The biome concept is difficult to apply to freshwater habitats (and marine environments), since biomes are a way to regionalize terrestrial life and are based on vegetation types that correspond to regional climate types. Macrophytes (plants visible to the naked eye) are relatively minor and inconspicuous parts of freshwater communities. What are commonly called biomes in the freshwater realm are the three main types of habitat recognized by scientists—lakes, streams, and wetlands.
While the influence of and the response to the regional climate is certainly important, the main environmental factors controlling the distribution of organisms living in freshwater are:
- substrate: rocky bottoms versus sandy or muddy bottoms
- depth of water
- the flow regime
- water chemistry
- water temperature, and
Life in freshwater biomes
I. Plants large enough to be seen by the unaided eye (macropytes) are generally limited to shallow, slow-moving or still water. The main growthforms are
- emergents: plants rooted in the bottom but with parts extending well above the water’s surface
- submerged aquatic plants: also rooted in the bottom, the leaves, stems, and reproductive parts of these plants are always below the water’s surface
- floating plants: these may be either rooted in the bottom but with leaves and flowers floating on the surface or free-floaters unattached to bottom materials.
II. Protista, part of the microflora, include numerous species of algae that may grow as epiphytes occurring on the submerged stems and leaves of macrophytes or as often slimy algal mats on hard surfaces on the bottom. These may accumulate in visible masses. Algae, along with cyanobacteria, also make up the phytoplankton, free-floating organisms of microscopic size that comprise the base of many aquatic food chains.
III. Animals. An abundance of insects, especially larval stages, distinguishes freshwater biomes from marine biomes. Common invertebrates include mollusks such as snails and mussels and crustaceans such as crayfish. Among vertebrates are to be found fish, amphibians (frogs and salamanders), and reptiles (snakes, turtles, and crocodilians). While many birds depend on freshwater habitats for shelter and food, none can be considered completely aquatic species. Very few mammals are confined to freshwater habitats; those that are include river dolphins and the Amazon manatee.
Time, change, and connectivity are important factors in the evolution and distribution of life forms in the freshwater biomes. Most bodies of freshwater are geologically young. (Exceptions include very ancient lakes such as Africa’s Great Lakes and Lake Baikal in Russia.) Seasonal changes, especially in the middle latitudes, in water volume and velocity are common occurrences, as are changes in area, substrate, water temperature, and/or water chemistry. Connectivity is the rule, with streams, wetlands, and lakes frequently transitioning geographically and over time into one another.
Each of the three major freshwater “biomes” will be described separately in more detail on other pages. Vernal pools will be discussed under “Wetlands” and saline bodies of water that are not oceans or seas are described under “Lakes.”