There is a lot of controversy about what defines a wetland, and some places define wetlands differently from others. In the US wetlands classification system, there are four types of wetlands; marshes, swamps, bogs, and fens.
A marsh is an area of nutrient-rich, low-lying wetland dominated by grasses and herbs, where water covers the ground for long periods of time. While a swamp is characterized by large trees, saturated soil, and standing water, both wetlands contain soils with high nutrient levels and are often destroyed, to turn their nutrient-rich soil into farmland.
Bogs and Fens while also being deep-water habitats are significantly different. Bogs are wetlands dominated by the presence of peat - decayed plant matter - and poor nutrient soil. Fens have less peat and significantly higher nutrient levels. And over time, fens usually become bogs.
Marshes and swamps are sometimes interchanged as they possess a lot of similarities. They are both wetlands - ecosystems that are constantly flooded by water. Marshes and swamps can be freshwater, brackish water, or saltwater. But they are significantly different.
Marshes are defined by the herbaceous plants – plants without woody stems above-ground – that flood their surroundings. There are three types of marshes; tidal freshwater marshes, inland freshwater marshes, and tidal salt marshes.
Tidal freshwater marshes are often found near rivers and lakes. But, they are close enough to the coast to be affected by tidal flows. They are nourished by freshwater streams with low salt content. Like most marshes, grasses and other herbaceous plants are what constitute their ecosystem, and some species of fish.
Further inland, non-tidal marshes such as inland freshwater marshes can be found in the shallow water along lakes and rivers. They contain varieties of grasses, plants, and animals. Wet meadows, a wetland with soils that are saturated throughout the growing season, are the perfect ecosystem for frogs and snakes, but cannot support some types of plant. The vegetation in these marshes depends largely on water. Thus plants only grow during the biannual flooding of freshwater marshes.
All marshes receive soil saturation from rains, floods, and surface water.
In contrast, Swamps are largely dominated by trees, which are unable to grow in marshes. Swamps are either freshwater swamps or saltwater swamps. Unlike marshes, they are defined by the types of trees which populate them. Some examples are hardwood swamps, cypress swamps, and forested swamps. Many swamps are covered completely by water with only their trees sticking out.
Freshwater forest swamps can be found inland, around lakes and streams. Swamps, like all wetlands, have water-tolerant vegetation to withstand the constant flooding and rain.
Different tree types grow in freshwater swamps including cypress trees, tupelo trees, and tiny plants called 'duckweed'. Frogs and alligators may live in freshwater swamps as their bodies have adapted to the water levels and mineralized soil.
Saltwater swamps form along tropical and subtropical coastlines such as the Gulf of Mexico. Woody, mangrove trees dominate saltwater swamps. These trees have the ability to withstand the tidal flooding that would rot the roots of most trees.
Bogs and Fens are also very similar classifications of wetlands. Fens can also be referred to as bog-like wetlands. They are both peatlands, dominated by the presence of decayed plant matter. Unlike marshes and swamps, they have low-nutrient soils. They get their water through precipitation rather than from rains and floods.
The water in bogs is very acidic due to the decayed plant matter and is unable to support plant-life. Bogs can last for thousands of years and accumulate layers of peat. Fuel is often made by burning peat. This gives them a very spongy ground that can easily absorb humans and animals like quicksand.
Fens on the other hand have less acidic soil and thus can grow some plant life. Their soil has relatively higher nutrient levels than bogs. Grasses and sedges are common in fens, and this gives them the appearance of a meadow. Fens lack the sponge-like texture of bogs. Like bogs, fens could take thousands of years to form. However, if the decaying plant matter in fens becomes too much, the acidic levels will rise and reduce the nutrients in the fen and turn it into a bog.
Each wetland contributes greatly to our ecosystem. Tidal marshes are extremely important to our environment. They provide shelter for migratory birds, slow coastal erosions, and act as a buffer for storms. Many coastal communities use tidal marshes to curb the impact of storms and hurricanes. Fens are homes to many rare plants and species. According to the US forest service, 'vegetation in all wetlands plays an important role in recycling nutrients, trapping eroding soil, and filtering out polluting chemicals such as nitrates.'