A nutrient-rich wetland fed by groundwater and dominated by sedges and orchids is a fen. Fens are peat-forming wetlands whose water supply comes largely from mineral-rich groundwater or surface water. This groundwater gives them higher mineral content and generally less acidic conditions than bogs.
The water table in a fen usually remains at or near the surface, leaving the soil saturated and low in oxygen. Sedges and other grass-like plants thrive in these conditions, while the relatively rich chemistry can support rare orchids and other specialized plants. Some fens are especially rich in calcium and may accumulate pale calcium-carbonate deposits called marl.
The most common mix-up is with a bog. Bogs are primarily fed by rainfall, tend to be more acidic and nutrient-poor, and often favor sphagnum moss. Marshes are usually dominated by grasses, reeds, or other herbaceous plants, while swamps are characterized mainly by woody vegetation. Over time, a fen can receive less groundwater as peat builds up, becoming more acidic and developing bog-like characteristics. Fens also store carbon and help transform or retain nutrients and pollutants in water.