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

Cold-water corals mainly thrive in waters with low aragonite saturation (Ωar). As several laboratory studies showed, long-term exposure to acidified conditions does not seem to negatively affect their growth rates but in situ studies on the response of cold-water corals to low Ωar are still scarce. The Comau Fjord in Chilean Patagonia is naturally stratified with vertical and horizontal pH gradients and high densities of the cosmopolitan cold-water coral Desmophyllum dianthus are found at Ωar ≤ 1. Previous studies revealed high growth rates of D. dianthus in summer but it is not known so far 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, corals were sampled at six stations at 20 m water depth along the horizontal pHT gradient of Comau Fjord and at one station at 300 m depth. In addition, corals were transplanted between the stations at the head and the mouth at the fjord and between stations at 20 m and 300 m depth. Linear extension rates (fluorescent microscopy), long-term growth (buoyant weight technique) and short-term calcification rates (alkalinity anomaly technique) of D. dianthus were compared with the physico-chemical conditions in the water column (pHT, Ωar, pCO2 and temperature) in austral summer 2016/2017 and winter 2017. In addition, the internal pH in the calcifying fluid (pHcf) was determined by measuring the skeletal boron isotopic composition (δ11B) using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Growth rates of D. dianthus differed between stations along the pHT gradient of Comau Fjord but growth rates could not be predicted from seawater pHT or temperature. No seasonal differences were found in linear extension and short-term calcification rates but long-term growth rates were higher in summer compared to winter likely due to seasonal differences in food availability. In addition, cross-transplanted corals between 20 m and 300 m showed that D. dianthus is able to acclimatise to undersaturated conditions with respect to aragonite (Ωar < 1). Furthermore, the present study shows that the pH up-regulation (ΔpH) of D. dianthus was higher at low pHT (Ωar < 1) compared to high pHT and higher in winter compared to summer despite lower food availability and with this less energy for pH up-regulation. The findings of the present study lead to the conclusion that other factors than Ωar primarily affect growth of D. dianthus in Comau Fjord as this species seems to be well adapted to natural low pHT conditions with highest growth rates at Ωar < 1