The term "coral reef" invokes, for most people, a vision of clear-water tropical seas. The clear water of this vision is not necessarily true. In Fanning Island Lagoon (3°54^primeN, 159°20^primeW) extensive thickets of Acropora and muddy sediments coexist at depths of 35 ft in water so turbid that a diver is not visible for more than 10-15 ft.

Three narrow passes connect the lagoon with the open ocean. A network of linear reefs divides the lagoon into several nearly isolated ponds. Lagoon waters are turbid except for an area around 1 of the passes. Coral abundance in the turbid-water area does not differ markedly from that in the clearer water. However, the corals of the clear water are mostly massive forms, while the turbid-water corals are ramose. Lush coral growth is present along the sides of the linear reefs as well as in thickets in the interreef ponds.

Linear reefs wider than about 50 ft have medial sand areas and there is a medium- to coarse-grained sand in the ponds along the reef edges. However, the major sediment in the lagoon is medium silt. The sediment particles are the result of physical and biologic abrasion of corals, mollusks, and calcareous red algae. Unlike many lagoons, Foraminifera and Halimeda are not important sediment contributors in Fanning Lagoon.

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