Reef-building corals thrive within hot-acidified and deoxygenated waters

Abstract

Coral reefs are deteriorating under climate change as oceans continue to warm and acidify and thermal anomalies grow in frequency and intensity. In vitro experiments are widely used to forecast reef-building coral health into the future, but often fail to account for the complex ecological and biogeochemical interactions that govern reefs. Consequently, observations from coral communities under naturally occurring extremes have become central for improved predictions of future reef form and function. Here, we present a semi-enclosed lagoon system in New Caledonia characterised by diel fluctuations of hot-deoxygenated water coupled with tidally driven persistently low pH, relative to neighbouring reefs. Coral communities within the lagoon system exhibited high richness (number of species = 20) and cover (24–35% across lagoon sites). Calcification rates for key species (Acropora formosa, Acropora pulchra, Coelastrea aspera and Porites lutea) for populations from the lagoon were equivalent to, or reduced by ca. 30–40% compared to those from the reef. Enhanced coral respiration, alongside high particulate organic content of the lagoon sediment, suggests acclimatisation to this trio of temperature, oxygen and pH changes through heterotrophic plasticity. This semi-enclosed lagoon therefore provides a novel system to understand coral acclimatisation to complex climatic scenarios and may serve as a reservoir of coral populations already resistant to extreme environmental conditions.

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Mangrove systems have dynamic temperature, pH and oxygen profiles, and as such, expose resident corals to very different physicochemical conditions than neighboring reefs (Camp et al, 2017). Mangroves are highly heterogeneous in nature, with freshwater input, organic content, system size and consequently water residency time, along with other benthic components (e.g., seagrass composition) influencing the local physicochemical conditions.…”

“…For example, work in the Caribbean (Yates et al, 2014) suggested that mangroves could act as a potential refuge for corals, through physical shading and the elevation of downstream A T as a result of carbonate-sediment dissolution (i.e., buffering potential similar to seagrass habitats), as well as frequent exposure to more variable temperatures. In other locations [e.g., the Seychelles, Indonesia (Camp et al, 2016a) and New Caledonia (Camp et al, 2017)] mangrove systems have exposed corals to simultaneously low pH, low oxygen and warmer waters relative to adjacent reefs. As such, these mangrove systems may precondition corals to future climates (Camp et al, 2016a) or house corals more resilient to future climate conditions (Figure 2, Camp et al, 2017).…”

“…In other locations [e.g., the Seychelles, Indonesia (Camp et al, 2016a) and New Caledonia (Camp et al, 2017)] mangrove systems have exposed corals to simultaneously low pH, low oxygen and warmer waters relative to adjacent reefs. As such, these mangrove systems may precondition corals to future climates (Camp et al, 2016a) or house corals more resilient to future climate conditions (Figure 2, Camp et al, 2017). In New Caledonia, over 20 coral species were regularly exposed to pH as low as 7.3, temperature 1-2 • C higher than adjacent reefs, and oxygen levels below 3 mg L −1 (Camp et al, 2017).…”

“…As such, these mangrove systems may precondition corals to future climates (Camp et al, 2016a) or house corals more resilient to future climate conditions (Figure 2, Camp et al, 2017). In New Caledonia, over 20 coral species were regularly exposed to pH as low as 7.3, temperature 1-2 • C higher than adjacent reefs, and oxygen levels below 3 mg L −1 (Camp et al, 2017). Initial research at this site suggests that physiological plasticity (e.g., enhanced respiration indicative of increased heterotrophy) is facilitating survival of diverse coral communities, which include numerous species typically thought to be more sensitive to stress, e.g., Acropora spp.…”

Mechanisms of Thermal Tolerance in Reef-Building Corals across a Fine-Grained Environmental Mosaic: Lessons from Ofu, American Samoa

“…What environmental conditions are associated with the presence of heat adapted alleles? Certainly, the extreme temperature profiles of the HV pool are not unique to Ofu; corals are found in a variety of extreme environments and are exposed to temperatures that would cause bleaching in their conspecifics from other areas (Coles and Riegl, 2013;Kline et al, 2015;Richards et al, 2015;Camp et al, 2017). Wide variation in thermal tolerance and genetic divergence has been reported across latitudes and at large-spatial scales (Middlebrook et al, 2008;Howells et al, 2013;Dixon et al, 2015;Thomas et al, 2017), and it is becoming increasingly clear that locally adapted thermally tolerant pockets of corals exist at fine-spatial scales within a variety of coral reef systems (Goreau and Macfarlane, 1990;Barshis et al, 2010;Castillo et al, 2012;Kenkel et al, 2013bKenkel et al, , 2015Schoepf et al, 2015).…”