The Royal National City Park contains many wetlands that are both natural and artificial. The majority were once sea inlets which through the land rise were cut-off and made shallow.
A wetland is an area where the water level is usually just above or just below ground level. There are many types of wetland, and they can be covered in trees or completely open, natural or artificial. In a somewhat wider context, even small watercourses, dams and shallower lakes count as wetlands. Wetlands are important living environments for many plant and animal species, including many insects and other invertebrates. Wetlands are often created to support animal life or for filtering purposes by capturing both nitrogen and phosphorus and thereby reducing the eutrophication of our oceans. By conscious restoration and care of the wetlands in the park, many species have returned, such as the lapwing.
Wetlands can also be subdivided into marshes and fens which are mainly fed by surrounding groundwater and bogs which are peat-building wetlands where the dead plants do not fully decompose but survive in the form of peat, owing to the oxygen-free conditions that occur. A bog can be created over dry land or by a lake drying up. A good example of a wooded marsh or fen is Ugglevikskärret, previously an inlet of the Baltic Sea. The area was first dominated by reeds but successively became a shrub marsh with many interesting plants and animals. In general, however, all boglands have disappeared and today only survive as occasional patches.
Sometimes wetlands can occur by accident, as when Lappkärret was formed from accidental damming of the ground water when houses were built nearby. Now Lappkärret is a minor lake with a rich bird-life.