Effects of Differential Fish Grazing on the Community Structure of an Intertidal Reef Flat at Enewetak Atoll, Marshall Islands

Abstract

Miller AC.1982. Effects of differential fish grazing on the community structure of an intertidal reef flat at Enewetak Atoll, Marshall Islands. Pac Sci 36(4): 467-482. The high and middle intertidal zones of the northeastern portion ("barren") of the limestone bench on the windward side of Enewetak Island, Enewetak Atoll, Marshall Islands, have a significantly higher number of herbivorous fishes grazing at high tide than the same intertidal zones of the reef flat 300 m to the southwest ("algae-covered"). This portion of the reef flat in the barren, heavily grazed area has a significantly lower coverage by erect, macroscopic algae and a lower algal biomass than the same portion of the reef flat in the algae-covered area. The removal of part of the limestone substratum by the grazing fishes as they feed and the reduced coverage by erect, macroscopic algae result in a lower topographic relief in the barren area than found in the algaecovered area. The heavily grazed area has a significantly lower number of mobile epifaunal invertebrate species and individuals per square meter than the lightly grazed area. Differences in infauna (sipunculans, polychaete worms, and tanaid crustaceans) are not so clear. When portions of the barren area were excluded from fish grazing activity for three months, the substratum under the exclosures had 100%coverage by an algal mat; the density of mobile invertebrate epifauna was an order of magnitude higher than in quadrats outside the exclosures. Although the high and middle intertidal community is subjected to apparently severe physical stresses (desiccation, insolation, wave shock, ultraviolet radiation, and osmotic stress from evaporation in the tide pools and rainfall), it appears to be principally structured by the grazing activities of herbivorous fishes. The high level of grazing in the barren area results in coverage by filamentous blue-green algae and a diatom-bacterial film, which may be a nutritionally more important food source to the fishes than the coralline algae in the algae-covered area.