A fen is one of the four main types of wetland, and is usually fed by mineral-rich surface water or groundwater. Fens have neutral or alkaline water, with relatively high dissolved mineral levels but few other plant nutrients. They are usually dominated by grasses and sedges, and typically have brown mosses in general including "Scorpidium" or "Drepanocladus". Fens often have a high diversity of other plant species including carnivorous plants such as "Pinguicula". They may also occur along large lakes and rivers where seasonal changes in water level maintain wet soils with few woody plants. The distribution of individual species of fen plants is often closely connected to water regimes and nutrient concentrations.