Flexible Canopies and Their Effect on Benthic Community Structure on Shallow Coral Reefs

On shallow coral reefs, arborescent taxa are increasing in abundance and produce forests with dense canopies. This study tests for the effects of such forests on understory invertebrates through analyses of macroalgae in Moorea, French Polynesia, and arborescent octocorals in St. John, US Virgin Islands. Algal and octocoral forests were quantified by the height and density of their component organisms and occlusion of down-welling irradiance. Community structure beneath the canopies was quantified from the abundance of scleractinians in Moorea, and scleractinians plus macroscopic invertebrates in St. John. In Moorea, algal forests were formed from moderately tall (4–12 cm) thalli of Turbinara ornata at 52–532 individuals m^-2 that together occluded 0–40% of down-welling light. In St. John, octocoral forests were formed from tall (20–67 cm) octocorals at 1–8 colonies m^-2 that occluded 0–67% of down-welling light. Eighteen scleractinians were found in the algal forests, and their community structure was unaffected by variation in the canopy effect. The invertebrate community in the octocoral forests included 26 scleractinians and members of at least six other phyla, and their community structure also was unaffected by variation in the canopy effects. Both types of forests created temporally heterogeneous light regimes that effected the photosynthetic induction time of Porites within algal-forests, but not within octocoral forests. Together, these results suggest that natural variation in algal- and octocoral- forests does not affect the community structure of understory invertebrates, although corals in forests of short and dense macroalgae were physiologically modified to better utilize temporally variable light regimes. Once formed, algal and octocoral forests do not appear to affect the community structure of understory invertebrates, possibly because autotrophic taxa physiologically respond to some of the modified physical conditions beneath the densest canopies.