Influx of large volumes of river water into the lagoon may lower the salinity. Abnormally high or low salinity will have an effect on the sediment-forming organisms present in the lagoon. Barrier reefs occur on a variety of scales. On a smaller scale, patch reefs often occur in the lagoon behind a barrier reef. Pinnacle reefs may also occur on the fore-reef side.
Atolls are areas of shallow water carbonate deposition in the open ocean.
Most have a volcanic core. An essential feature is the central lagoon. The diagram below shows the evolution from fringing reef to barrier reef to atoll with time on oceanic volcanic islands. This is Charles Darwin's model, based on his experiences on the voyages of the Beagle . The oceanic lithospheric plate subsides as it moves away from the ridge and cools.