Environmental Context. Coral reefs have now been highlighted as significant sources of dimethylsulfide and other organic sulfur compounds, which may be important for the formation of clouds over the ocean and climate regulation. However, no studies have reported the seasonal and diurnal cycles of these organic sulfur substances in reef waters. This study describes the cycling of dimethylsulfide and related organic substances at One Tree Reef, in the southern part of the Great Barrier Reef, as well as their production from staghorn coral in chamber experiments. The results suggest that coral reefs are significant sources of dimethylsulfide to reef waters and possibly the reef atmosphere, but the effect of this substance on the radiative climate over the Great Barrier Reef is unknown.

Abstract. Seasonal and diurnal studies conducted at One Tree Reef lagoon in the Great Barrier Reef (GBR) highlight increased production of dissolved dimethylsulfide, atmospheric dimethylsulfide, dissolved and particulate dimethylsulfoniopropionate, and dimethylsulfoxide during the summer months, and when the lagoon is moated with inter-reefal water. This production is due to both the growth of phytoplankton populations within the lagoon, and benthic corals and algae in or on sediments and attached to solid substrates. The relative importance of these two processes was not determined in this study, although benthic production from corals was shown to be significant in chamber experiments. The diurnal cycles of the organic sulfur substances measured at One Tree Reef provide unequivocal evidence that coral reefs produce significant quantities of these organic sulfur substances during the day and suggest that coral reefs could be significant sources of atmospheric dimethylsulfide.