In situ effects of low pH and elevated HCO3- on juvenile massive Porites spp. in Moorea, French Polynesia.

Juvenile colonies of massive Porites spp. were exposed to manipulated pH and bicarbonate ([HCO3-]) in situ to test the hypothesis that ocean acidification (OA) does not affect respiration and calcification. Incubations lasted 28 h and exposed corals to ambient temperature and light with ecologically relevant water motion. Three treatments were applied: (1) ambient conditions of pH 8.04 and 1751 μmol HCO3- kg(-1) (Treatment 1), (2) pCO2-induced ocean acidification of pH 7.73 and 2011 μmol HCO3- kg(-1) (Treatment 2), and (3) pCO2 and HCO3--enriched seawater of pH 7.69 and 2730 μmol HCO3- kg(-1) (Treatment 3). The third treatment providing elevated [HCO3-] was used to test for stimulatory effects of dissolved inorganic carbon on calcification under low pH and low saturation of aragonite (Ωarag), but it does not reflect conditions expected to occur under CO2-driven OA. Calcification of juvenile massive Porites spp. was affected by treatments, with an 81% elevation in Treatment 3 versus Treatment 1, but no difference between Treatments 1 and 2; respiration and the metabolic expenditure concurrent with calcification remained unaffected. These findings indicate that juvenile massive Porites spp. are resistant to short exposures to OA in situ, and separately, that they can increase calcification at low pH and low Ωarag if [HCO3-] is elevated. Juvenile Porites spp. may therefore be limited by dissolved inorganic carbon under ambient pCO2 conditions.