Potential Applications of Light Absorption Coefficients in Assessing Water Optical Quality: Insights from Varadero Reef, an Extreme Coral Ecosystem

Coral reefs exposed to chronically turbid conditions challenge conventional assumptions about the optical environments required for reef persistence and productivity. This study investigates the utility of light absorption coefficients as indicators of optical water quality in Varadero Reef, an extreme coral ecosystem located in Cartagena Bay, Colombia. Field campaigns were conducted across three seasons (rainy, dry, and transitional) along a transect from fluvial to marine influence. Absorption coefficients at 440 nm were derived for particulate (ap(440)) and chromophoric dissolved organic matter (aCDOM(440)) to assess their contribution to underwater light attenuation. Average values across seasons show that ap(440) reached 0.466 m−1 in the rainy season (September 2021), 0.285 m−1 in the dry season (February 2022), and 0.944 m−1 in the transitional rainy season (June 2022). Meanwhile, mean aCDOM(440) values were 0.368, 0.111, and 0.552 m−1, respectively. These coefficients reflect the dominant influence of particulate absorption under turbid conditions and increasing aCDOM(440) relevance during lower turbidity periods. Mean Secchi Disk Depth (ZSD) ranged from 0.6 m in the rainy season to 3.0 m in the dry season, aligning with variations in Kd PAR, which averaged 2.63 m−1, 1.13 m−1, and 1.08 m−1 for the three campaigns. Chlorophyll-a concentrations at 1 m depth also varied significantly, with average values of 2.3, 2.7, and 6.2 μg L−1, indicating phytoplankton biomass peaks associated with seasonal freshwater inputs. While particulate absorption limits light penetration, CDOM plays a potentially photoprotective role by attenuating UV radiation. The observed variability in these optical constituents reflects complex hydrodynamic and environmental gradients, providing insight into the mechanisms that sustain coral functionality under suboptimal light conditions. The absorption-based approach applied here, using standardized spectrophotometric methods, proved to be a reliable and reproducible tool for characterizing the spatial and temporal variability of IOPs. We propose integrating these indicators into monitoring frameworks as cost-effective, component-resolving tool for evaluating light regimes and ecological resilience in optically dynamic coastal systems.