Seasonal growth of the cold-water coral Desmophyllum dianthus along an in situ aragonite saturation gradient

Cold-water corals (CWC) are thought to be especially vulnerable to ocean acidification. However, in situ studies on the response of CWC to low aragonite saturation (Ωar) are still scarce. The Comau Fjord in southern Patagonia (Chile) is naturally stratified with vertical and horizontal pH gradients and high abundances of the cosmopolitan CWC Desmophyllum dianthus at Ωar ≤ 1. Previous studies reveal high growth rates of D. dianthus in summer but it is not known if skeletal growth and the ability of internal pH up-regulation show seasonal fluctuations due to changes in Ωar and/or food supply. In the present study, we compare D. dianthus skeletal carbonate accretion (buoyant weight technique) and linear extension rates (fluorescent microscopy) with the physico-chemical conditions in the water column (T, Ωar) in austral summer 2016/2017 and winter 2017. Corals were sampled at six stations at 20m water depth along the presumed horizontal pH gradient of Comau Fjord.