The isthmus of A Lanzada, while joining O Grove with the continent, is also the meeting point of two ecosystems with a rich marine biodiversity. In open sea the protected space embraces the archipelago of the Ons in a great area of sea water (See National Park). Cetaceans like the porpoise, the dolphin and the bottle nose dolphin can all be found in this area. On the inner side, there is a wide plain which is subject to the tides and which on some maps appears with the name of O Vao cove, and which includes the island of A Toxa and the famous beaches of O Grove. The inter-tidal complex is at the mouth of the distinctive river Umia, a vast area of mud, sandy bottoms and fields of marine vegetation, caused by the waters’ low tide.
The Spanish Society of Ornithology (SEO/Birdlife) regards the region as one of the most important winter migration areas and routes for waterfowl on Spain’s north coast. The number of over-wintering waterfowl has been estimated at fifteen thousand, including an average of almost four thousand anatidae and ten thousand limicolae.
The protected space also includes the coastal stretch of the O Grove peninsula, between Reboredo and San Vicente do Mar. The Bodeira lagoon, unique in this coastal region of the southern coast of Galicia, is located here. Here also, one can ascend the Siradella mountain, the highest point, where from its modest 159 metres of height one can enjoy the best views of this first-class nature complex.
In the estuary’s interior the route over the Carreirón point, in the peace of A Illa de Arousa, makes for a pleasant walk.