How do corals from a suboptimal environment survive? The story of the Varadero reef in Colombia

The Varadero Reef is a relic of a wider reef system located close to one of the straits that connect the Bay of Cartagena in Colombia to the Caribbean Sea. This reef has remained relatively healthy despite the poor environmental conditions associated to significant freshwater discharges with high sediment loads from the Magdalena River through the Canal del Dique. The aim of this study was to evaluate how massive reef-building corals survive in this particular environment, opposite to the typical environment in which most coral reefs thrive. Preliminary results from a reciprocal transplant experiment revealed that corals from Varadero developed a “low light” phenotype with higher survivorship rates, despite being located at nearly 3 m depth. Our experiment also demonstrated a severe but highly variable attenuating effect of the Canal del Dique plume on the underwater light in Varadero, with periods of both drastically high and low light attenuation coefficient (Kd) in the water column. Models of daily photosynthetic productivity suggests that the complex dynamics of the light field in Varadero can favor the survivorship of massive corals in the shallow areas of the reef but at the same time impose a constraint for these corals to thrive in deeper areas. This study demonstrates that the dynamics of the underwater light field is a critical component on the photo-acclimatory condition and the survivorship of corals; its adequate knowledge at a particular site could increase the effectiveness of restoration or recovery actions. Further studies in the Varadero reef can reveal other unforeseen mechanisms and adaptation/acclimation strategies of corals under stressed environments.