Within the backreef habitat of Moorea, French Polynesia there are disjunct distributions of two dominant macroalgal species, where a more palatable species, Sargassum pacificum, is found predominantly on the reefcrest and a less palatable species, Turbinaria ornata, dominates the backreef. This project tested the hypothesis that these distributions are a function of herbivory. The impact of herbivores was reduced in spatial refugia, where herbivore access was limited, and when preferred species were associated with algal species that are less palatable. Spatial refugia vary across this habitat but are most apparent on the reefcrest and tops of coral bommies where feeding assays indicated that both species were consumed less compared to individuals in the backreef or on the seafloor. In the backreef, Sargassum often was found in association with Turbinaria. Field experiments compared the loss of Sargassum individuals alone to treatments where Sargassum was associated with two densities of Turbinaria. Results indicated that significantly less mass was lost when in association with Turbinaria. Although Turbinaria limited herbivory on Sargassum, the hypothesis was tested that the association may result in a physiological cost for Sargassum as a result of shading by neighboring Turbinaria thalli. Results of a short-term study that quantified physiological changes in Sargassum indicated that although light was decreased in aggregations of Turbinaria, Sargassum was able to mitigate that cost by increasing its photosynthetic efficiency. These results suggest that the distribution of chemically and morphologically defended macroalgae is a function of herbivore access, limiting the distribution and abundance of some macroalgal species which has important consequences in a coral reef environment where competition for space is the primary limiting factor for benthic species.
Thesis or Dissertation
Department of Biology, CSU Northridge