Seawater carbonate chemistry, nutrients and growth rate during experiments with coral Astrangia poculata and field observations, 2010

Zooxanthellate colonies of the scleractinian coral Astrangia poculata were grown under combinations of ambient and elevated nutrients (5 µM NO, 0.3 µM PO4, and 2nM Fe) and CO2 (780 ppmv) treatments for a period of 6 months. Coral calcification rates, estimated from buoyant weights, were not significantly affected by moderately elevated nutrients at ambient CO2 and were negatively affected by elevated CO2 at ambient nutrient levels. However, calcification by corals reared under elevated nutrients combined with elevated CO2 was not significantly different from that of corals reared under ambient conditions, suggesting that CO2 enrichment can lead to nutrient limitation in zooxanthellate corals. A conceptual model is proposed to explain how nutrients and CO2 interact to control zooxanthellate coral calcification. Nutrient limited corals are unable to utilize an increase in dissolved inorganic carbon (DIC) as nutrients are already limiting growth, thus the effect of elevated CO2 on saturation state drives the calcification response. Under nutrient replete conditions, corals may have the ability to utilize more DIC, thus the calcification response to CO2 becomes the product of a negative effect on saturation state and a positive effect on gross carbon fixation, depending upon which dominates, the calcification response can be either positive or negative. This may help explain how the range of coral responses found in different studies of ocean acidification can be obtained.

本研究将共生有虫黄藻的石珊瑚平展星珊瑚（Astrangia poculata）群落，置于不同营养盐（环境浓度与升高浓度：5 µM 硝酸盐（NO）、0.3 µM 磷酸盐（PO4）与2 nM 铁（Fe））与二氧化碳（780 ppmv）的组合处理条件下连续培养6个月。通过浮力称重法估算的珊瑚钙化速率结果显示：在环境二氧化碳浓度条件下，适度升高的营养盐对珊瑚钙化速率无显著影响；而在环境营养盐浓度条件下，高二氧化碳浓度会显著抑制珊瑚钙化。然而，在升高营养盐结合高二氧化碳浓度处理下培养的珊瑚，其钙化速率与环境对照条件下的珊瑚无显著差异，这表明二氧化碳富集可能会引发虫黄藻共生珊瑚出现营养盐限制现象。本研究提出了一个概念模型，用以阐释营养盐与二氧化碳如何协同调控虫黄藻共生珊瑚的钙化过程：当珊瑚受营养盐限制时，由于营养盐已成为生长限制性因子，珊瑚无法利用溶解无机碳（dissolved inorganic carbon, DIC）浓度的提升，此时高二氧化碳浓度对碳酸钙饱和状态的影响将主导钙化响应；在营养盐充足的条件下，珊瑚具备利用更多DIC的能力，因此二氧化碳对钙化的影响为“饱和状态抑制效应”与“总碳固定促进效应”的共同结果，二者相对强弱将决定钙化响应为正向或负向。这一机制或可解释不同海洋酸化研究中观测到的珊瑚响应差异。