The Papagayo upwelling system is a natural laboratory for studying ecosystems' response to ocean acidification (OA). We measured pH and pCO₂ in situ with high temporal resolution and compared them with data available from upwelling season. Local coral reefs are exposed to acidic and undersaturated waters in upwelling and non-upwelling events. These restrictive conditions occur alongside local stressors, potentially decreasing reefs' resilience and increasing their vulnerability under future OA.