Seawater carbonate chemistry and larval growth response of the Portuguese oyster (Crassostrea angulata) in a laboratory experiment

Rising anthropogenic carbon dioxide (CO2) dissolving into coastal waters is decreasing the pH and carbonate ion concentration, thereby lowering the saturation state of calcium carbonate (CaCO3) minerals through a process named ocean acidification (OA). The unprecedented threats posed by such low pH on calcifying larvae of several edible oyster species have not yet been fully explored. Effects of low pH (7.9, 7.6, 7.4) on the early growth phase of Portuguese oyster (Crassostrea angulata) veliger larvae was examined at ambient salinity (34 ppt) and the low-salinity (27 ppt) treatment. Additionally, the combined effect of pH (8.1, 7.6), salinity (24 and 34 ppt) and temperature (24 °C and 30 °C) was examined using factorial experimental design. Surprisingly, the early growth phase from hatching to 5-day-old veliger stage showed high tolerance to pH 7.9 and pH 7.6 at both 34 ppt and 27 ppt. Larval shell area was significantly smaller at pH 7.4 only in low-salinity. In the 3-factor experiment, shell area was affected by salinity and the interaction between salinity and temperature but not by other combinations. Larvae produced the largest shell at the elevated temperature in low-salinity, regardless of pH. Thus the growth of the Portuguese oyster larvae appears to be robust to near-future pH level (> 7.6) when combined with projected elevated temperature and low-salinity in the coastal aquaculture zones of South China Sea.

人为活动排放的二氧化碳（carbon dioxide, CO2）不断溶解进入近海水域，导致水体pH值下降、碳酸根离子浓度降低，进而通过被称为海洋酸化（ocean acidification, OA）的过程降低了碳酸钙（calcium carbonate, CaCO3）矿物的饱和状态。此类低pH环境对多种食用牡蛎物种的钙化幼体构成的前所未有的威胁，目前尚未得到充分探究。本研究在环境盐度（34 ppt）与低盐度（27 ppt）两种处理条件下，考察了低pH（7.9、7.6、7.4）对葡萄牙牡蛎（Crassostrea angulata）面盘幼虫（veliger larvae）早期生长阶段的影响。此外，本研究采用析因实验设计，考察了pH（8.1、7.6）、盐度（24与34 ppt）及温度（24 ℃与30 ℃）三者的联合效应。令人意外的是，从孵化至5日龄面盘幼虫的早期生长阶段，无论盐度为34 ppt还是27 ppt，幼虫对pH 7.9与pH 7.6均表现出较高耐受性。仅在低盐度处理组中，pH 7.4条件下的幼虫壳面积才显著更小。在三因素实验中，幼虫壳面积仅受盐度以及盐度与温度的交互作用影响，其余组合因素无显著效应。无论pH水平如何，在低盐度条件下升温时，幼虫的壳面积均达到最大。因此，当结合南海近岸养殖区预计将出现的升温与低盐度环境时，葡萄牙牡蛎幼虫的生长对近未来的pH水平（>7.6）表现出较强的耐受性。