Diel versus tidal cycles of chromophoric dissolved organic matter (CDOM) and radon in a coral reef in the Great Barrier Reef

Excessive ultra-violet (UV) exposure acts synergistically with ocean warming to trigger coral bleaching. Photoinhibition in corals may be reduced by chromophoric dissolved organic matter (CDOM) absorbing UV light. This study assesses whether submarine groundwater discharge (SGD) delivers CDOM to the coral reef lagoon of One Tree Island (Great Barrier Reef). SGD was traced using radon (222Rn), a naturally occurring tracer. Time series observations revealed tidal radon and diel CDOM trends at One Tree Island. CDOM exhibited a strong negative correlation with PAR (R2= 0.50), and a weak positive correlation with 222Rn (R2= 0.13). Removal of time and light as drivers of CDOM resulted in residuals linked to radon at low tide and at night, but not at high tide or during the day. We conclude that CDOM was driven primarily by PAR fluctuations and light-driven oxidation with only minor contributions via 222Rn-traced SGD in this lagoon.