Coral and algal changes after the 1998 coral bleaching: interaction with reef management and herbivores on Kenyan reefs

Interaction between the El Niño and Indian Ocean dipole ocean–atmosphere quasi-periodic oscillations produced one of the warmest seawater temperatures on record in 1998. During the warm northeast monsoon in March and April, Kenya's shallow coral reefs experienced water temperatures between 30 and 31 °C and low winds. This caused large-scale bleaching of hard and soft corals at the end of March, which extended into the cooler months of May and June. Direct observations of coloration in the Mombasa Marine National Park found that the coral genera Acropora, Millepora, Pocillopora, branching Porites and Stylophora showed rapid bleaching and high mortality by the end of May 1998. Other hard coral genera that bleached significantly included Echinopora, Favia, Favites, Galaxea, Hydnophora, Goniopora, Leptoria, Montipora, Platygyra and massive Porites, but mortality was variable among these genera. Astreopora, Coscinarea, Cyphastrea and Pavona were the least responsive genera, with some paling, but little evidence of full bleaching or significant mortality. We compared changes in reef ecology in four national parks (protected from fishing) with four non-park areas (heavy fishing) to determine how coral mortality and herbivory interact under the two management regimes. Benthic studies using line transects in 16 sites spread across ~150 km of coastline were completed before and 6 to 13 months after the bleaching event and found that the cover of nine hard coral genera including Acropora, Alveopora, Favites, Goniopora, Platygyra, Pocillopora, branching Porites, Stylophora and Tubipora decreased significantly (p<0.04) after the event, usually by >85%, and soft coral cover decreased by ~75%. One year after the bleaching, sites in the national parks experienced 88 and 115% increases in turf and fleshy algal cover, respectively, while reefs outside the parks had a 220% increase in fleshy algal cover with no appreciable change in turf-forming algal cover. There was, however, high spatial variation and no statistically significant difference in the change in fleshy algal cover between sites in and out of the national parks. Estimates of herbivory by both fish and sea urchins at each site were a good predictor of the change in fleshy algal cover over the 1-year post-bleaching period in about half of the sites. The other half of the sites did not exhibit large changes in fleshy algal cover, and algal cover at these sites was not clearly influenced by herbivory. The number of coral genera per transect at each site decreased significantly by 31 and 44% in and out of the national parks respectively. Larger-scale search sampling to determine the presence/absence of genera in study sites found consistent losses of coral genera from sites, but produced smaller differences than the line-transect method, suggesting that some genera persisted, but at very low population densities and small colony sizes.