Hysteresis between coral reef calcification and the seawater aragonite saturation state

Some predictions of how ocean acidification (OA) will affect coral reefs assume a linear functional relationship between the ambient seawater aragonite saturation state (Ω a ) and net ecosystem calcification (NEC). We quantified NEC in a healthy coral reef lagoon in the Great Barrier Reef during different times of the day. Our observations revealed a diel hysteresis pattern in the NEC versus Ω a relationship, with peak NEC rates occurring before the Ω a peak and relatively steady nighttime NEC in spite of variable Ω a . Net ecosystem production had stronger correlations with NEC than light, temperature, nutrients, pH, and Ω a . The observed hysteresis may represent an overlooked challenge for predicting the effects of OA on coral reefs. If widespread, the hysteresis could prevent the use of a linear extrapolation to determine critical Ω a threshold levels required to shift coral reefs from a net calcifying to a net dissolving state.