Effects of Simulated Eutrophication and Overfishing on Coral Reef Invertebrates, Algae and Microbes in the Red Sea

Abstract

Besides the main climate change consequences, ocean warming and acidification, local disturbances such as overfishing and eutrophication are major threats to coral reefs worldwide. Despite its relatively healthy coral reefs that are increasingly faced with growing coastal development, the Red Sea is highly under-investigated, particularly outside the Gulf of Aqaba. This thesis therefore aims to contribute to the understanding of eutrophication and overfishing effects on Red Sea coral reefs by answering the following three key questions: (1) How do different grazer groups contribute to herbivory, and is herbivory therefore susceptible to overfishing? (2) What are the individual and combined effects of eutrophication and overfishing on the development of important reef organisms? (3) What are the potential consequences for reef functioning when local threats increase? The thesis consists of five chapters that are framed between a general introduction and synoptic discussion. At the beginning, a review summarizes the current state of knowledge on marine eutrophication (Chapter 1), an important anthropogenic threat for coral reefs that are highly adapted to very oligotrophic conditions. Further, a series of in situ experiments with settling tiles and coral fragments in the Egyptian and Saudi Arabian Red Sea were used to investigate not only the contribution and influence of herbivory on benthic macroalgae development (Chapters 2 and 3), but also the individual and combined effects of simulated eutrophication and overfishing on settlement of benthic macroalgae (Chapter 3), sessile invertebrates (Chapter 4), and a coral with its associated bacterial community (Chapter 5). Findings revealed that: (a) among the two dominating grazer groups, herbivorous fish were fivefold more effective in reducing algal biomass than sea urchins. (b) the simulation of eutrophication did not affect algal biomass, but decreased coral settlement and caused specific Alphaproteo-, Sphingo-, and Epsilonproteobacteria to emerge in the coral holobiont. (c) the simulation of overfishing exhibited stronger effects than that of eutrophication. It caused algal cover shifting from communities dominated by encrusting algae with low biomass and oxygen consumption in controls to communities containing less calcifying algae, with high-biomass and oxygen consumption. The brown algae Padina sp. and Hydroclathrus clathrathus, along with filamentous algae, benefitted most from this treatment. Coral settlement was absent, while that of polychaetes increased, and specific Deltaproteobacteria were found within the coral holobiont. (d) the combined treatment produced stronger and longer lasting effects on algae than overfishing alone. Settlement of bryozoans and bivalves increased and specific Alphaproteobacteria emerged. In summary, this study underlines the ecological importance of herbivorous fishes, the high susceptibility of herbivory to overfishing, and it provides - for the first time - comprehensive information on how Red Sea coral reefs respond to eutrophication and overfishing. Findings recommend that both stressors, but particularly overfishing, should be prevented in pristine reefs and reduced in already affected reefs to avoid potential phase-shifts from dominance by hard corals to that by brown and filamentous algae, or other invertebrates such as polychaetes, bryozoans, and bivalves. The appearance and composition of algae and invertebrates may be used as bioindicators for local reef monitoring and management measures.