Net ecosystem production, calcification and CO2 fluxes on a reef flat in Northeastern Brazil

• First assessment of carbonate system in a South Atlantic reef.
• Extreme high nighttime total alkalinity, dissolved inorganic carbon and pCO2.
• Metabolism of sediment and water advection major controls for net daily CO2 evasion.
• Benthic metabolism in reef flat pools during low tide promote diurnal CO2 invasion.
• Evasion to atmosphere can be an important component of reef carbon budget.

The carbon cycle in coral reefs is usually dominated by the organic carbon metabolism and precipitation-dissolution of CaCO3, processes that control the CO2 partial pressure (pCO2) in seawater and the CO2 fluxes through the air–sea interface. In order to characterize these processes and the carbonate system, four sampling surveys were conducted at the reef flat of Coroa Vermelha during low tide (exposed flat). Net ecosystem production (NEP), net precipitation-dissolution of CaCO3 (G) and CO2 fluxes across the air–water interface were calculated. The reef presented net autotrophy and calcification at daytime low tide. The NEP ranged from −8.7 to 31.6 mmol C m−2 h−1 and calcification from −13.1 to 26.0 mmol C m−2 h−1. The highest calcification rates occurred in August 2007, coinciding with the greater NEP rates. The daytime CO2 fluxes varied from −9.7 to 22.6 μmol CO2 m−2 h−1, but reached up to 13,900 μmol CO2 m−2 h−1 during nighttime. Carbon dioxide influx to seawater was predominant in the reef flat during low tide. The regions adjacent to the reef showed a supersaturation of CO2, acting as a source of CO2 to the atmosphere (from −22.8 to −2.6 mol CO2 m−2 h−1) in the reef flat during ebbing tide. Nighttime gas release to the atmosphere indicates a net CO2 release from the Coroa Vermelha reef flat within 24 h, and that these fluxes can be important to carbon budget in coral reefs.