Herbivory effects on benthic algal composition and growth on a coral reef flat in the Egyptian Red Sea

TLDR: Investigation of the impact of herbivory as a potential key factor controlling algal growth on a reef flat in the Egyptian northern Red Sea suggests that herbivorous fish act as an important top-down factor controlling both benthic algal biomass and composition at the study location.

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.