Effects of Herbivore Exclusion and Degree of Bleaching on Coral-algal Community Dynamics

Abstract

In recent years, the disturbances and coral bleaching frequently occur and directly influence benthic communities shifting coral to algal dominated communities. Overfishing has been known as the one of factors driving coral-algal phase shifts. The aim of this study was to investigate the effect of herbivory and degree of bleaching on coral-algal community dynamics. These effects were experimentally tested at the subtidal reef crest at Koh Taen, Mu Ko Thale Tai National Park, Nakhon Si Thammarat Province, Gulf of Thailand. To test the effect of herbivore, cages (cage size was 25 × 25 × 25 cm3 with 2 × 2 cm2) were used to exclude herbivore. There were three categories; fully cage, partial cage, and no-cage. To investigate the effect of degree of bleaching, patches; healthy coral (initially having 100% cover of live coral), natural condition (initially having 10% cover of live coral), and cleared dead coral patches (initially having 100% of dead corals). Percentage covers of corals and algae and succession were monitored for a year. In this study, twenty six algal species were found with three dominant groups: red turf algae, Padina, and Lobophora variegata. Red turf algae were dominated group and persisted for a year. It might be because red turf algae have a fast growing ability and rapid colonization on cleared spaces. In addition, red turf algae have a high competitive ability that might reduce growth rate and suppress the recruitment of the later species. For the effect of herbivore, there was significant difference among herbivore excluded treatments (p <0.05). Fan shaped Padina was dominant inside the cages while Vaughaniella stage of Padina had a low cover in uncaged patches. Decreasing in coral percentage cover was showed in full caged of healthy coral and natural patches, however, there was no significance among caged treatments (p>0.05). Algal abundance was not apparently affected by herbivory. This result might be because of the territorial aggressive behaviour of damselfish excluding other herbivorous fishes from their territories and maintaining dense of turf algae. There was coral juvenile settlement after one month of clearing and died after one month of settlement. Algal colonization can contribute to the failure, delay, and inhibit corals to recover from the disturbances. So, herbivory and degree of bleaching can influence coral-algal community structure.