Space competition between coral and algae — effect of two functional groups of algae on juvenile Acropora corals

Abstract

Overharvesting of herbivorous fishes is assumed to be one of the causes for phase shift on coral reefs from coral- to macroalgal-dominated communities by reducing inhibitor of algal growth. In order to reveal the effect of herbivorous fishes on algae and juvenile acroporid corals, field experiment was conducted in Okinawa, southern Japan. Grazer-exclusion cages were established where small (2 cm in length) coral branchlets of Acropora tenuis were transplanted both inside and outside the cages. During the exclusion experiment, algal biomass, survival and growth of the transplanted corals were monitored. The cages effectively excluded herbivorous fishes that resulted in significantly greater algal biomass inside the cages than outside. While algal biomass continued to increase within the cages, algal species composition has changed drastically at the middle of the experimental period. During the first half period (3 months) when encrusting turf algae covered substrate adjacent to coral branchlets inside the cages, coral branchlets could not expand their attachment area on the substrata. In contrast, during the second half period (3 months) corals started rapid growth after turf algae disappeared and frondose macroalgae dominated. We conclude that turf algae may prevent the growth of juvenile acroporid corals especially in the early stages of horizontal expansion prior to the vertical growth.