Field Validation of Chromophoric Dissolved Organic Matter (CDOM) Absorbance as a Proxy for Dissolved Organic Carbon (DOC) in Tropical Coastal Waters Influenced by Aquaculture Effluent

Abstract Aquaculture effluent is a growing source of organic pollution in tropical coastal waters, yet cost-effective and scalable monitoring tools remain limited. This study evaluated the feasibility of using chromophoric dissolved organic matter (CDOM) absorbance, particularly at 254 nm (a₂₅₄), as a proxy for dissolved organic carbon (DOC) in aquaculture-impacted marine environments. Field surveys were conducted at three distinct aquaculture systems in North Bali, Indonesia, including marine finfish cages, shrimp pond discharge, and seaweed farms, using spatial transects, vertical profiling, and temporal sampling across tidal phases and feeding cycles. CDOM absorbance and DOC concentrations were measured at multiple distances from effluent discharge points and depths, while controlled laboratory dilution experiments established quantitative a₂₅₄-DOC relationships for each effluent source.Results showed strong linear correlations between a₂₅₄ and DOC across all systems (R² ≥ 0.99), with relatively consistent regression slopes, indicating a conserved optical yield of DOC among tropical aquaculture effluents. CDOM and DOC concentrations declined with increasing distance from source, and increased following feeding activity, demonstrating responsiveness to operational and hydrodynamic conditions. Spectral slope (S₂₇₅–₂₉₅) and the E₂/E₃ ratio consistently indicated a shift toward higher molecular weight and more aromatic DOM near discharge zones. Principal component analysis further distinguished aquaculture-influenced waters from background conditions based on CDOM-DOC signatures and ammonium loading.These findings confirm that CDOM absorbance is a sensitive, rapid, and cost-effective tool for tracking organic pollution from aquaculture. The integration of field and laboratory data across multiple systems enhances the potential for regional application and supports the adoption of optical proxies in sustainable coastal aquaculture management.