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

Disturbance in coral reef environments commonly results in an algal communi ty dominated by highly productive, small filamentous forms and cyanobacteri a, collectively known as algal turf. Research on the types of disturbance r esponsible for this community structure has concentrated mainly on biologic al 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 hat in Kaneohe Bay, Oahu, Hawaii, algal turfs grow primarily upon cora l rubble that tumbles with passing swells. We manipulated the frequency of rubble tumbling in field experiments to mimic the effects of physical distu rbance by abrasion and light reduction on algal biomass, canopy height, and community structure. Treatments approximated a gradient of disturbance int ensities and durations that occur on the reef flat. Although sea urchins an d herbivorous fishes are not widespread and abundant on the reef flat, biol ogical disturbances to algal turf communities in the form of herbivory by s mall crabs and abrasion by tough macroalgae contributed significantly to th e variation in algal turf biomass. Within all experiments increasing distur bance significantly reduced algal biomass and canopy heights and the commun ity structure shifted to more disturbance-tolerant algal forms. This study shows that the chronic physical disturbances from water motion and biologic al disturbances other than grazing from large herbivores can control algal communities in coral reef environments. (C) 2000 Elsevier Science B.V. All rights reserved.