A study of six key coral species, conducted by Hennige et al. (2010), examined coral distribution along environmental gradients within the Wakatobi Marine National Park reef system. Though the total number of coral species decreased from optimal coral reef sites to marginal sites, massive corals such as Goniastrea aspera tended to dominate in marginal sites, along with the symbiotic microalgae Symbiodinium type D, indicating more resilience to varying environmental conditions. In sites with optimal reef conditions, branching corals, such as P. cylindrica, were more abundant with the type C Symbiodinium. Marginal reef habitats are predicted to expand due to climate change; therefore, the success of massive coral species in marginal conditions could decrease branching coral diversity in reefs, changing the coral composition of the Indonesian and world reef system. ¾ Rachel King
Hennige, S., Smith, D., Walsh, S., McGinley, M., Warmer, M., Suggett, D., 2010. Acclimation and adaptation of scleractinian coral communities along environmental gradients within an Indonesian reef system. Journal of Experimental and Marine Biology 391, 143–152.
The authors selected five sites to study that provided a range of environmental conditions, with habitats classified from “optimal” to “marginal”. “Optimal” sites refer to areas of high coral diversity and abundance, while “marginal” sites exhibit highly variable conditions from optimal growth sites. Temperature and light were measured over two field seasons using HOBO temperature (ºC) and light (lux) loggers that were deployed for one-week periods to obtain minima and maxima data. The lux data were also used to determine daily changes in light level between sites. The wavelength-specific light attenuation coefficient, Kd(l), was calculated from radiometer data at each site, and the average, Kd (site), was used to calculate optical depth. This data enabled sites to be ranked from optimal to marginal growth locations. Marginal sites had higher values of Kd and higher diurnal range of light and temperature, while optimal sites had low light and temperature variability and low Kd (low turbidity) values.
To determine the coral species present at each site, 50 m continuous line-intercept transects were used, but this method was modified at marginal sites to 50 x 2 belt transects. Metabolic and daily productivity assessments were performed by measuring oxygen concentration during respiration and photosynthesis. The oxygen drift per unit time was used to calculate hourly rates of respiration, net photosynthesis, and gross photosynthesis. Using a previous model, the authors calculated the maximum gross productivity, PG(D), of the corals, and they also calculated the daily respiration rate. Portions of the coral samples were used to genetically identify the Symbiodinium in the coral.
The data from the light analysis of the sites, the Kd values and optical depth, demonstrated that marginal corals experienced higher and more variable light intensities than con-specific species on the optimal reefs. The authors’ results also illustrated a decrease in the number of coral species from optimal to marginal reefs, and a change in the morphological features of the corals. Optimal reefs had a higher presence of branching corals, while the marginal reefs contained only massive coral species. The metabolic data collected showed a species-specific trend for PG(D) compared to optical depth: PG(D) either increased or remained the same with lower optical depths. The results also showed a change in Symbiodinium community structure in optimal versus marginal reefs. Type C was observed in all the optimal sites, but as the sites became more marginal, type D became increasingly prominent until it was the only Symbiodinium identified at the most marginal site. G. aspera was the only coral reported to have different Symbiodinium clades at different sites.
The authors propose several reasons for the dominance of massive corals at marginal sites. Massive corals are more stable than branching species, and some massive corals such as G. aspera have certain adaptations, such as heat shock proteins, to help protect themselves from marginal environments. These advantages, as well as the presence of type D Symbiodinium, a more resilient symbiont, all contribute to the prevalence of massive corals at marginal sites. The higher abundance of branching corals at optimal sites was attributed to the coral’s high reproductive rate and fast growth rate, so they can out compete massive corals under certain conditions. However, this study explains that if there is an increase in the amount of marginal coral locations, the massive corals that are more suited to tolerate a widely varying environment will probably dominate. This would result in a decrease in branching coral diversity, affecting reef biodiversity and ecosystem services and reef metabolism and accretion rates. Studying current marginal locations provides a good opportunity to examine possible future coral reef community structures in the face of climate change.