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 (10°18’23.3”N, 75°35’08.0”W). 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. Three study sites with contrasting light regimes were considered in order to evaluate the role of the light-environment perturbation associated with the Dique channel freshwater plume on the photosynthetic performance of corals from Varadero: 1) Varadero reef at 3.5m depth close to the Dique channel mouth (10°18’23.3”N, 75°35’08.0”W), 2) Rosario reef at 12m depth as clear-control site 21 km southwest from Varadero (10°11'12.1"N, 75°44'43.0"W), and 3) Cartagena Bay at 3m depth, the closest  site to the Dique channel mouth and most turbid among the three sites (10°18'5.80"N, 75°34'37.10"W).

The values of the photosynthetic efficiency (alpha) are expressed in umol O2 umol quanta-1, the compensating irradiance (Ec) in umol quanta m-2 s-1, the saturating irradiance (Ek) in umol quanta m-2 s-1, the average respiration rate (R-avg) as well as the respiration pre- and post-illumination (R0 and Rf) are expressed in umol O2 m-2 s-1, and the net and gross maximum photosynthesis rates (Pmax(n) and Pmax(gross)) are expressed in umoles O2 m-2 s-1.

Photosynthetic parameters of corals were obtained from PE (photosynthesis vs. irradiance) curves conducted under laboratory conditions. A custom-made water-jacket acrylic chamber with four independent hermetic compartments (~650 ml each) was used to run the PE curves, maintaining a constant temperature of 28 °C, close to natural conditions, with an external circulating water bath (Isotemp, Fisher Scientific). During each incubation, corals were submerged on filtered seawater (0.45 µm) under constant agitation by magnetic stirrers. Ten levels of irradiance between 0 and ~1400 µmol quanta m-2 s-1 were supplied at 10-min intervals with four 26 W LED bulbs (UL PAR38, LED Wholesalers Inc, USA). The light intensity was controlled with a custom-made software. The LEDs were operated in continuous mode with a multifunction I/O card (USB-6001, National Instruments Corp., USA) to avoid potential artefacts related to the effect of different pulsating frequencies on photosynthesis. Oxygen concentrations inside the compartments within the chamber were measured with a 4-channel fiber optical oxygen meter system (FireSting, Pyroscience, Germany). The photosynthetic efficiency (α), compensating irradiance (Ec), saturating irradiance (Ek), respiration rates (Rd), and maximum photosynthetic rates (Pmax), were calculated from the light-limited and light-saturated regions of the PE curves. Chlorophyll a (Chl a) content per unit of coral surface area was determined after obtaining coral tissue slurries with the help of an air gun connected to a scuba tank. Pigment extraction was performed in acetone/dimethyl sulfoxide (95:5 vol/vol) after homogenizing the slurries with a Tissue-Tearor Homogenizer (BioSpec Inc, USA). Chl a density was estimated spectrophotometrically with a modular spectrometer (Flame-T-UV-VIS, Ocean Optics Inc., USA). The specific absorption coefficient of Chl a (a*Chl a) was calculated using the equation: a*Chl a = (D675/ρ) · ln(10), where D675 is the estimated absorbance value of corals at 675 nm, calculated from reflectance (R) measurements as [D675 = log (1/R675)], and ρ is the pigment content per projected surface area (mg Chl a m-2).

The software Pyro Oxygen Logger was used to operate the fiber optical oxygen meter system. The software OceanView was used to operate there modular spectrometer.