Spatial Distribution of CO 2 , CH 4 , and N 2 O in the Great Barrier Reef Revealed Through High Resolution Sampling and Isotopic Analysis

Methane (CH 4 ) and nitrous oxide (N 2 O) dynamics in coastal coral reef areas are poorly understood. We measured dissolved carbon dioxide (CO 2 ) and CH 4 (with δ 13 C‐CO 2 and δ 13 C‐CH 4 isotope fractions) and N 2 O in the Great Barrier Reef (GBR) to determine spatial distributions and emissions. CO 2 (379–589 μatm) was oversaturated due to calcification and riverine sources, as indicated by depleted δ 13 C‐CO 2 values. CH 4 (1.5–13.5 nM) was also oversaturated from nearshore biogenic sources indicated by depleted δ 13 C‐CH 4 and probable offshore aerobic production. N 2 O (5.5–6.6 nM) was generally undersaturated, with uptake highest near the coast. Daily CO 2 emissions were 5826 ± 1191 tonnes, with CO 2 equivalent ( eq ) N 2 O uptake (191 ± 44 tonnes) offsetting 3.3% of CO 2 or 89% of CH 4 eq (214 ± 45 tonnes) emissions based on 20‐year global warming potentials. The GBR was a slight CO 2 and CH 4 source and N 2 O sink during our study. However, further work is required to constrain diurnal, seasonal, and spatial dynamics. The oceans absorb carbon dioxide from the atmosphere but can emit the more potent greenhouse gases of methane and nitrous oxide. Large uncertainties remain in oceanic greenhouse gas budgets due to variation in regional emissions from environmental factors such as upwelling, oxygen depletion, continental nutrient inputs and sedimentary processes in coastal areas. Here, we measured dissolved carbon dioxide, methane, and nitrous oxide in the Great Barrier Reef lagoon to characterize spatial distributions, drivers, and emissions. The lagoon was a minor source of carbon dioxide and methane to the atmosphere, with higher emissions near the coast. The primary carbon dioxide source was calcification, where carbon dioxide is produced when organisms build calcium carbonate skeletons. Methane concentrations were highest along the coast, likely from river and groundwater inputs and in‐situ production. Nitrous oxide was slightly undersaturated in the lagoon waters compared to atmospheric concentrations. Overall, the nitrous oxide uptake offset 3.2% of the combined carbon dioxide and methane emissions from the lagoon during the study period. These findings indicate coral reef greenhouse gas dynamics contrast with most open oceanic systems. The Great Barrier Reef lagoon was a source of CO 2 and CH 4 and a N 2 O sink CH 4 was supersaturated despite well oxygenated seawaters N 2 O (CO 2 equivalent) uptake offset 3.2% of the combined CO 2 and CH 4 em