Species-specific responses to climate change and community composition determine future calcification rates of Florida Keys reefs

Anthropogenic climate change compromises reef growth as a result of increasing temperatures and ocean acidification. Scleractinian corals vary in their sensitivity to these variables, suggesting species composition will influence how reef communities respond to future climate change. Because data are lacking for many species, most studies that model future reef growth rely on uniform scleractinian calcification sensitivities to temperature and ocean acidification. In order to address this knowledge gap, calcification of twelve common and understudied Caribbean coral species was measured for two months under crossed temperatures (27°C, 30.3°C) and CO2 partial pressures (pCO2) (400, 900, 1300 µatm). Mixed effects models of calcification for each species were then used to project community-level scleractinian calcification using Florida Keys reef composition data and IPCC AR5 ensemble climate model data. Three of the four most abundant species, Orbicella faveolata, Montastraea cavernosa, and Porites astreoides, had negative calcification responses to both elevated temperature and pCO2. In the business-as-usual CO2 emissions scenario, reefs with high abundances of these species had projected end-of-century declines in scleractinian calcification of >50% relative to present-day rates. Siderastrea siderea, the other most-common species, was insensitive to both temperature and pCO2 within the levels tested here. Reefs dominated by this species had the most stable end-of-century growth. Under more optimistic scenarios of reduced CO2 emissions, calcification rates throughout the Florida Keys declined <20% by 2100. Under the most extreme emissions scenario, projected declines were highly variable among reefs, ranging 10 to 100%. Without considering bleaching, reef growth will likely decline on most reefs, especially where resistant species like S. siderea are not already dominant. This study demonstrates how species composition influences reef community responses to climate change and how reduced CO2 emissions can limit future declines in reef calcification.人为气候变化通过全球升温和海洋酸化损害珊瑚礁生长。石珊瑚（Scleractinian corals）对这些环境变量的敏感性存在种间差异，这表明物种组成将影响珊瑚礁群落对未来气候变化的响应方式。由于多数珊瑚物种的相关数据较为匮乏，绝大多数模拟未来珊瑚礁生长的研究均采用了统一的石珊瑚钙化（calcification）敏感性参数来表征其对升温和海洋酸化的响应。为填补这一知识空白，本研究在交叉设置的温度梯度（27°C、30.3°C）与二氧化碳分压（pCO₂，400、900、1300 µatm）条件下，对12种常见且此前研究较少的加勒比海珊瑚物种的钙化速率进行了为期两个月的测定。随后，基于针对各物种构建的钙化混合效应模型（Mixed effects models），结合佛罗里达礁岛群（Florida Keys）的珊瑚群落组成数据以及政府间气候变化专门委员会第五次评估报告（IPCC AR5）集合气候模式数据，对群落水平的石珊瑚钙化率进行了预估。本研究涉及的4种优势珊瑚物种中，叶状团块珊瑚（Orbicella faveolata）、巨形脑珊瑚（Montastraea cavernosa）与星孔珊瑚（Porites astreoides）这3种的钙化速率对升温和升高的pCO₂均呈现负响应。在“一切照旧”的二氧化碳排放情景下，以这些物种为优势类群的珊瑚礁，其本世纪末的石珊瑚钙化率相较当前水平预估将下降50%以上。而另一种常见物种平坦星珊瑚（Siderastrea siderea）在本试验设置的温度与pCO₂梯度下均未表现出显著响应；以该物种为优势类群的珊瑚礁，其本世纪末的生长最为稳定。在更为乐观的二氧化碳减排情景下，至2100年佛罗里达礁岛群整体的珊瑚钙化率预估下降幅度将低于20%；而在最极端的排放情景下，不同珊瑚礁的钙化率下降幅度差异极大，区间为10%至100%。若不考虑珊瑚白化（bleaching）效应，多数珊瑚礁的生长大概率会出现衰退，尤其是在平坦星珊瑚这类耐受力较强的物种尚未占据优势的礁区。本研究阐明了物种组成如何影响珊瑚礁群落对气候变化的响应，以及降低二氧化碳排放如何能够限制未来珊瑚礁钙化率的衰退幅度。