In situ growth rates of the cold-water coral Desmophyllum dianthus are highest in aragonite undersaturated waters

Cold-water corals (CWC) were long thought to be particularly sensitive to ocean acidification (OA). However, previous laboratory studies indicate no negative effect of low aragonite saturation (Ωar) on growth rates of CWC while the few in situ studies show the same result. The CWC Desmophyllum dianthus is ubiquitous in Comau Fjord (southern Chile), a small but deep (> 400 m) semi-enclosed stratified basin with pronounced horizontal and vertical pH gradients. High densities of D. dianthus can be found below the aragonite saturation horizon (Ωar < 1) but it is not known so far if seasonal changes in Ωar lead to seasonal differences in growth rates. Corals were sampled along the pH gradients of Comau Fjord (Ωar = 0.65-1.45) and cross-transplanted between stations. Skeletal carbonate accretion (buoyant weighing technique) and calcification rates (alkalinity anomaly technique) were measured in austral summer 2016/2017 and winter 2017 and compared to physico-chemical conditions in the water column (T, Ωar). Higher growth rates were found in summer than in winter. Surprisingly, growth of D. dianthus was highest in undersaturated waters in both seasons (Ωar = 0.65 and 0.83) and cross-transplanted specimens were able to acclimatise to Ωar < 1. We conclude that Ωar is a poor predictor of D. dianthus growth and more factors (like plankton food supply) need to be taken into account when investigating the impact of OA on CWC in the future