Sediment topography enhances the response of coral reef carbonate sediment dissolution to ocean acidification

The interaction between current flow and topography (e.g., surface ripples) in shallow, permeable coral reef carbonate sediments establishes pressure gradients that increase the rate of sediment-water solute exchange relative to fluid shear along a flat bottom. It is currently unknown how this effect from surface ripples will modify the rate at which the sediment porewater is exposed to future chemical changes in the overlying water column, such as elevated pCO2 that is causing ocean acidification (OA). To address this question, this study used a series of 22-hour incubations in flume aquaria with permeable calcium carbonate sediment communities and examined the interactive effect of pCO2 (400 and 1000 µatm) and surface topography (flat and rippled sediments) on carbonate sediment metabolism and dissolution. According to dissolved oxygen optode image analysis, the presence of surface ripples increased the oxygenated area below the sediment surface by 295% relative to flat sediments. This was reflected in the sediment-to-water column fluxes of dissolved oxygen, where rippled sediments exhibited rates of respiration (R) and gross primary production (GPP) that were ~ 45% and ~ 50% higher, respectively, than flat sediments. An increase in pCO2 shifted the sediments in the flat flumes from net calcifying (Gnet > 0) to net dissolving (Gnet < 0), an effect that was amplified an additional ~ 60% in rippled sediments. These results suggest that current estimates of coral reef carbonate sediment Gnet may be underestimating the dissolution response to OA where the carbonate sediment environment exhibits ripples in the topography.

浅海渗透性珊瑚礁碳酸盐沉积物中，水流与地形（如表面波纹）的相互作用会形成压力梯度，相较于平底环境下的流体剪切力，该梯度可提升沉积物-水体间的溶质交换速率。目前尚不清楚表面波纹带来的这一效应，会如何改变沉积物孔隙水暴露于上覆水柱的未来化学变化的速率——例如引发海洋酸化（OA, Ocean Acidification）的二氧化碳分压升高。为解答这一科学问题，本研究在搭载渗透性碳酸钙沉积物群落的水槽水族箱中开展了一系列时长22小时的培养实验，探究了二氧化碳分压（pCO2，400与1000 µatm）与表面地形（平底与波纹底沉积物）对碳酸盐沉积物代谢与溶解过程的交互影响。通过溶解氧光极图像分析可知，相较于平底沉积物，表面波纹的存在可使沉积物表层下方的含氧区域扩大295%。这一结果在溶解氧的沉积物-水柱通量中得到印证：波纹底沉积物的呼吸速率（R）与总初级生产力（GPP, Gross Primary Production）分别较平底沉积物高出约45%与50%。二氧化碳分压升高会使平底水槽中的沉积物从净钙化状态（Gnet > 0）转变为净溶解状态（Gnet < 0），而这一效应在波纹底沉积物中还会额外被放大约60%。上述结果表明，当前针对珊瑚礁碳酸盐沉积物净钙化速率（Gnet）的估算，可能低估了地形存在波纹的碳酸盐沉积物环境对海洋酸化（OA）的溶解响应。