Declining coral calcification in massive Porites in two nearshore regions of the northern Great Barrier Reef

Abstract Temporal and spatial variation in the growth parameters skeletal density, linear extension and calcification rate in massive Porites from two nearshore regions of the northern Great Barrier Reef (GBR) were examined over a 16‐year study period. Calcification rates in massive Porites have declined by approximately 21% in two regions on the GBR ∼450 km apart. This is a function primarily of a decrease in linear extension (∼16%) with a smaller decline in skeletal density (∼6%) and contrasts with previous studies on the environmental controls on growth of massive Porites on the GBR. Changes in the growth parameters were linear over time. Averaged across colonies, skeletal density declined over time from 1.32 g cm −3 (SE = 0.017) in 1988 to 1.25 g cm −3 (0.013) in 2003, equivalent to 0.36% yr −1 (0.13). Annual extension declined from 1.52 cm yr −1 (0.035) to 1.28 cm yr −1 (0.026), equivalent to 1.02% yr −1 (0.39). Calcification rates (the product of skeletal density and annual extension) declined from 1.96 g cm −2 yr −1 (0.049) to 1.59 g cm −2 yr −1 (0.041), equivalent to 1.29% yr −1 (0.30). Mean annual seawater temperatures had no effect on skeletal density, but a modal effect on annual extension and calcification with maxima at ∼26.7 °C. There were minor differences in the growth parameters between regions. A decline in coral calcification of this magnitude with increasing seawater temperatures is unprecedented in recent centuries based on analysis of growth records from long cores of massive Porites . We discuss the decline in calcification within the context of known environmental controls on coral growth. Although our findings are consistent with studies of the synergistic effect of elevated seawater temperatures and p CO 2 on coral calcification, we conclude that further data on seawater chemistry of the GBR are required to better understand the links between environmental change and effects on coral growth.