DataSheet_1_High Coral Recruitment Despite Coralline Algal Loss Under Extreme Environmental Conditions.docx

The crucial role of crustose coralline algae (CCA) in inducing hard coral larval settlement and ensuring the replenishment of coral reefs is widely accepted, and so are the negative effects of anthropogenic CO2 emissions on both CCA abundance and coral development. However, diversified and well-developed coral reef communities have been recently discovered in natural conditions where CCA and corals would not be expected to thrive. Back-reef pools, volcanic CO2 vents, mangrove estuaries, and semi-enclosed lagoons systems can present seawater pH, temperature, and dissolved oxygen values reaching or even exceeding the conditions currently predicted by the Inter Panel on Climate Change (IPCC) for 2100. In the semi-enclosed lagoon of Bouraké (New Caledonia, southwest Pacific Ocean), seawater pHT, dissolved oxygen, and temperatures regularly fluctuate with the tide reaching respectively minimum values of 7.23 pHT units, 2.28 mg O2 L-1, and maximum of 33.85°C. This study reports the effect of such extreme environmental conditions on hard coral recruitment and CCA originally settled at a forereef on artificial substrates that were transplanted over two years in two fringing reef and at the Bouraké lagoon. Our data emphasize the negative effects of the extreme conditions in our study sites on the CCA, which decreased in cover by ca. 80% and lost in the competition with turf algae, which, in turn, increased up to 162% at the end of the two years. Conversely, hard coral recruitment remained high at Bouraké throughout the study, three-fold higher than at two sites located outside Bouraké where environmental conditions were typical for coastal fringing reefs. Our findings show that while such extreme, climate change like-conditions have a direct and adverse effect on CCA abundance, and despite a certain persistence, coral larvae settlement was not affected. Based on previous findings from Bouraké, and the present observations, both coral recruits and adults seem to be unaffected despite the extreme environmental conditions. This study supports previous research illustrating how extreme natural and variable environments may reveal unexpected and positive insights on the processes underlying coral acclimatization and adaptation to global change.

结壳珊瑚藻（crustose coralline algae, CCA）在诱导硬珊瑚幼虫附着、保障珊瑚礁种群补充中发挥的关键作用已得到学界广泛认可，人为二氧化碳排放对结壳珊瑚藻丰度及珊瑚生长的负面影响亦获一致认同。然而，近期研究在原本认为结壳珊瑚藻与珊瑚难以存活的自然环境中，发现了物种多样且发育成熟的珊瑚礁群落。礁后池、火山二氧化碳喷口、红树林河口以及半封闭泻湖系统，其海水pH、温度与溶解氧水平可达到甚至超出政府间气候变化专门委员会（Inter Panel on Climate Change, IPCC）对2100年的预估环境条件。在西南太平洋新喀里多尼亚的布拉克（Bouraké）半封闭泻湖内，海水pH值、溶解氧含量与温度随潮汐周期性波动，最低分别可达7.23 pH单位、2.28 mg O₂ L⁻¹，最高则达到33.85℃。本研究调查了此类极端环境条件对硬珊瑚补充量与原本定植于礁前人工基质的结壳珊瑚藻的影响：研究将上述样本移植至两处岸礁及布拉克泻湖内，为期两年。研究数据证实，本研究样地的极端环境对结壳珊瑚藻产生了显著负面影响：其盖度下降约80%，并在与石藻的竞争中落败；而石藻盖度在两年研究周期结束时最高提升至162%。与之相反，布拉克泻湖内的硬珊瑚补充量在整个研究周期中始终维持高位，为布拉克外部两处环境符合沿岸岸礁典型特征的样地的3倍。本研究结果显示，尽管此类类气候变化的极端环境对结壳珊瑚藻丰度具有直接负面影响，且结壳珊瑚藻虽具备一定存续能力，但硬珊瑚幼虫附着并未受到影响。结合布拉克泻湖的既往研究结果与本次观测数据，尽管环境条件极端，珊瑚幼体与成体似乎均未受到影响。本研究佐证了既往研究成果，即极端自然且动态变化的环境或可揭示珊瑚对全球变化的驯化与适应过程中此前未被发现的重要机制。