Breathing reefs: carbon exchange in tropical coral reef ecosystems

Carbon is continually exchanged between the air-sea surface interface in response to disparities in the partial pressures between CO2 levels in the atmosphere and ocean, respectively. This process is also impacted by the presence of marine organisms and the relative metabolic pulse of communities. Of those assemblages, coral reefs are known to contribute significantly to local CO2 exchange at the air-sea surface and it is widely accepted that the biological activity on reefs makes them a net source of CO2 to the atmosphere (Ware et al., 1992). However, impacts of climate change are altering reef community compositions with phase shifts from coral to algal dominated systems now documented throughout all oceanic regions (Bruno, et al., 2001). Alterations to the community structures on reefs have huge implications for local and regional uptake of carbon above these ecosystems, as calcifying and algal communities are dominated by different primary processes, affecting water carbonate chemistry differently. Research suggests that macroalgal dominated systems may reverse the role of coral reefs from a net source to a net sink to the atmosphere. A lab-based mesocosm experiment (26-hours) was run and measurements of the relative CO2 efflux above key benthic communities (n =3) was measured. Communities emulated past (calcifying; coral and CCA; coral dominated), present (mixed; coral, CCA, macroalgae; no species dominance) and future (macroalgal; macroalgae and CCA; macroalgae dominated) reef communities and differences between CO2 evasion investigated. In addition, water chemistry measurements were collected concurrently with flux data to record community-specific metabolism and help define the causal effects of the diel patterns observed. No significant differences between the CO2 efflux above communities were found. All communities experienced negative net ecosystem productivity and reduced rates of calcification throughout the experiment, with no significant differences detected between communities or time of day, suggesting reduced organism functionality and low levels of metabolic activity in tanks. In contrast, significant differences were detected for pCO2 and dissolved oxygen levels, contradicting findings of reduced metabolic activity in tanks indicated by low NEP and calcification rates. In conclusion, although CO2 efflux did not significantly differ in relation to the benthic community in this experiment, the contradicting results describing metabolic activity in tanks mean differences in CO2 efflux between communities may exist, although this study would need to be repeated to gain a conclusive answer