CROSS-REEF DIFFERENCES IN BASELINE THERMAL EXPOSURE RENDER FOREREEF CORALS MORE SUSCEPTIBLE TO WARMING THAN BACKREEF AND NEARSHORE CORALS
Thirteen coral cores were extracted at water depths of 4-5 m from forereef (7), backreef (3) and nearshore (3) reef zones using a pneumatic drill. 6-mm-thick slabs were sectioned vertically from the center of each core and X-rayed. Skeletal extension rates were estimated from the thickness of high-density and low-density annual growth couplets using Coral XDS software. Core chronologies were established by counting annual growth bands backwards from the 2008 growth band at the top of the core. Local SST records for the forereef, backreef, and nearshore reef zones were obtained from NOAA's Optimum Interpolation Sea Surface Temperature database (0.25 deg).
Our findings reveal that between 1982 and 2008 on the Mesoamerican Reef in the western Caribbean Sea, skeletal extension rates of forereef colonies declined by 1.7 ± 0.2 mm yr-1 as seawater temperature increased by 1.3°C, whereas extension rates of backreef and nearshore colonies were not impacted by comparable warming. This apparent resilience of backreef and nearshore corals to recent ocean warming may have evolved from their exposure to greater baseline diurnal and seasonal thermal fluctuations than exist for forereef colonies. A similar pattern emerged within the forereef zone, where colonies from the more thermally stable northern portions of the forereef zone (more proximal to the open ocean) exhibited a more negative calcification response to recent seawater warming than colonies from the more thermally variable southern portions of the forereef zone (more distal from the open ocean). These findings reveal how corals have responded to recent anthropogenic warming, offer insights into how corals will respond to future warming, and shed light on how coral biomineralization has evolved in response to cross-reef differences in baseline thermal exposure.