores of large lakes and rivers. Some animals, such as bears, come to streams and rivers to feed on fish while others, such as muskrats, may live their whole lives in ponds and along rivers. Frogs prefer ponds and swamps, and salamanders like wetlands. Alligators, turtles and snakes are less particular and can be found in any of the types of habitat except fast-flowing streams and rivers.
Some Birds Have Adapted to Water
Waterfowl haven't abandoned the air, but when they are not flying, they spend most of their time on and in the water. They tend to avoid fast-flowing water because it is hard to swim against strong currents, but they can be found wherever else there is fresh water. While they often feed while swimming, they have to come out of the water to build nests and hatch their eggs, mostly in reeds or grasses that grow in wetlands or stagnant water. Ducks and geese are common on lakes and rivers, but insect-eating birds such as swallows are often found near swamps and ponds because the insects there are a great source of food.
Insects Are Everywhere
Many kinds of insects, from biting ones such as mosquitoes to elegant dragonflies to bees, wasps and water striders, all live close to fresh water. Insects depend on other animals, other insects or plants for food, so they are typically found in places where animals and plants are plentiful. They tend to congregate in swamps, around ponds and along streams and rivers, but many can fly long distances, so they may fly across large lakes as well. Their favorite habitat tends to be a freshwater ecosystem that has some standing water but also some dry bits of land. These areas are often so full of insects they are characterized by a persistent hum of insect activity.
Freshwater Habitats Are Diverse
Freshwater habitats are characterized by an intermingling of water and land that gives rise to a very diverse ecosystem. In addition to major animal groups such as fish, mammals and reptiles, there are usually many other less obvious animals present. Crustaceans such as shrimp or tiny water fleas and molluscs such as mussels or snails can often be found in great numbers. The key to maintaining such ecosystems is to avoid polluting them with manmade products and to allow the natural flow of water into and out of the habitats.