Patterns of algal overgrowth on scleractinian corals in light of environmental and biological parameters around Dahab, Egypt

Steve Gerber

Both algal cover and tourism pressures have been steadily increasing in Dahab, Egypt, during the last decade. Therefore, patterns of algal overgrowth on corals were investigated in relation to environmental parameters, including sewage input and number of visitors. Sewage input exhibited a mild effect on the presence of overgrowth, with a higher occurrence at sites with lower sewage input. However, estimations on sewage input relied largely on visual estimates due to a general lack of information and may not have been representative in retrospect. Overall, the shape of the coral colony, i.e. the growth form, was the most important variable in explaining the presence of overgrowth as well as the corresponding contributions of algal functional groups. Branching corals were overgrown more often, potentially explained by a reduction of water flow and inaccessibility to larger herbivores within the structure, both benefitting algae. The findings on growth form contradict previous studies, hence further work is needed investigating the persistence of this pattern in the reefs around Dahab. As no repeated measures of coral-algae interactions were conducted, competitive outcomes of the observed interactions are not known and therefore, conclusions related to ongoing ecological changes, such as phase shifts, cannot be made.