Evaluating measurements of coral reef net ecosystem calcification rates

Monitoring the rates and drivers of coral reef net ecosystem calcification (NEC) under anthropogenic environmental change is critical for predicting associated changes in reef structures and ecosystem services. However, NEC studies to date show weak agreement between studies and notably reveal no relationship between NEC and benthic calcifier cover. In this study, we tested for the sensitivity of calculated NEC to uncertainties in seawater depths and residence times (+/- 83% relative to 6 m and 6 h, respectively) using a coral reef total alkalinity (AT) simulator (reefCATS) and found that these errors can interact to drive large asymmetric uncertainties ranging from - 91% to ? 1000% in NEC. Furthermore, numerical simulations of hypothetical NEC for coral populations occupying reefs with increasing structural complexity (rugosity = 1-4) showed that the effects of reef-scale rugosity on NEC can be as important as benthic community composition. As a result, uncertainties in seawater depth, residence time, and/or reef structural complexity are enough to mask any potential real correlation between NEC and percent calcifier cover in the field. To improve comparability and validity of NEC studies, we recommend that future studies place a high degree of scrutiny on measurements of seawater hydrodynamics, report all NEC equation parameters +/- uncertainties, and ideally include benthic community composition and structural complexity data to further explore the relationship between NEC and calcifier cover.