Abstract
One of the major threats facing coral reefs is intense benthic algal growth that can result in overgrowth and mass mortality of corals if not controlled by herbivore grazing. Unlike the well-studied coastlines of the Caribbean, there is currently a lack of knowledge regarding the effects of herbivory on benthic communities in the Red Sea. This is particularly relevant today as the local impacts in the Red Sea are increasing due to growing population and tourism. Over 4 mo, this study investigated the impact of herbivory as a potential key factor controlling algal growth on a reef flat in the Egyptian northern Red Sea. The main experiment consisted of in situ deploy- ment of exclosure cages in combination with quantification of sea urchins and herbivorous fish. When all herbivores were excluded, our findings showed a significant 17-fold increase of algal dry mass within 4 mo. Although herbivorous fish occurred in much lower abundance (0.6 ± 0.1 ind. m �2 ; mean ± SE) compared to sea urchins (3.4 ± 0.2 ind. m �2 ), they were 5-fold more efficient in reducing algal dry mass and 22-fold more efficient in reducing autotrophic production of nitrogen. A significant shift from benthic turf to macroalgae (mostly Padina sp. and Hydroclathrus clathrathus) was observed when grazers were excluded. These algae may serve as early warning indicators for overfishing. Findings suggest that herbivorous fish act as an important top-down factor controlling both benthic algal biomass and composition at the study location. Results also indicate the potential of rapid benthic community change at the study site if herbivory is impeded.
Highlights
Coral reefs belong to the most complex and diverse marine ecosystems in the world
Over 4 mo, this study investigated the impact of herbivory as a potential key factor controlling algal growth on a reef flat in the Egyptian northern Red Sea
Algal development and occurrence in healthy coral reefs is usually impeded by low nutrient concentrations and the activity of herbivores (Burkepile & Hay 2006) that are beneficial to corals by controlling algae or promoting coral recruitment (Hughes et al 2010)
Summary
Coral reefs belong to the most complex and diverse marine ecosystems in the world. The main reef ecosystem engineers are scleractinian corals which provide habitats for associated organisms and generate and transform inorganic and organic materials (Wild et al 2011). Scleractinian corals stand in direct competition with algae for space and light. Algal development and occurrence in healthy coral reefs is usually impeded by low nutrient concentrations and the activity of herbivores (Burkepile & Hay 2006) that are beneficial to corals by controlling algae or promoting coral recruitment (Hughes et al 2010). Fish and echinoids are usually the most abundant herbivores (Jennings & Polunin 1996). The most important herbivorous fish groups in Red Sea coral reefs contain Siganidae (rabbit fish), Chaetodontidae (butterfly fish), Balistidae (trigger fish), Acanthuridae (surgeon fish), and Scaridae (parrot fish) (Vine 1974).
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