Growth responses of the reef-building coral Montastraea annularis along a gradient of continental influence in the southern Gulf of Mexico

Skeletal growth variables [density (g cm⁻³), extension rate (cm yr⁻¹), and calcification rate (g cm⁻² yr⁻¹)] were determined for annual density bands in skeletal slabs of the reef building coral Montastraea annularis. Colonies growing at the same depth (10 m) were collected from six coral reefs distributed along an environmental gradient of continental influence within the southern Gulf of Mexico. No significant differences in growth variables were evident within each reef for the period of time represented by the slabs. Variations in calcification rate are more closely linked with variations in extension rate than with variations in density. Differences among reefs were significant for the three growth variables and appeared to relate to the environmental gradient dominated by turbidity and sediment load. Density and calcification rate increased from high to low turbidity and sediment load, while extension rate followed an inverse trend. The data suggest that, as corals experience a harsher environment, they respond by extending their skeleton more with the same or less calcium carbonate, with a concomitant reduction of skeletal density. This 'stretching' modulation of skeletal growth may be a wide-spread phenomenon.