The Varadero Reef is located south-west of the Cartagena Bay close to the southern strait that connects the Bay to the Caribbean Sea in Colombia. The Bay is a receiving estuary from the Magdalena River through the Canal del Dique, a man-made channel whose construction and operation dates back almost a century. The bathymetric distribution of the Orbicella species complex was evaluated at two reference sites with contrasting exposure to the Dique channel freshwater plume: 1) Varadero reef close to the Dique channel mouth (10°18’23.3”N, 75°35’08.0”W), and 2) Rosario reef located 21 km southwest of Varadero, significantly less exposed to the turbid plume from the Dique channel (10°11'12.1"N, 75°44'43.0"W).

To estimate the potential negative effect of perturbing the optical properties of water and light penetration on coral survival, we performed ecological surveys of Orbicella spp. populations. Populations were sampled along the whole depth range at which Orbicella spp. were observed. Random dives near the experimental sites allowed us to define the depth limits of distribution. The number of colonies of each species was recorded on 10x1 m belt transects deployed perpendicular to the reef slope at six depths in Varadero (2, 3, 4.5, 6, 7.5 and 9m) and eight in Rosario (3, 5, 7, 9, 13, 17, 21 and 25m). Colony size (length D1, wide D2 and height H, in centimeters) and percentage of mortality (both recent and old mortality, using the AGRAA protocol) were also recorded at all transects.

A diving computer Leonardo Cressi Sub was used to determine the depths of surveys. A 50m measuring tape was used to determine the length of the belt transect. A 1m PVC pipe carried by the diver was used to determine the width of each transect.