Ecological Impact of thermal stress in reefs of Zanzibar following the 2016 elevated higher sea surface temperatures

Abstract

This study has assessed the reef community characteristics so as to describe the community change following a severe threat and adaptively manage coral reefs in response to the 2016 and future bleaching events. The study was conducted in Misali, Bawe, Chumbe, and Mnemba reefs in Zanzibar. Misali reef is located in Pemba Island, whereas the remaining three reefs are located in Unguja Island. The composition of the benthic cover within plots was assessed using the 20-m-long line intercept method. Each site had two plots at least 150 m apart. Four transects were randomly placed within each plot. A total of 63 benthic categories, summarized into 8 major groups, hard corals (49 categories), soft corals, sponges, algae (5 categories), corallimorpharians, hard substrates (4 categories), soft substrate, and others, were monitored. Hard corals were assessed at genus level to improve biodiversity change detection. Coral bleaching levels and coral size class distribution were assessed. A belt transect of 25 m × 1 m was used to sample for coral colonies larger than 10 cm in diameter, whereas sampling for corals smaller than 10 cm, which were considered as recruits, was done using six 1 m2 quadrats located along the transect, at fixed intervals of 0, 5, 10, 15, 20, and 25 m. Four transects per plot were sampled out. The results showed that prevalence of bleaching among surveyed reefs varied between 74% (Misali) and 88% (Bawe). However, the effect of bleaching to coral cover loss was more pronounced in Bawe (37%) and Chumbe reef (19%). Genus Acropora was the most affected coral with higher relative proportions of dead colonies for Misali (85%), Chumbe (63%), and Mnemba (43%) and Porites for Bawe (39%). Genus Porites at Bawe and Acropora at Misali and Mnemba have temporarily lost their dominance to Echinopora, Galaxea, and Porites, respectively. Coral diversity was highest in Misali for both adult and recruit populations and was lowest in Bawe reef. Geographical location, coral community composition, diversity, and reef topography are suggested to be among the determinants of variability in coral bleaching prevalence and impact to coral cover loss.