These coastal ecosystems are shifting mosaics of different habitats, depending on the shape of the local coast, and proportional inflow of freshwater and seawater. Combined nutrients from marine, freshwater and land-based sources support very high productivity. Transient large animals like dugongs, dolphins, turtles and shorebirds feed on abundant fish, invertebrates and plant life, and they commonly serve as sheltered nursery areas for fish. Many organisms are adapted to large variations in salinity.