Differential roles of anthropogenic CO2 in mediating seasonal amplitudes of ocean acidification metrics over a coastal coral habitat

Abstract

Seasonal-scale local forcings sharply reduce the coastal pH and aragonite saturation state (Ω_aragonite). However, habitat-specific seasonality and control change signatures under increasing atmospheric CO₂ are still poorly characterized. Here, we investigated carbonate system parameter dynamics over a Dongshan coral habitat that is greatly influenced by seasonal current patterns on the western Taiwan Strait coast. Specifically, relatively low pH and Ω_aragonite were observed in the trial zone throughout the seasons. Using a first-order Taylor decomposition considering biological carbon metabolism, we suggest that the higher net aerobic respiration related to intense local human activities produced worse ocean acidity in the trial zone. Seasonally, a decreasing Ω_aragonite trend was observed from the transition to the northeast monsoon seasons, mainly controlled by dissolved inorganic carbon (DIC) divergence among seasons. The pH/hydrogen ion concentration ([H⁺]) seasonal cycle was determined by both DIC and temperature components, revealing the lowest/highest value in the southwest monsoon season. Based on ocean acidification scenario modeling forced with a business-as-usual emissions scenario, the Ω_aragonite seasonal amplitude attenuation was projected to exceed 30% during the 21st century. However, [H⁺] seasonal amplitude was amplified over 170%. The attenuation in the Ω_aragonite seasonal amplitude mainly resulted from an increase in anthropogenic CO₂ seasonal divergence. The increase in [H⁺] seasonal amplitude mostly followed from an increase in the [H⁺] sensitivities to DIC and temperature changes.