Growth response of an early successional assemblage of coralline algae and benthic diatoms to ocean acidification

The sustained absorption of anthropogenically released atmospheric CO2 by the oceans is modifying seawater carbonate chemistry, a process termed ocean acidification (OA). By the year 2100, the worst case scenario is a decline in the average oceanic surface seawater pH by 0.3 units to 7.75. The changing seawater carbonate chemistry is predicted to negatively affect many marine species, particularly calcifying organisms such as coralline algae, while species such as diatoms and fleshy seaweed are predicted to be little affected or may even benefit from OA. It has been hypothesized in previous work that the direct negative effects imposed on coralline algae, and the direct positive effects on fleshy seaweeds and diatoms under a future high CO2 ocean could result in a reduced ability of corallines to compete with diatoms and fleshy seaweed for space in the future. In a 6-week laboratory experiment, we examined the effect of pH 7.60 (pH predicted to occur due to ocean acidification just beyond the year 2100) compared to pH 8.05 (present day) on the lateral growth rates of an early successional, cold-temperate species assemblage dominated by crustose coralline algae and benthic diatoms. Crustose coralline algae and benthic diatoms maintained positive growth rates in both pH treatments. The growth rates of coralline algae were three times lower at pH 7.60, and a non-significant decline in diatom growth meant that proportions of the two functional groups remained similar over the course of the experiment. Our results do not support our hypothesis that benthic diatoms will outcompete crustose coralline algae under future pH conditions. However, while crustose coralline algae were able to maintain their presence in this benthic rocky reef species assemblage, the reduced growth rates suggest that they will be less capable of recolonizing after disturbance events, which could result in reduced coralline cover under OA conditions.

海洋持续吸收人为排放的大气二氧化碳，正在改变海水碳酸盐化学体系，这一过程被称为海洋酸化（OA）。到2100年，最严峻的情景下，海洋表层海水平均pH值将下降0.3个单位，降至7.75。海水碳酸盐化学的改变预计会对诸多海洋生物产生负面影响，尤以钙化生物如珊瑚藻为甚；而硅藻、肉质海藻等物种受影响较小，甚至可能从海洋酸化中获益。既往研究提出假说：未来高CO₂海洋环境中，壳状珊瑚藻会受到直接负面影响，而肉质海藻与硅藻则会获得直接正向影响，这可能导致珊瑚藻未来与硅藻、肉质海藻争夺空间的竞争力下降。本研究开展了为期6周的室内实验，对比了pH 8.05（当前海洋pH水平）与pH 7.60（预计2100年之后出现的酸化后pH水平）对以壳状珊瑚藻和底栖硅藻为优势类群的早期演替冷温带物种组合侧向生长速率的影响。两种pH处理下，壳状珊瑚藻与底栖硅藻均保持了正向生长速率。但pH 7.60条件下，珊瑚藻的生长速率仅为当前pH条件下的三分之一；而硅藻生长速率虽出现非显著性下降，使得实验期间两类功能群的占比未发生明显变化。本研究结果不支持“未来酸化环境中底栖硅藻将胜过壳状珊瑚藻”的假说。不过，尽管壳状珊瑚藻能够在底栖岩礁物种组合中维持自身存在，但其生长速率的下降表明，它们在扰动事件后的再定植能力将有所减弱，这可能导致海洋酸化条件下珊瑚藻的盖度降低。