Mellerstön is one of the biggest islands in the Piteå archipelago; the Site consists of part of this island and of the neighbouring Lill-Räbben island, and the shallow marine water between them. This representative example of a natural wetland complex archipelago in the Baltic Sea within the European Boreal region is characterized by rocky shores, coastal saline lagoons, intertidal marshes and a mosaic of mires and deciduous forests. The shallow marine areas, mud flats and sand banks are very important feeding, staging and breeding grounds for waterbirds. The site supports nationally-threatened species including ruddy turnstone Arenaria interpres and northern pintail Anas acuta. Endemic flora subspecies of Artemisia campestris are also present. The Site is part of a nature reserve and has local importance for various tourism and recreational activities. The main land use is nature conservation; there are no major threats to the ecological character of the Site.