Top-down and bottom-up effects on Red Sea coral reef algae

Abstract

Overfishing and eutrophication can lead to a reduction in herbivory and an increase in algae growth, respectively. Therefore, they are among the most significant local pressures on coral reefs. In particular Red Sea coral reefs, with their high degree of endemism, are increasingly threatened by effects of coastal development and overfishing. Here, we evaluated the independent and combined effects of the top-down factor herbivory and the bottom -up factor inorganic nutrient availability in an offshore reef of the Central Red Sea. We deployed exclosure cages and fertilizer sources in situ over 4 months (with 5 temporal samplings) to study development of dry mass and fu nctional groups of algae on installed terracotta tiles. Our findings indicate that exclusion of herbivores had a stronger impact than eutrophication alone, and the combined treatment had the highest impact. The effect of elevated nutrients alone had no impact on composition of functional algae groups, dry mass, and the development of organic carbon (Corg) and nitrogen (N) on the tiles. However, when larger herbivores were excluded, tile surface cover changed drastically from patchy non-coralline crusts to filamentous algae. Dry mass increased up to 300 times, and with additional nutrient enrichment up to 500 times, though strong successional patterns could be observed. Our study is the first from the Red Sea that evaluates top-down and bottom-up factors on algae development and underlines the importance of addressing both of these factors in management measures to preserve the local coral reef resources.