As mentioned above, the overall biodiversity rating is good for the coastal zone of mid Norway according the Nature Index of Norway (Gundersen et al., Expansion of kelp forest and associated species has led to an increase in the index, however a decline in coastal populations of (e.g. coastal cod), mammals (e.g. grey seal, Halichoerus grypus) and birds (e.g. common eider), has led to an index decrease (Gundersen et al., The mean depth of the Baltic Sea is 55 meters and the deepest parts are approximately 400 meters. A strong salinity gradient effects both biodiversity and ecosystem function.