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Concentrations of dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS), and DMS flux are reported for the Great Barrier Reef (GBR), Great Barrier Reef Lagoon (GBRL), and Coral Sea. Generally higher concentrations of dimethylsulfoniopropionate and DMS occurred in coral reef waters, compared with GBRL concentrations. DMS flux from GBR coral reefs in summer ranged from nondetectable to 153molm(-2) d(-1) (mean 6.4), while winter fluxes ranged from 0.02 to 15molm(-2) d(-1) (mean 2.4). No significant seasonal difference in DMS flux occurred for the GBRL. High DMSw concentrations and DMS fluxes periodically occur at coral reefs during very low tides and elevated sea surface temperatures (SSTs). For the GBRL and GBR coral reefs there was a significant correlation between seawater DMSw concentrations and SST (p<0.001), up to temperatures of 30 degrees C. During coral bleaching DMS flux from reefs almost completely shuts down when SSTs are >30 degrees C. The GBRL and associated coral reefs emit 439 and 32MmolS per year, respectively. Cyclones on average produce 170MmolS to the GBR atmosphere in summer. This amount can markedly increase during severe cyclones such as severe tropical Cyclone Debbie in March 2017. Overall, the annual DMS emission estimate from the GBRL and coral reefs in the GBR is 0.64GmolS, with cyclones contributing 27% or greater of the annual emission estimate, depending on the cyclone intensity. Oxidation of atmospheric DMS can potentially affect solar radiation, SSTs, low-level cloud cover, and rainfall causing cooling and warming of the climate in the GBR region as recent modeling predicts.

Plain Language Summary This study reports the first detailed measurements of the flux and emission of dimethylsulfide (DMS, a trace sulfur gas thought to be involved in climate regulation), from the Great Barrier Reef Lagoon, Coral Sea, and coral reefs that form the Great Barrier Reef (GBR). Concentrations of DMS and its main precursor dimethylsulphoniopropionate are greatest at fringing and microatoll reefs, followed by midshelf and barrier reefs on the continental shelf. Seawater DMS and DMS flux are significantly correlated with SST up to temperatures of 30 degrees C, when production of DMS markedly decreases during coral bleaching. Very low tides increase seawater DMS and dimethylsulphoniopropionate concentrations and can significantly increase the flux of DMS to the atmosphere. There is now increasing evidence that coral reef production of DMS to the atmosphere of the GBR and its effect on the aerosol optical depth could have a significant effect on the climate of NE Australia.