Grazing pressure on coral reefs decreases across a wide depth gradient in the Gulf of Aqaba, Red Sea

Brokovich E, Ayalon I, Einbinder S, Segev N, Shaked Y, Genin A, Kark S, Kiflawi M

Journal: MARINE ECOLOGY-PROGRESS SERIES Volume: 399 Year: 2010 Pages: 69-80

Abstract Algae communities are an essential component of any healthy coral reef and serve as the base of the food chain and as a living habitat for a myriad of organisms. Herbivorous fish play an important role in coral reef ecology by mediating competition between fast-growing algae and relatively slow-growing corals. However, because of diving-related difficulties, the importance of herbivory in deep, mesophotic coral reefs (30-150 m) remains largely unknown. Here, we examined the relationship between herbivores and algae along a depth gradient down to 65 m, and evaluated grazing pressure along the depth gradient. We visually assessed the abundance of herbivorous fishes along the depth gradient and estimated fish biomass. We measured in-situ turf algae growth rates and performed experimental assessments of grazing pressure using settlement plates and bioassays. We discovered that while both algal growth and fish density decrease with depth the decrease in grazing pressure is faster; with a consumption of less than 20% of turf-algae production at 65m vs. 40-60% at 5-20 m. Concomitantly, a decrease in fish biomass and density along the depth gradient reduced potential grazing pressure on macro-algae. Our findings suggest a less important role of herbivorous fish in maintaining algal-free substrate for coral recruitment and growth in deep zones compared with the shallow reef.

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