Seawater carbonate chemistry, photosynthesis and calcification rate during experiments with coral Acropora muricata, 2011@en

The effect of decreasing aragonite saturation state (Omega Arag) of seawater (elevated pCO2) on calcification rates of Acropora muricata was studied using nubbins prepared from parent colonies located at two sites of La Saline reef (La Réunion Island, western Indian Ocean): a back-reef site (BR) affected by nutrient-enriched groundwater discharge (mainly nitrate), and a reef flat site (RF) with low terrigenous inputs. Protein and chlorophyll a content of the nubbins, as well as zooxanthellae abundance, were lower at RF than BR. Nubbins were incubated at ~27°C over 2 h under sunlight, in filtered seawater manipulated to get differing initial pCO2 (1,440-340 µatm), Omega Arag (1.4-4.0), and dissolved inorganic carbon (DIC) concentrations (2,100-1,850 µmol kg-1). Increasing DIC concentrations at constant total alkalinity (AT) resulted in a decrease in Omega Arag and an increase in pCO2. AT at the beginning of the incubations was kept at a natural level of 2,193 +- 6 µmol kg-1 (mean +- SD). Net photosynthesis (NP) and calcification were calculated from changes in pH and AT during the incubations. Calcification decrease in response to doubling pCO2 relative to preindustrial level was 22% for RF nubbins. When normalized to surface area of the nubbins, (1) NP and calcification were higher at BR than RF, (2) NP increased in high pCO2 treatments at BR compared to low pCO2 treatments, and (3) calcification was not related to Omega Arag at BR. When normalized to NP, calcification was linearly related to Omega Arag at both sites, and the slopes of the relationships were not significantly different. The increase in NP at BR in the high pCO2 treatments may have increased calcification and thus masked the negative effect of low Omega Arag on calcification. Removing the effect of NP variations at BR showed that calcification declined in a similar manner with decreased Omega Arag (increased pCO2) whatever the nutrient loading.本研究以印度洋西部留尼汪岛拉盐礁（La Saline reef）两个采样点位的亲本珊瑚群落培育的断枝（nubbins）为实验材料，探究海水文石饱和状态（aragonite saturation state, Omega Arag）降低（即二氧化碳分压pCO2升高）对棘鹿角珊瑚（Acropora muricata）钙化速率的影响。两个采样点位分别为受富营养化地下水排放（主要为硝酸盐）影响的礁后坪（back-reef site, BR），以及陆源输入量较低的礁滩坪（reef flat site, RF）。实验断枝的蛋白质、叶绿素a含量，以及虫黄藻（zooxanthellae）丰度均在RF点位低于BR点位。将断枝置于约27℃的过滤海水中，在光照条件下培养2小时，通过人工调控获得不同初始pCO2（1440~340 μatm）、Omega Arag（1.4~4.0）以及溶解无机碳（dissolved inorganic carbon, DIC）浓度（2100~1850 μmol kg⁻¹）。在总碱度（total alkalinity, AT）恒定的条件下提升DIC浓度，会导致Omega Arag降低、pCO2升高。实验初始阶段的总碱度维持在自然水平：2193±6 μmol kg⁻¹（平均值±标准偏差）。通过培养过程中pH与总碱度的变化计算净光合速率（net photosynthesis, NP）与钙化速率。相较于工业化前水平，当pCO2翻倍时，RF组断枝的钙化速率下降22%。将实验数据标准化至断枝表面积后，可观察到以下结果：（1）BR组的净光合速率与钙化速率均高于RF组；（2）相较于低pCO2处理组，BR组在高pCO2处理下的净光合速率有所提升；（3）BR组的钙化速率与Omega Arag无显著相关性。当将数据标准化至净光合速率后，两个点位的钙化速率均与Omega Arag呈线性相关，且二者的回归斜率无显著差异。BR组在高pCO2处理下的净光合速率提升，可能会促进钙化，从而掩盖了低Omega Arag对钙化的负面影响。消除BR组净光合速率变化的影响后，无论营养负荷如何，钙化速率均随Omega Arag降低（即pCO2升高）呈相似程度的下降趋势。