Swamps are forested wetlands that are seasonally or permanently flooded. In North America, swamps are dominated by tree-sized plants such as bald cypress or silver maple. Bogs are wetlands that develop in relatively cool but wet climates. They tend to be acidic and, therefore, biologically unproductive. Bogs depend on nutrients obtained from the atmosphere, and are typically dominated by species of sphagnum moss.
Fens also develop in cool and wet climates, but they have a better nutrient supply than bogs.
Consequently, they are less acidic and more productive than bogs. The number of organisms supported by these factors is small and can be compared to some of the least productive terrestrial biomes, such as deserts. The lowest level of food webs in the ocean are occupied by tiny organisms known as phytoplankton. Various species of phytoplankton range in size from extremely small bacteria to larger algae that consist of a single cell and may or may not live in large colonies. The phytoplankton are grazed upon by small crustaceans known as zooplankton.