Understanding individual and combined effects of ocean acidification, warming and coastal runoff on marine calcifying organisms on tropical coral reefs

Anthropogenically induced greenhouse gas emissions result in two major environmental changes on the global scale for coral reefs: ocean acidification (OA) and ocean warming (OW). Additionally, increasing levels of terrestrial runoff, that introduce fertilizer, sewage, sediments and other contaminants into coastal areas, can decrease water quality on the local scale. Consequently, photosynthesizing and calcifying coral reef organisms are affected by OA, OW and coastal runoff, but knowledge about the interactive effects of these stressors on key coral reef organisms is scarce. The aim of this thesis was to investigate how OA individually, and in combination with OW or local stressors (i.e. decreased light availability and inorganic eutrophication), affects important calcifying coral reef organisms. A series of field- and laboratory-based experiments were conducted on the Great Barrier Reef and at natural volcanic carbon dioxide seeps in Papua New Guinea. A range of response parameters, including growth, calcification and photosynthesis, were investigated at the species and community level. OA showed no negative impact on three large benthic foraminiferal species in the short term (Chapter 2) and several calcifying green algae species of the genus Halimeda in the long term (Chapter 3). OA combined with decreased light availability resulted in additive negative effects on the coral Acropora millepora in the short term (Chapter 4), while OA combined with inorganic eutrophication did not exhibit any significant effects on the corals Acropora tenuis and Seriatopora hystrix and on the calcifying green alga Halimeda opuntia in the short term (Chapter 5). In the long term, the combination of OA and OW resulted in decreased growth and calcification of epilithic algal communities, particularly in crustose coralline red algae (Chapter 6). The different sensitivity of the species investigated to global and local stressors, suggests that changes will occur in coral reef community structures in the near future. Reduced light availability may amplify negative effects of OA on corals and thereby contribute to shifts from coral to algae dominated communities on reefs affected by coastal runoff. The scientific findings of the present thesis indicate that OA in combination with other stressors may reduce the abundance of calcifying organisms and thus lower the calcium carbonate production on coral reefs in the future. This may lead to reduced reef growth, increased brittleness and reduced recovery potential after acute disturbances. Ultimately, declining reef habitat will likely lead to a reduction in biodiversity and may thus have implications on the people, who are dependent on coral reefs for their livelihood