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.
\nThe high and middle intertidal zones of the northeastern portion
\n("barren") of the limestone bench on the windward side of Enewetak Island,
\nEnewetak Atoll, Marshall Islands, have a significantly higher number of herbivorous
\nfishes grazing at high tide than the same intertidal zones of the reef flat
\n300 m to the southwest ("algae-covered"). This portion of the reef flat in the
\nbarren, heavily grazed area has a significantly lower coverage by erect, macroscopic
\nalgae and a lower algal biomass than the same portion of the reef flat in the
\nalgae-covered area. The removal of part of the limestone substratum by the
\ngrazing fishes as they feed and the reduced coverage by erect, macroscopic algae
\nresult in a lower topographic relief in the barren area than found in the algaecovered
\narea. The heavily grazed area has a significantly lower number of
\nmobile epifaunal invertebrate species and individuals per square meter than the
\nlightly grazed area. Differences in infauna (sipunculans, polychaete worms, and
\ntanaid crustaceans) are not so clear.
\nWhen portions of the barren area were excluded from fish grazing activity for
\nthree months, the substratum under the exclosures had 100%coverage by an
\nalgal mat; the density of mobile invertebrate epifauna was an order of magnitude
\nhigher than in quadrats outside the exclosures.
\nAlthough the high and middle intertidal community is subjected to apparently
\nsevere physical stresses (desiccation, insolation, wave shock, ultraviolet radiation,
\nand osmotic stress from evaporation in the tide pools and rainfall), it
\nappears to be principally structured by the grazing activities of herbivorous
\nfishes. The high level of grazing in the barren area results in coverage by
\nfilamentous blue-green algae and a diatom-bacterial film, which may be a
\nnutritionally more important food source to the fishes than the coralline algae in
\nthe algae-covered area.