Net community calcification and production rates from Palmyra Atoll using a boundary layer gradient flux approach

Coral reefs are a highly dynamic system, where large variability in environmental conditions (e.g. pH) occurs on timescales of minutes to hours. Yet, techniques that are capable of monitoring reef calcification rates without artificial confinement on the same frequency are scarce. Here, we present a 2 week time series of sub-hourly, in situ benthic net community production (P_net) and net community calcification (G_net) rates from a reef terrace at Palmyra Atoll using the Benthic Ecosystem and Acidification Monitoring System (BEAMS). The net metabolism rates reported here are measured under natural conditions, without any alterations to the environment (e.g. light, flow, pH). The BEAMS measures the chemical gradient and the current velocity profile in the benthic boundary layer using autonomous sensors to calculate the chemical flux from the benthos. The O₂ and total alkalinity (TA) fluxes were used to calculate P_net and G_net, respectively; TA gradients were calculated from pH and O₂ measurements. G_net can be constrained to better than 3 mmol CaCO₃ m^-2 hr^-1 using this approach, based on three simultaneous BEAMS deployments. A clear diel cycle of G_net was observed, where the peak day time G_net and average nighttime G_net were 14 and 1 mmol CaCO₃ m^-2 hr^-1, respectively. Integrated daily G_net ranged from 76 to 219 mmol CaCO₃ m^-2 d^-1, with an average of 107 ± 14 mmol CaCO₃ m^-2 d^-1. Light had the strongest control over G_net, with current velocity having a smaller yet noticeable effect. During the deployment, pH varied by 0.16 (ranged between 7.92 and 8.08), and a significant positive relationship was observed between pH and G_net. However, pH was also positively correlated with current velocity and P_net, making it difficult to determine if natural variability in pH was significantly affecting G_net on the timescale of days to weeks.