Collection and sample preparation:
This study was carried out in August-October 2013 in Moorea, French Polynesia, using organisms collected from the back reef of the north shore at ~1-2m depth. The organisms were used to construct communities in outdoor flumes matching the contemporary (in 2013) mean cover of a back reef in Moorea (Carpenter, 2014; Edmunds, 2014). Coral communities were built from the four dominant coral taxa found on the back reefs of Moorea: massive Porites spp. (11% cover), Porites rus (6%), Montipora spp. (3 %), and Pocillopora spp. (2 %), that together accounted for 98% of the coral cover. In addition to corals, 6% of the surface comprised crustose coralline algae that consisted of 66% Porolithon onkodes and 33% Lithophyllum flavescens. After collection of corals and algae (10cm×10 cm), they were returned to the Richard B. Gump South Pacific Research and attached to plastic supports using epoxy glue. Following preparation, samples were left to recover in a seawater table for 3 d. Sediments were collected from the lagoon on the north shore, ~200m from the reef crest, at 2m depth using 24 custom made boxes (0.4m×0.3m×0.3m). Sediment boxes were inserted into the sediment and left in situ for 4 d to allow sediment stratification to be established naturally before transfer to the flumes.

The 4 outdoor flumes consisted of a working section measuring 5.0m×0.3m×0.3m. Water was re-circulated using water pumps (W. Lim Wave II 373 J s-1) to obtain a 10 cm s-1 flow. Flow was measured across the working section of the flume using a Nortek Vectrino Acoustic Doppler Velocimeter. At each end of the flume seawater passed through an 88 cm transition section (rectangular to circular) that housed 20 cm (length) flow straighteners made of stacked, 3 cm diameter PVC pipe, and then into a 12.5 cm return section. Fresh sand-filtered seawater, pumped from Cook’s Bay at 12m depth, was dispensed continuously into the flume at 5 L min-1. Flumes experienced natural sunlight that was attenuated using screen to maintain irradiances similar to ambient irradiances in the back reefs of Moorea (daily maximum of ~1500 µmol photons m-2 cm-1 over the incubation period determined with a 4 quantum sensor LI-193 and a LiCor LI-1400 meter).

Carbonate chemistry control and measurements:
Two flumes were maintained at ambient conditions and two at a pCO2 expected by the end of the present century under a pessimistic scenario (Representative Concentration Pathway 8.5, ~1300 µatm, Moss et al., 2010). Control of the pCO2 was accomplished using a pH-stat (Aquacontroller, Neptune systems, USA) and pH was maintained 0.1 unit lower at night (from 18:00:00 to 6:00:00 LT) than during the day to match the natural diel variation in pH in the back reef of Moorea. pH was measured daily using a portable pH meter (Orion 3-stars, Thermo-Scientific, USA) fitted with a DG 115-SC pH probe (Mettler Toledo, Switzerland) calibrated every other day with Tris/HCl buffers (Dickson et al., 2007). pH also was measured spectrophotometrically using m-cresol dye (Dickson et al., 2007) at regular intervals. Measurement of total alkalinity (AT) was made using open-cell potentiometric titrations (Dickson et al., 2007) using 50mL samples of seawater collected every 2-3 d. Parameters of the carbonate system in seawater were calculated using the R package seacarb (Lavigne and Gattuso, 2013).

Calcification measurements:
Calcification rates were measured using the total alkalinity anomaly method (Chisholm and Gattuso, 1991). Calcification measurements were made every 7 d on the constructed community, and in the analysis of sediments alone, after 7, 30, and 56 d incubation. During incubations, the addition of seawater was stopped so that each flume was a closed loop; seawater samples for AT were taken every 3 h during the day and every 6 h at night. To maintain AT and nutrients close to ambient levels, water in the flumes was refreshed every 6 h for 30 min. Nutrient changes in the flumes were monitored during 4 incubations and the changes in nitrate and ammonium during incubations were < 2 µmol L-1. To conduct incubations with sediments alone, corals and coralline algae were removed from the flumes for 24 h and held in a separate tank where conditions were identical to those in the flumes. Corals and coralline algal calcification was calculated by subtracting the mean light and dark net calcification of the sediment from the community calcification. For both corals and algae, buoyant weight (Davies, 1989) was recorded before and after incubation and converted to dry weight to quantify the contribution of each functional group to the calcification budget.