Marine & Environmental Sciences Faculty Articles

Title

Spatial distribution of CO2, CH4 and N2O in the Great Barrier Reef revealed though high resolution sampling and isotopic analysis

Document Type

Article

Publication Date

7-30-2021

Publication Title

Geophysical Research Letters

Keywords

Air/sea interactions, Coral reef systems (4916), Physical and biogeochemical interactions, Continental shelf and slope processes (3002), Gases, Carbon dioxide, Methane, Nitrous oxide, Greenhouse gas, Coral reef, Continental shelf

ISSN

1944-8007

Volume

48

First Page

e2021GL092534

Abstract

Methane (CH4) and nitrous oxide (N2O) dynamics in coastal coral reef areas are poorly understood. We measured dissolved carbon dioxide (CO2), and CH4 (with δ13C-CO2 and δ13C-CH4 isotope fractions) and N2O in the Great Barrier Reef (GBR) to determine spatial distributions and emissions. CO2 (379-589μatm) was oversaturated from calcification and riverine sources, as indicated by depleted δ13C-CO2 values. CH4 (1.5-13.5nM) was also oversaturated from nearshore biogenic sources indicated by depleted δ13C-CH4 and probable offshore aerobic production . N2O (5.5-6.6nM) was generally undersaturated, with uptake highest near the coast. Daily CO2 emissions were 5826 ±1191 tonnes, with CO2 equivalent (eq) N2O uptake (191 ±44 tonnes) offsetting 3.3% of CO2 or 89% of CH4eq (214 ±45 tonnes) emissions based on 20-year global warming potentials. The GBR was a slight CO2 and CH4 source and N2O sink during our study. However, further work is required to constrain diurnal, seasonal, and spatial dynamics.

Plain Language Summary

The oceans absorb carbon dioxide from the atmosphere but can emit the more potent greenhouse gases of methane and nitrous oxide. However, 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 characterise 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 can contrast with most open oceanic systems.

Comments

We acknowledge funding received from the Australian Research Council [LE120100156, FT170100327 (IRS), and DE180100535 (DT)] and the Great Barrier Reef Foundation. We also express our gratitude to the crew of the vessel RV Cape Fergusson and Australian Institute of Marine Science for use of the vessel. We wish to thank colleagues from Southern Cross University's 'School of Environment Science and Engineering; and 'Southern Cross Geoscience' for technical and logistical support.

Additional Comments

This article is protected by copyright. All rights reserved.

ORCID ID

0000-0003-3556-7616

DOI

10.1029/2021GL092534

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