Colony size and turbulent flow speed modulate the calcification response of the coral Pocillopora verrucosa to temperature

For poikilothermic organisms in aquatic environment, temperature strongly affects metabolism and modulates the extent to which it is mass transfer limited. The effects of temperature on metabolism are translated into organism performance depending on the organism size, morphology, and the flow regime to which they are exposed. Environmental forcing of metabolism is particularly germane to tropical reef corals, because they increasingly are exposed to upward temperature perturbations to which they display variable responses that remain incompletely understood. In this study, we tested the effects of colony size and seawater turbulence on the response of the common Indo-Pacific branching coral, Pocillopora verrucosa, to different seawater temperatures. Using whole-colony calcification as a response variable, 12 tanks (each 150 l) were used in two trials lasting 14 days to contrast the effects of seawater turbulence (two levels) and temperature (25.5 vs 29.5 degrees C) on colonies varying in size from similar to 4 to 13-cm diameter. Turbulence in the tanks was measured as the root mean square (rms) turbulent flow speed (q(rms)), which quantified the variation in speed of complex and non-linear flow, regardless of direction. Treatments contrasted low (q(rms) = 0.63 cm s(-1)) and high (q(rms) = 2.07 cm s(-1)) turbulence that were ecologically relevant for shallow coral reefs. The effects of turbulent flow speed and temperature on calcification (mg day(-1)) differed among colony sizes: at 25.5 degrees C, calcification increased with tissue surface area at a slower rate under high, compared to low turbulent flow speeds; at low turbulent flow speeds, calcification increased with tissue surface area more slowly at 29.5 versus 25.5 degrees C. These results show that the response of branching corals to temperatures depends on colony size and turbulent flow speed. This outcome suggests that spatial heterogeneity in turbulent flow regimes across a coral reef may provide opportunities for branching corals to exploit colony size to modulate susceptibility to rising temperature.