Community-level sensitivity of a calcifying ecosystem to acute in situ CO2 enrichment

The rate of change in ocean carbonate chemistry is a vital determinant in the magnitude of effects observed. Benthic marine ecosystems are facing an increasing risk of acute CO2 exposure that may be natural or anthropogenically derived (e.g. engineering and industrial activities). However, our understanding of how acute CO2 events impact marine life is restricted to individual organisms, with little understanding for how this manifests at the community level. Here, we investigated in situ the effect of acute CO2 enrichment on the coralline algal ecosystem—a globally ubiquitous, ecologically and economically important habitat, but one which is likely to be sensitive to CO2 enrichment due to its highly calcified reef-like structures engineered by coralline algae. Most notably, we observed a rapid community-level shift to favour net dissolution rather than net calcification. Smaller changes from net respiration to net photosynthesis were also observed. There was no effect on the net flux of DMS/DMSP (algal secondary metabolites), nor on the nutrients nitrate and phosphate. Following return to ambient CO2 levels, only a partial recovery was seen within the monitoring timeframe. This study highlights the sensitivity of biogenic carbonate marine communities to acute CO2 enrichment and raises concerns over the capacity for the system to 'bounce back' if subjected to repeated acute high-CO2 events.

海洋碳酸盐化学的变化速率是决定观测效应强度的关键因素。海洋底栖生态系统正面临日益升高的急性二氧化碳暴露风险，此类暴露可源于自然过程或人为活动（如工程与工业活动）。然而，当前学界对急性二氧化碳事件如何影响海洋生物的认知仅局限于单个生物体层面，几乎缺乏对该影响在群落尺度下具体表现的了解。本研究通过原位实验，探究了急性二氧化碳富集对珊瑚藻生态系统的影响——该生态系统是全球广泛分布、兼具重要生态与经济价值的栖息地，但其由珊瑚藻构建的高度钙化礁状结构，使其对二氧化碳富集极为敏感。尤为显著的是，我们观测到群落层面发生了快速转变，整体代谢转向净溶解而非净钙化；同时还观测到净呼吸向净光合的小幅变化。二甲基硫/二甲基巯基丙酸内盐（DMS/DMSP，藻类次生代谢物）的净通量，以及硝酸盐、磷酸盐营养盐，均未受到显著影响。当系统恢复至背景二氧化碳水平后，在本次监测周期内仅观测到部分恢复。本研究凸显了海洋生物成因碳酸盐群落对急性二氧化碳富集的敏感性，并引发学界担忧：若该生态系统反复遭遇急性高二氧化碳事件，其是否具备"回弹恢复"的能力。