(fig. 4) Shelf lagoons are very large and are not necessarily found in the tropical zone of 30 degrees north latitude to 30 degrees south latitude. Their barriers are quite far from the landmass to the inner and middle segments of the shelf. Shelf lagoons may have, at one time, been one of the other types of lagoons. An example of a shelf lagoon is Southern Florida. (fig. 5) Finally and most uniquely, the atoll lagoon's barriers are barrier reefs with landmass in the vicinity. There are also combination-type lagoons that are difficult to place in a single category.
The existence of barriers on at least one side of a lagoon is an essential feature. Lagoonal barriers considerably lessen oceanic currents, swell, and even wind-driven waves. Consequently, the energy of lagoons is usually low or at least lower then near by high-energy environments. Good examples of this are the back-reef lagoons; moderate to high wave and current energies occur on the opposite side of a high-energy reef complex. This complex lessens the pounding surf and large ocean swells before it reaches the lagoon. Back-reef lagoons are the highest energy form of lagoons, but are still less energetic then the near by environment.
Barriers on lagoon margins restrict water circulation patterns and, like in Florida Bay, may significantly alter lagoon salinities. Salinity is usually lower near the land and greater near the sea. Because lagoons are shallow, surface and bottom waters often mix so the salinity is the same at different depths. In humid climates, lagoon barriers permit less mixing of fresh and marine waters by confining fresh-water runoff from swamps. In arid climates, the presence of extensive barriers permits deposition of evaporites, like gypsum, by concentrating and precipitating minerals that are dissolved in seawater of normal salinities. Some lagoons may also have areas of hypersalinity, where the salt content is higher than normal seawater, due to evaporation.
