Short periods of decreased water flow may modulate long-term ocean acidification in reef-building corals

Abstract

Ocean acidification (OA) poses a major threat to reef-building corals. Although water flow variability is common in coral reefs and modulates coral physiology, the interactive effects of flow and OA on corals remain poorly understood. Therefore, we performed a three-month OA experiment investigating the effect of changes in flow on coral physiology. We exposed the reef-building coralsAcropora cytherea,Pocillopora verrucosa, andPorites cylindricato control (pH 8.0) and OA (pH 7.8) conditions at moderate flow (6 cm s-1) and monitored OA effects on growth. Throughout the experiment, we intermittently exposed all corals to low flow (2 cm s-1) for 1.5 h and measured their photosynthesis:photosynthesis (P:R) ratio under low and moderate flow. On average, corals under OA calcified 18 % less and grew 23 % less in surface area than those at ambient pH. We observed species-specific interactive effects of OA and flow on coral physiology. P:R ratios decreased after 12 weeks of OA inA. cytherea(22 %) andP. cylindrica(28 %) under moderate flow, but were unaffected by OA under low flow. P:R ratios were stable inP. verrucosa. These results suggest that short periods of decreased water flow may modulate OA effects on some coral species, indicating that flow variability is a factor to consider when assessing long-term effects of climate change.

Effects of water flow and ocean acidification on oxygen and pH gradients in coral boundary layer

“…Similarly to P. verrucosa, the lack of substantial elevation of surface pH observed here in P. cylindrica is remarkable, considering its mild metabolic response to OA 91 . However, given that P. cylindrica may maintain a constant pH of the calcifying fluid under OA 104 , acid-base regulation could potentially differ between these two species 97 .…”

“…The smallest pH elevation was observed in P. verrucosa under OA, which also had the largest pH reduction in darkness. However, this did not underlie a larger reduction in growth under OA (investigated in a study that assessed the OA-induced metabolic changes in the same coral fragments as studied here 91 ), which was overall similar between P. verrucosa and P. cylindrica, and larger in A. cytherea. These results suggest that coral OA susceptibility is not determined by the magnitude of pH fluctuation at the coral surface, consistent with previous findings that greater seawater pH variability does not systematically enhance OA resistance 92 .…”

“…In addition, A. cytherea was also the only species with OA effects on the thickness of the O 2 CBL, highlighting the strong relationship between O 2 flux and CBL thickness. Small-scale changes could, therefore, be one of the underlying factors of the OA-induced decrease in net photosynthesis of the studied coral fragments of A. cytherea 91 and of other acroporids (e.g. 112,113 ).…”