Thousands of plant species live in freshwater habitats around the world: along edges, on the surface, or at the bottom of shallow lakes and ponds; in temporarily flooded low areas and meadows; at seeps and springs (cienegas) in hill or montane regions; in flowing water of streams and rivers; rooted in waterlogged soils; and along any other natural or human-produced drainage system. "Freshwater wetlands" occur from below sealevel to some very lofty alpine habitats, where water may persist throughout the year or where it can be very ephemeral. Normally we classify a freshwater wetland as a place where at least half of the species found there are truly aquatic plant species.
Many species of aquatic plants are essentially cosmopolitan, meaning that they are widely distributed around the world. Some of the widest distributions are attributable to human activities. Humans have accidentally (sometimes intentionally) transported seeds, fruits, or vegetative clones from one pond or watershed to another, but many of the cosmopolitan distributions are attributable instead to birds, particularly waterfowl, which inadvertently transport the plant propagules when lodged in their features or trapped in mud on the feet.
Characteristics of a Freshwater Environment
- Water is plentiful, at least during the growing season.
- PFD (wavelengths of sunlight used for photosynthesis) is low for submerged leaves, because light penetration through the water column is very much reduced. At the water surface there often is unobstructed full sun for a photosynthetic organ floating, and an emergent canopy may intercept high PFD.
- Concentration of carbon dioxide dissolved in water is low (higher in water strongly acidic or strongly basic than in neutral pH solutions).
- Oxygen concentration of oxygen in the water and in thick tissues of the underwater plant is low.
- Minerals and nutrients are scarce or dilute within the water medium, as compared with drier soil.
- Moving water (currents and waves) can be damaging to the organs of the plant.
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