High‐resolution carbon budgets on a Palau back‐reef modulated by interactions between hydrodynamics and reef metabolism

Abstract

Diel trends in carbon chemistry and hydrodynamic regimes were measured on a coral‐dominated back‐reef community in the Republic of Palau, western Pacific. Observed diel ranges over 5 d were pH 7.92–8.09, partial pressure of carbon dioxide 33.4–52.7 Pa, measured total alkalinity 2080–2272 µmol kg−1, total dissolved inorganic carbon 1763–1939 µmol kg−1, aragonite saturation state 2.84–3.98, and calcite saturation state 4.44–5.67. The combination of reef metabolism and hydrodynamic regimes drives this variability. We report net community calcification (NCC) and net community production (NCP) rates of −20.4 (± 7.8) to 41.2 (± 37.7) mmol CaCO3 m−2 h−1 and 77.3 (± 95) to 64 (± 22) mmol C m−2 h−1, respectively. Using a control volume approach, we show that NCC and NCP rate estimates are quite sensitive to the effective control volume dimensions. At this site, tracking vertical mixing of water masses within the control volume shows that active water mixing is often confined to a region 2–3 m above the corals. Vertical carbon mass fluxes into and out of the benthic environment were unexpectedly large for the slow current flow rates observed, suggesting that coral morphology or reef rugosity may affect mass transport of carbon and nutrients. The ranges of NCC and NCP values highlight the need for additional field work on various reefs to tune and optimize the control volume approach to achieve its full potential for reef metabolism monitoring.