Effects of physical and biological disturbances on algal turfs in Kaneohe Bay, Hawaii.

Disturbance in coral reef environments commonly results in an algal community dominated by highly productive, small filamentous forms and cyanobacteria, collectively known as algal turf. Research on the types of disturbance responsible for this community structure has concentrated mainly on biological disturbance in the form of grazing, although physical and other forms of biological disturbances may be important in many coral reef areas. On the reef flat in Kaneohe Bay, Oahu, Hawaii, algal turfs grow primarily upon coral rubble that tumbles with passing swells. We manipulated the frequency of rubble tumbling in field experiments to mimic the effects of physical disturbance by abrasion and light reduction on algal biomass, canopy height, and community structure. Treatments approximated a gradient of disturbance intensities and durations that occur on the reef flat. Although sea urchins and herbivorous fishes are not widespread and abundant on the reef flat, biological disturbances to algal turf communities in the form of herbivory by small crabs and abrasion by tough macroalgae contributed significantly to the variation in algal turf biomass. Within all experiments increasing disturbance significantly reduced algal biomass and canopy heights and the community structure shifted to more disturbance-tolerant algal forms. This study shows that the chronic physical disturbances from water motion and biological disturbances other than grazing from large herbivores can control algal communities in coral reef environments.