Biogeochemical consequences of organic matter release by reef-building scleractinian corals

For my parents, for introducing me to the wonders of coral reefs from an early age and for their everlasting support " Coral reefs are threatened, not doomed – if we can avoid extreme climate change, runoff and over-fishing. Prevention is better than cure, but recovery is possible. The decisions we make now, or don't make, will have profound long-term consequences. " und Alex, die mich zu jeder Zeit unterstützt haben. Thesis abstract This thesis is composed of 12 chapters presenting a series of biogeochemical and ecological investigations relevant to coral reef ecosystems. The major focus lies on the consequences of organic matter release by benthic reef organisms, in particular reef-building scleractinian corals, for biogeochemical element cycles of fringing coral reefs, globally the most common reef type. Warm water coral reefs, while thriving in oligotrophic marine environments, are nonetheless characterized by high primary production rates facilitated by efficient utilization, recycling and conservation of organic matter within reef biogeochemical processes. Scleractinian corals contribute substantially to biogeochemical processes and general reef ecosystem engineering by several synergistic features, which include (1) the construction of complex reef frameworks enhancing habitat and species diversity, (2) the erosion-induced supply of biogenic calcareous sands functioning as catalytic filter systems for organic matter recycling, and (3) the continuous release of organic matter, e.g. as mucus, initiating reef element cycles and conservation of essential nutrients via particle trapping. Within this thesis, the ecosystem engineering feature of organic matter release by reef corals is examined by successive investigations on distinct pathways of this material within fringing reef ecosystems: ranging from its initial release and function as an energy and nutrient carrier, to its degradation and recycling by reef-associated organisms, in particular microbes. Quantitative investigations provide the first comprehensive overview of organic matter release rates by dominant scleractinian coral taxa and elucidate its substantial contribution to organic matter pools in reef-surrounding waters. In situ and laboratory studies exhibit the efficient microbial degradation of coral-derived organic matter in reef waters and sandy reef sediments, emphasizing its significant input to pelagic and benthic reef metabolism. Studies on the trapping efficiency of particulate coral mucus for picoplankton organisms indicate its potential role in benthic-pelagic coupling processes and biomass conservation within fringing reef ecosystems. Further, the function of coral-derived organic material in reef trophic interactions is demonstrated by tracer studies revealing its uptake by coral-associated organisms. These combined findings confirm organic matter release as a …