Abstract
Abstract. According to current experimental evidence, coral reefs could disappear within the century if CO2 emissions remain unabated. However, recent discoveries of diverse and high cover reefs that already live under extreme conditions suggest that some corals might thrive well under hot, high-pCO2, and deoxygenated seawater. Volcanic CO2 vents, semi-enclosed lagoons, and mangrove estuaries are unique study sites where one or more ecologically relevant parameters for life in the oceans are close to or even worse than currently projected for the year 2100. Although they do not perfectly mimic future conditions, these natural laboratories offer unique opportunities to explore the mechanisms that reef species could use to keep pace with climate change. To achieve this, it is essential to characterize their environment as a whole and accurately consider all possible environmental factors that may differ from what is expected in the future, possibly altering the ecosystem response. This study focuses on the semi-enclosed lagoon of Bouraké (New Caledonia, southwest Pacific Ocean) where a healthy reef ecosystem thrives in warm, acidified, and deoxygenated water. We used a multi-scale approach to characterize the main physical-chemical parameters and mapped the benthic community composition (i.e., corals, sponges, and macroalgae). The data revealed that most physical and chemical parameters are regulated by the tide, strongly fluctuate three to four times a day, and are entirely predictable. The seawater pH and dissolved oxygen decrease during falling tide and reach extreme low values at low tide (7.2 pHT and 1.9 mg O2 L−1 at Bouraké vs. 7.9 pHT and 5.5 mg O2 L−1 at reference reefs). Dissolved oxygen, temperature, and pH fluctuate according to the tide by up to 4.91 mg O2 L−1, 6.50 ∘C, and 0.69 pHT units on a single day. Furthermore, the concentration of most of the chemical parameters was 1 to 5 times higher at the Bouraké lagoon, particularly for organic and inorganic carbon and nitrogen but also for some nutrients, notably silicates. Surprisingly, despite extreme environmental conditions and altered seawater chemical composition measured at Bouraké, our results reveal a diverse and high cover community of macroalgae, sponges, and corals accounting for 28, 11, and 66 species, respectively. Both environmental variability and nutrient imbalance might contribute to their survival under such extreme environmental conditions. We describe the natural dynamics of the Bouraké ecosystem and its relevance as a natural laboratory to investigate the benthic organism's adaptive responses to multiple extreme environmental conditions.
Highlights
Atmospheric carbon dioxide (CO2) has steadily increased over the industrial period (Gattuso et al, 2015), leading to ocean warming, acidification, and deoxygenation
There, we found an unprecedented number of benthic species, including two New Caledonian endemics, thriving under chronic suboptimal conditions that fluctuate with the tide
We found that the Bouraké lagoon is mostly N-limited (DIN : DIP < 10 and Si(OH)4 : dissolved inorganic nitrogen (DIN) > 1), which confirms the findings of Justicet al. (1995) and is similar to the conditions observed in other New Caledonian mangroves (Jacquet et al, 2006)
Summary
Atmospheric carbon dioxide (CO2) has steadily increased over the industrial period (Gattuso et al, 2015), leading to ocean warming, acidification, and deoxygenation. Shallow-water volcanic CO2 seeps, low pH springs, semi-enclosed bays, mangrove habitats, shallow sheltered-bay reefs, macrotidal environments, and low-pH upwelling areas are all potential study systems where the surrounding seawater is subject to a localized or widespread increase in either pCO2 or temperature and eventually a decrease in dissolved oxygen (DO; Camp et al, 2018) At these sites, general observations suggest ecosystem-level consequences of lifelong exposure to extreme conditions, such as reduced biological diversity, especially among calcifying organisms, decreased rates of coral calcification, and high rates of bioerosion (e.g., Hall-Spencer et al, 2008; Manzello et al, 2008; Fabricius et al, 2011; Crook et al, 2013; Kroeker et al, 2011; IglesiasPrieto et al, 2014; Milazzo et al, 2014; Paytan et al, 2014). Combined with strong currents and turbid waters, diverse and probably resilient coral reefs have been described there (Dandan et al, 2015; Schoepf et al, 2015)
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