Eddy Covariance measurements of CO2 exchange during monsoon and winter conditions over Heron Reef, southern Great Barrier Reef, Australia

Coral reefs cover approximately 2.8 to 6.0 x 10⁵ km² of the Earth's tropical and subtropical oceans and are among the most biologically diverse and economically important ecosystems on the planet. However relatively little information exists on the role of coral reefs in air-sea CO₂ exchange. It was thought that coral reefs were sinks of CO₂ because precipitation of CaCO₃ results in sequestering of carbon. However, the associated change in pH results in a release of CO₂ which has been estimated to be equal to 0.4 to 1.4% of anthropogenic CO₂ emissions. While these values are not great they need to be confirmed through direct measurement so that the role of coral reefs in carbon budgets can be accurately quantified.

In this paper we present results from the first in-situ Eddy Covariance measurements of air-sea CO₂ exchange conducted over the 344,400 km² Great Barrier Reef, Australia. Measurements were made in June 2009 (austral winter) and February 2010 (austral summer) using pontoon mounted eddy covariance systems at Heron Reef (23°26.577S, 151°26.577E) in the Capricorn Bunker Group of the southern Great Barrier Reef. Results show that during summer under monsoonal conditions the shallow reef flat was found to be a net source of CO₂, particularly at night while the shallow and deep lagoons of Heron Reef were net sinks. Concurrent measurements of CO₂ exchange over the shallow reef flat and ocean adjacent to Heron Reef found the ocean to be a CO₂ sink. During winter net CO₂ sequestration was found to be significantly greater than in summer. A simple CO₂ budget based on these results is presented indicating that reefs such as Heron Reef in the southern Great Barrier Reef act as CO₂ sinks.