Study of the Spatial Distributions of Nitrogen and Phosphorus Diffusion Fluxes at the Water-sediment Interface in the Fresco Lagoon, Côte d’Ivoire

Nutrients (nitrogen and phosphorus in particular) from sediments, after diffusion, can lead to an increase in their concentrations in the overlying waters, promoting the proliferation of aquatic plants (algae, macrophytes, etc.). This could present an ecological risk due to eutrophication. Thus, the objective of the present study is to evaluate the spatial distribution of diffusion fluxes of four nutrients (NH4+-N, NO2--N, NO3--N, PO43--P) at the water-sediment interface in the Fresco Lagoon. To do this, the physicochemical parameters of the bottom waters were measured using a multiparameter. The nutrient contents of the bottom and interstitial waters were determined by the spectrophotometric method. The diffusion fluxes of nutrients at the water-sediment interface were evaluated using Fick's first diffusion law. Over the study period, the results revealed relatively high temperatures (approximately 29.55 ± 1.37°C) in the open waters, due to the shallow depth of the environment, which is favorable to the penetration of sunlight. The open waters were reducing (-23.30 ± 9.78 mV) and poorly oxygenated (2.38 ± 0.57 mg/L) due to the oxidation of organic matter. The rates of organic matter in the sediments were relatively high (20.44 ± 1.54%). The evaluation of nutrient diffusion fluxes showed that the fluxes of NH4+-N (0.036 ± 0.016 mg.m-2.d-1), NO2--N (0.002 ± 0.001 mg.m-2.d-1) and PO43--P (0.05 ± 0.01 mg.m-2.d-1) were positive and those of NO3--N were negative (-0.11 ± 0.43 mg.m-2.d-1). This would mean that NH4+-N, NO2--N and PO43--P diffused into the water column while NO3--N diffused into the sediments. It would appear that the sediments were a source of nutrients for the water column; this would be a serious threat to the aquatic environment. Research into decontamination techniques is envisaged to improve the quality of open waters.