Local drivers of the seasonal carbonate cycle across four contrasting coastal systems

Four contrasting coastal systems in Ireland, each with shellfish production activities, were studied to provide a first evaluation of the spatial and seasonal influences on the local carbonate system. The study sites included; (1) a coastal system with sandstone bedrock and minimal freshwater sources, (2) an estuarine system with a catchment limestone bedrock, (3) an estuarine system with a catchment granite bedrock, and (4) a karst groundwater-fed estuary. The type of bedrock was the dominant control on regional carbonate chemistry, where the calcium carbonate catchment bedrock was a strong source of both dissolved inorganic carbon and total alkalinity input in the two limestone regions, which are supersaturated with respect to atmospheric CO₂ throughout the year. Primary production played an important role in the non-limestone regions, where both systems were CO₂-undersaturated during productive months. Minimum aragonite saturation (Ω_aragonite) was observed at all sites during winter when productivity is lowest; surface winter Ω_aragonite is <1.5 close to the mussel farms in Kinvara Bay and Bantry Bay. Ω_aragonite was recalculated to account for the higher calcium concentrations in the River Suir from limestone dissolution, which increases Ω_aragonite by ∼0.5 in the mid estuary and >2 in the inner estuary. The substrate-to-inhibitor ratio (SIR), an alternative indicator of ecosystem vulnerability to acidification, was positively correlated to Ω_aragonite in all systems, however with more variability in the two limestone regions. Results highlight challenges of assessing local ecosystem vulnerability to future acidification and the importance of understanding the local spatio-temporal biogeochemistry.