Effects of Ocean Acidification on the Ballast of Surface Aggregates Sinking through the Twilight Zone

The dissolution of CaCO3 is one of the ways ocean acidification can, potentially, greatly affect the ballast of aggregates. A diminution of the ballast could reduce the settling speed of aggregates, resulting in a change in the carbon flux to the deep sea. This would mean lower amounts of more refractory organic matter reaching the ocean floor. This work aimed to determine the effect of ocean acidification on the ballast of sinking surface aggregates. Our hypothesis was that the decrease of pH will increase the dissolution of particulate inorganic carbon ballasting the aggregates, consequently reducing their settling velocity and increasing their residence time in the upper twilight zone. Using a new methodology for simulation of aggregate settling, our results suggest that future pCO2 conditions can significantly change the ballast composition of sinking aggregates. The change in aggregate composition had an effect on the size distribution of the aggregates, with a shift to smaller aggregates. A change also occurred in the settling velocity of the particles, which would lead to a higher residence time in the water column, where they could be continuously degraded. In the environment, such an effect would result in a reduction of the carbon flux to the deep-sea. This reduction would impact those benthic communities, which rely on the vertical flow of carbon as primary source of energy.

碳酸钙（CaCO₃）的溶解是海洋酸化可能极大影响海洋聚集体压载物的途径之一。压载物的减少会降低聚集体的沉降速率，进而改变输送至深海的碳通量。这意味着抵达海底的难降解有机质含量将有所降低。本研究旨在探明海洋酸化对沉降性表层聚集体压载物的影响。我们提出的假说为：pH值降低会增加作为聚集体压载的颗粒无机碳的溶解量，从而降低聚集体的沉降速度，并延长其在表层弱光层的停留时间。本研究采用一种全新的聚集体沉降模拟方法，结果表明未来的二氧化碳分压（pCO₂）环境将显著改变沉降性聚集体的压载组分组成。聚集体组成的变化会影响其粒径分布，使聚集体向更小尺寸偏移。同时，颗粒的沉降速率也会发生改变，这将导致其在水柱中的停留时间更长，在此期间可被持续降解。在自然环境中，此类效应将导致输送至深海的碳通量减少，进而对依赖垂直碳流作为主要能量来源的底栖生物群落造成影响。