TITLE:
Optical Control of Phytoplankton Primary Production by Colored Dissolved Organic Matter (CDOM) in Urban Lakes of Yamoussoukro
AUTHORS:
Koan Olivier Bagui, Kalou Chrislaire Toh, Jocelyne Mamaket Bosson, Assoumou S. Doria Yamoa, Kouabenan Anicet Kouakou, Jérémie Thouakesseh Zoueu
KEYWORDS:
CDOM, Optical Control, Primary Production, Phytoplankton, Critical Threshold, Tropical Urban Lakes
JOURNAL NAME:
Journal of Water Resource and Protection,
Vol.18 No.9,
September
29,
2026
ABSTRACT: Colored dissolved organic matter (CDOM) strongly influences light penetration and biogeochemical processes in inland waters. This study investigates the role of optical control exerted by CDOM on phytoplankton primary production in thirteen urban lakes of Yamoussoukro (Côte d’Ivoire), spanning a wide absorption gradient (a355 = 0.01 - 1.90 m−1). A total of 117 water samples were collected during four monthly campaigns (July-October) with 2 to 3 sampling stations per lake according to lake size and anthropogenic influence. In situ bio-optical measurements, dissolved nutrient analyses, and primary production estimates were combined with nonlinear mixed-effects statistical models accounting for the nested data structure (lake × campaign) and controlling for environmental covariates. The results reveal a significant unimodal relationship between CDOM and primary production (marginal R2 = 0.61; p 2 = 0.72). Segmented regression identified a critical optical threshold at a355 ≈ 0.42 m−1 (95% bootstrap CI: 0.31 - 0.53), beyond which light attenuation becomes the dominant limiting factor, leading to a significant decline in productivity. Sensitivity analysis on photosynthetic parameters (±20% to 50%) and independent decoupling tests (constant Kd, direct Chl-a correlation) confirmed that this relationship is not an artefact of the modeling framework. These findings demonstrate that CDOM acts as a nonlinear optical regulator structuring the trophic dynamics of tropical urban lakes and provide a mechanistic framework for their ecological monitoring.