High resolution carbon dioxide, methane, and nitrous oxide concentrations and emissions measurements in the Great Barrier Reef lagoon, Australia

Greenhouse gas emission studies from Coral Reef settings remain sparce compared to open ocean, rivers, and estuaries. Here we sampled the greenhouse gases of carbon dioxide (CO2) with isotopic fractions (δ13C-CO2), methane (CH4) with isotopic fractions (δ13C-CH4), and nitrous oxide (N2O) in high resolution (905 individual georeferenced 5 minute averaged samples) from ship based cruises throughout the southern half of the Great Barrier Reef Australia. Time and location matched high-resolution physio-chemical data (salinity, temperature, dissolved oxygen, turbidity and chlorophyll a) were also measured to determine driving factors of greenhouse gases in the lagoon. Overall, the lagoon was oversaturated in CO2 which is consistent with previous coral reef area studies. CH4 was also oversaturated despite highly oxygenated waters that are normally unconducive for CH4 production. N2O was undersaturated and up-taking N2O from the atmosphere which contrasts with the few coral reef and continental shelf area studies. Dissolved greenhouse gases were measured with a Picarro G2201-i Cavity Ring Down Spectrometer for CO2 (200 ppb ±0.05%) and CH4 (5 ppb ± 0.05%) with respective δ12C and δ13C stable isotope fractions and a Picarro G2308 Cavity Ring Down Spectrometer for N2O concentrations (<3.5 ppb ± 0.008%). Both Cavity Ring Down Spectrometer instruments were connected in a closed loop with twin showerhead exchangers and a gas drying agent (Drierite). The showerhead exchangers received seawater from the vessel cold water intake. Both instruments averaged 300 measurements over a five-minute cycle for each greenhouse gas sample. Salinity, temperature, and turbidity were also recorded at the inlet of the seawater system with a Seabird SBE 21 (salinity ± 0.005 psu, temperature ± 0.002 °C, turbidity ± 0.04% NTU). Dissolved oxygen, pH, and Chlorophyll were recorded using respective Hydrolab MS5 multi-parameter sonde (dissolved oxygen 0.1 ± 0.02 mg L-1), SAMI-pH (<0.001 ± 0.003 pH units) and ECO 3-Triplet fluorometer (Chl-a 0.016 ± 0.025 μg L-1) instruments fitted with inline flow cells.