A multi-tracer model approach to estimate reef water residence times

We present a new method for obtaining the residence time of coral reef waters and demonstrate the successful application of this method by estimating rates of net ecosystem calcification (NEC) at four locations across the Bermuda platform and showing that the rates thus obtained are in reasonable agreement with independent estimates based on different methodologies. The contrast in Be-7 activity between reef and offshore waters can be related to the residence time of the waters over the reef through a time-dependent model that takes into account the rainwater flux of Be-7, the radioactive half-life of Be-7, and the rate of removal of Be-7 on particles estimated from Th-234. Sampling for Be-7 and Th-234 was conducted during the late fall and winter between 2008 and 2010. Model results yielded residence times ranging from 1.4 (+/- 0.7) days at the rim reef to 12 (+/- 4.0) days closer to shore. When combined with measurements of salinity-normalized total alkalinity anomalies, these residence times yielded platform-average NEC rates ranging from a maximum of 20.3 (+/- 7.0) mmolCaCO(3) m(-2) d(-1) in Nov 2008 to a minimum of 2.5 (+/- 0.8) mmolCaCO(3) m(-2) d(-1) in Feb 2009. The advantage of this new approach is that the rates of NEC obtained are temporally and spatially averaged. This novel approach for estimating NEC rates may be applicable to other coral reef ecosystems, providing an opportunity to assess how these rates may change in the context of ocean acidification.