Scaling-up carbon and carbonate metabolism of coral reefs using in-situ data and remote sensing

Areal up-scaling at reef-scale of organic and inorganic metabolism is possible using in-situ measurements and remote sensing data providing the extent of each bottom type inside the reef. Using a SPOT image and published values of metabolism, the gross production (93,560᎒³ kg C year^-1), excess production (10,017᎒³ kg C year^-1) and calcification (165,348᎒³ kg CaCO₃ year^-1) over 35 km² of coral reef environment in Moorea Island (French Polynesia) are estimated. While the computations are straightforward, certain assumptions must be made in order to conduct the scaling exercise. The exercise is valid only if the metabolism of reef benthos is additive through increasing spatial scale. Despite the difficulty of quantitatively assessing our extrapolations, spatial additivity seems to represent the reality. The other limitation is that the reef must be considered as a closed system, in an equilibrium state supposedly accurately described by the few available in-situ measurements. To consider the reef an open system, long-term metabolic measurements coupled with knowledge of oceanic and land forcing processes are required. These theoretical considerations point to the necessity of integrated multi-scale studies based on both remote sensing and in-situ data in order to better understand the productivity and calcification of reefs in the current global change context.