Abstract
It is proposed that emissions of volatile sulfur compounds by coral reefs contribute to the formation of a biologically-derived feedback on sea surface temperature (SST) through the formation of marine biogenic aerosol (MBA). The direction and strength of this feedback remains uncertain and constitutes a fundamental constraint on predicting the ability of corals to cope with future ocean warming. We investigate the effects of elevated SST and irradiance on satellite-derived fine-mode aerosol optical depth (AOD) throughout the Great Barrier Reef, Australia (GBR) over an 18-year time period. AOD is positively correlated with SST and irradiance and increases two-fold during spring and summer with high frequency variability. As the influence of non-biogenic and distant aerosol sources are found to be negligible, the results support recent findings that the 2,300 km stretch of coral reefs can be a substantial source of biogenic aerosol and thus, influence local ocean albedo. Importantly however, a tipping point in the coral stress response is identified, whereby thermal stress reaches a point that exceeds the capacity of corals to influence local atmospheric properties. Beyond this point, corals may become more susceptible to permanent damage with increasing stress, with potential implications for mass coral bleaching events.
Highlights
Ocean warming poses one of the greatest threats to coral reefs worldwide, with the incidence of coral bleaching and mortality increasing as sea surface temperature (SST) continues to rise[1,2,3,4]
The results suggest that aerosol concentration over the GBR is strongly influenced by irradiance levels, which are compounded by high SST in spring and summer and extremely low tides in winter
The results presented here suggest that corals in the GBR can act as a substantial source of biogenic aerosol and support recent findings that corals can influence local atmospheric properties through stress-induces emissions of DMS and other VOCs8,15,63
Summary
Ocean warming poses one of the greatest threats to coral reefs worldwide, with the incidence of coral bleaching and mortality increasing as sea surface temperature (SST) continues to rise[1,2,3,4]. DMSP is the precursor to dimethylsulfide (DMS), a volatile gas which is oxidised by hydroxyl radicals (OH) in the atmosphere to sulfur dioxide (SO2), methane sulfonic acid (MSA) and sulfuric acid (H2SO4)[27] These aerosol precursor compounds may contribute to the growth of existing particles or nucleate to form new non-sea salt sulfate (nss-SO4) particles which can act as efficient cloud condensation nuclei (CCN). In pristine coral reefs such as the Western Pacific Warm Pool (WPWP), DMS emissions and enhanced LLC formation are thought to be the key drivers behind an ocean thermostat which suppresses ocean warming below coral thermal tolerance thresholds (~30 °C)[31,32,33] Despite corals in this region living close to their thermal maxima, few coral bleaching events have been recorded over the past 25 years[31]. Seasonal increases in DMS emissions from the GBR have been observed during spring and summer, during low tides and periods of high rainfall when aerial exposure and hypo-salinity affect coral physiology[15,38]
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