Internal Phosphorus and Nitrogen Loading of a Tropical Lagoon System : Combined Effects of pH, Temperature, and Batch Hydrodynamic Perturbation

Abstract

Internal nutrient loading from sediments is a major driver of eutrophication in aquatic environments, particularly in shallow tropical lagoon systems subject to intense anthropogenic pressures. While the individual roles of pH, temperature, and hydrodynamic disturbance on the sedimentary mobilization of phosphorus (P) and nitrogen (N) are well established in temperate ecosystems, their interactions in oxic tropical environments remain poorly documented. This study provides an original experimental quantification of these combined effects through batch incubations (closed system, 168 h) conducted on sediments from Grand Nokoué (southern Benin), using a 3×2×2 factorial design combining three pH levels (5, 7, 10), two temperature levels (ambient 24.02 °C; 33.5 °C), and two disturbance intensities (0 and 50 rpm). Cumulative concentrations of total phosphorus (TP), orthophosphates ( ), total nitrogen (TN), nitrates ( ), nitrites ( ), and ammonium ( ) were determined by UV-Visible spectrophotometry (at t = 0, 6, 24, 120, and 168 h), and net fluxes were calculated by mass balance adapted to batch mode. Sequential fractionation of sedimentary phosphorus (P-exch, P-Fe/Mn, P-Fe, P-Al, P-Ca, P-org, P-res) was performed before and after nutrient release into the water column under selected conditions according to combined method of sequential fractionation of sedimentary phosphorus; a total of 180 samples were obtained from 3 sampling sites. The results establish a pH > Temperature > Perturbation control hierarchy on the batch net fluxes. The highest net TP fluxes are achieved under acidic conditions at room temperature without disturbance (condition 010: 124.4 μg m⁻2 j-1), while the combined action of the three factors accelerates the release kinetics up to 6-fold, reaching the peak as early as 6 h. Fractionation reveals that the P-Fe and P-Fe/Mn fractions govern short-term mobilization and that perturbation enriches the post-incubation P-exch fraction, demonstrating a high potential for recurrent remobilization. These results pave the way for electrochemical recovery of these nutrients to preserve aquatic ecosystems and ensure sustainable agriculture.