A tidal zone may contain an estuary, where a freshwater drainage meets the ocean. An estuary is a deepwater tidal habitat and its adjacent tidal wetlands. It is usually semi-enclosed by land but with open, partly obstructed or sporadic access to the open ocean. The ocean water is at least occasionally diluted from freshwater runoff from the land and the estuarine waters form an interface between fresh and salt waters. Estuarine areas sustain a rich abundance of fish, bird, and invertebrate species, and habitats such as mangroves, seagrass beds, and saltmarshes. Estuaries provide important nursery grounds for marine fish, sustain fisheries, control flood events, provide a rich feeding ground for birds, and are a necessary component for the life-cycle of many invertebrate species.