In A Nutshell
The world’s wetlands are ecosystems in themselves, and are defined by the flora and fauna they support. Marshes are nutrient-rich wetlands that support a variety of reeds and grasses, while swamps are defined by their ability to support woody plants and trees. Bogs are characterized by their poor soil and high peat content, while fens have less peat and more plant life than a bog.
The Whole Bushel
One of the defining characteristics of a marsh is that they are consistently flooded with water from one source or another. Many marshes are freshwater, and exist in areas with poor drainage—along streambeds, lakes, and ponds. Because the soil is consistently wet from flooding, it is also extremely nutrient-rich, and can support a wide variety of plant and animal life. Marshes may also be tidal; saltwater marshes can be found along the oceans, and are saturated every time the tide comes in. While some marshes are also fed by groundwater, all receive most of their soil saturation from surface water like tides and rains. Some marshes, such as those that form when potholes and large depressions in the earth catch melting snow, can be temporary.
Most of the vegetation in marshes are along the lines of cattails and reeds, while swamps are defined by the trees which have adapted to live in standing water or constantly saturated dirt. This waterlogged dirt is high in nutrient content. Trees like the cypress and some varieties of maple and oak can survive in these wet areas that would rot the roots of other trees. Wetlands that support woody plants like the buttonbush or the swamp rose are considered shrub swamps. Mangroves are shrubby trees that thrive in this wet environment, doing so well that there is a sub-type of swamp called the mangrove swamp. Many swamps and marshes have been destroyed to turn their nutrient-rich soils into farmland.
While other types of wetlands are very nutrient-rich, bogs are clearly defined by their lack of nutrients and their relative inability to support large plant life. A bog is created over hundreds or thousands of years, formed when plant matter decays in a lake and fills it. This creates layers and layers of peat, which is often drained before being harvested and burned as a heat source or used as insulation. Bogs are freshwater, and in spite of the large amounts of decaying plant matter, they are very poor in nutrients because of the slow rate of decay. Most of the plant life around a bog is along the lines of fungi, mosses, and small shrubs. Many carnivorous plants, such as the pitcher plant and the sundew, have evolved in bogs to combat the low nutrient levels in the soil. Bogs are infinitely valuable in their ability to store carbon, removing this greenhouse gas from the atmosphere.
Fens are very similar to bogs, and can contain much of the same decaying plant matter and peat. The difference is how they are formed. Fens are created by a water table that is very close to the surface and keeps the ground saturated. The water level in a fen can rise and fall slightly with changes to the water table, but fens are characterized by having flowing water year around. They tend to have a higher nutrient content than bogs and can support a wider variety of plant life. However, if the decaying plant matter reaches too high a level, it can strangle the nutrient levels of the fen and turn it into a bog.
Show Me The Proof
EPA: Wetland Types
National Geographic: Bog
What is a fen?