To estimate site specific net CaCO3 accretion rates and to relate calcification/dissolution to natural variability in pH, the SeaFET sensors were co-located with Calcification/Accretion Units (CAUs) (Fig. S1 ). A CAU consisted of a pair of roughly sanded PVC plates (10×10 cm) stacked 1 cm apart and plate pairs (N = 5 per site) were affixed to reef pavement at each site (>0.5 m apart and 10 cm above the substrate) using stainless steel rods and marine epoxy. Immediately after collection, all four surfaces of each CAU were photographed to determine early-successional community structure using the image analysis software PhotoGrid 1.0 (25 stratified random points analyzed per surface); organisms were sorted into ecological functional groups to look for patterns structuring the communities on the benthos and on the CAUs. Plates were then preserved in 8% formalin for subsequent measures of calcification rates.
To quantify the mass of CaCO3 accumulated on a CAU, the plates were dried to a constant weight at 60°C and then weighed. Subsequently, CAUs were submerged in 5% HCl for 48 hrs or until all CaCO3 had dissolved. The remaining fleshy tissue was scraped onto pre-weighed 11 µm cellulose filter paper, vacuum filtered, dried, and weighed to determine the difference in calcified to fleshy biomass on CAU surfaces. Finally, the acidified, scraped, and dried CAU plates were weighed. Calcimass was determined by subtracting the weight of the fleshy tissue and PVC plates from the total mass of the CAU. For all taxa recruiting to CAUs, the polymorph of CaCO3 deposited is known. Thus, the relative net accretion for each polymorph (calcite, aragonite, high Mg calcite) was calculated by multiplying the net calcification rate by the relative abundance of each calcifying taxa of known mineralogy.
To quantify the mass of CaCO3 accumulated on a CAU, the plates were dried to a constant weight at 60°C and then weighed. Subsequently, CAUs were submerged in 5% HCl for 48 hrs or until all CaCO3 had dissolved. The remaining fleshy tissue was scraped onto pre-weighed 11 µm cellulose filter paper, vacuum filtered, dried, and weighed to determine the difference in calcified to fleshy biomass on CAU surfaces. Finally, the acidified, scraped, and dried CAU plates were weighed. Calcimass was determined by subtracting the weight of the fleshy tissue and PVC plates from the total mass of the CAU. For all taxa recruiting to CAUs, the polymorph of CaCO3 deposited is known. Thus, the relative net accretion for each polymorph (calcite, aragonite, high Mg calcite) was calculated by multiplying the net calcification rate by the relative abundance of each calcifying taxa of known mineralogy.
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