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.
Highlights
Atmospheric CO2 levels have nearly doubled since the industrial revolution resulting in unprecedented climatic change
We describe a semi-enclosed lagoon system surrounded by mangroves (Bouraké, New Caledonia), where diverse and relatively abundant coral populations persist under the combined stress of elevated temperature, low pH and low dissolved oxygen, periodically fluctuating according to tidal and diurnal cycles
The lagoon mouth led to a channel bordered by a shallow coral reef platform running through the lagoon to connect a series of sheltered shallow (1–2 m depth) bays
Summary
Samples were allowed to recover for 4-h after collection in aquaria using a recirculating water system containing seawater from the sites of collection at controlled light (240 μmol m−2 s−1) and temperature (within 0.5 °C of in situ habitat temperature) conditions. Respiration and calcification were determined for all samples through 1-h light and 1-h dark incubations. Normalised rates of calcification (μmol CaCO3 cm[2] h−1) were calculated by standardising for chamber seawater volume, incubation time and coral surface area as: G(t). Net photosynthesis (PN) and respiration (R) were determined by changes in oxygen for each incubation chamber during the light and dark incubations respectively, corrected for any changes in oxygen of the three seawater controls. Coral metabolic parameters (net light (GL) and dark calcification (GD), photosynthesis and respiration) were compared for species between sites using a t-test. Statistical analyses were conducted using Graphpad Prism[52], R studio[53], and PRIMER (version 6)
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