Seasonal variability of the effect of coral reefs on seawater CO2 and air—sea CO2 exchange

Abstract

There are complex physical and biological processes controlling the exchange of carbon dioxide (CO2) between the ocean and atmosphere. In coral reef ecosystems, the balance of biological processes such as calcium carbonate (CaCO3) formation and organic carbon production can either lead to CO2 being retained in the oceanic environment (i.e., oceanic sink of CO2) or returned to the atmosphere through gas exchange (oceanic source of CO2). What remains uncertain is the fate of CO2 in reefs subject to seasonal change and the annual balance of air—sea CO2 flux in such systems. Here it is shown that the Bermuda coral reef acts as a sourc of CO2 to seawater overlying the reef. The magnitude of this source of CO2 varies seasonally in response to changes in the reef community between coral‐ and macroalga‐dominated states, reflecting changes in the net balance between calcification and organic carbon production. With knowledge of the calcification rate (~5.6 to 10.6 g CaCO3 m−2 d−1) and observed modification in seawater fCO2 by reef metabolism, rates (—0.6 to 3.3 g C m−2 d−1) and seasonal patterns of macroalgal productivity were estimated. Whether the Bermuda coral reef system acts as an oceanic sink or source of CO2 to the atmosphere not only depends on this seasonal variation, but, more importantly, depends on the pre‐existing air‐sea CO2 disequilibrium of open ocean waters surrounding the reef system. The Bermuda coral reef system serves as a useful model for understanding the fate of CO2 in other reefs, particularly those reefs changing because of environmental stress.