Ocean acidification, reduced silicate availability and declines in Ocean productivity

Understanding rising atmospheric CO₂ effects on the chemistry of the world's ecosystems remains nascent. CO₂ dissolved in water forms carbonic acid, thereby lowering pH (ocean acidification [OA]). Diatoms, which generate 40% of ocean primary productivity, depend on dissolved silicon to construct their silica frustules; lower pH reduces silica (SiO₂) solubility in seawater. Recent experiments document enhanced silica sedimentation under elevated CO₂, implying reduced Si recycling and availability to support diatom production at the ecosystem scale. We tested this hypothesis using long-term data from a productive Pacific coastline undergoing rapid pH decline. Over two decades, as pH declined, dissolved Si:N and water column chlorophyll concurrently declined strongly. Accounting for variation in potential Si transport vectors did not change these relationships. Our results support the OA Si loss hypothesis and indicate that this process is already ongoing in some productive ocean regions, with implications for the carbon cycle, coastal food webs, and human well-being.