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 densit y bands in skeletal slabs of the reef building coral Montastraea annularis. Colonies growing at the same depth (10 m) were collected from six coral re efs distributed along an environmental gradient of continental influence wi thin the southern Gulf of Mexico. No significant differences in growth vari ables were evident within each reef for the period of time represented by t he slabs. Variations in calcification rate are more closely linked with var iations in extension rate than with variations in density. Differences amon g reefs were significant for the three growth variables and appeared to rel ate to the environmental gradient dominated by turbidity and sediment load. Density and calcification rate increased from high to low turbidity and se diment load while extension rate followed an inverse trend. The data sugges t that, as corals experience a harsher environment, they respond by extendi ng their skeleton more with the same or less calcium carbonate, with a conc omitant reduction of skeletal density. This 'stretching' modulation of skel etal growth may be a widespread phenomenon.