A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetlands along with marshes, swamps, and bogs. Bogs and fens, which are both peat-forming ecosystems, are also known as mires. Fens are characterized by their unique water chemistry, which is a result of the ground or surface water input. Typically, this input results in higher mineral concentrations and a more basic pH than bogs. As peat accumulates in a fen, groundwater input can be reduced or cut off, making the fen ombrotrophic rather than minerotrophic. In this way, fens can become more acidic and transition to bogs over time.
