This work aims to assess the simultaneous effects of the climate change and the hydromorphological modification of Vridi channel on its hydrochemistry. To best estimate, this fact, the seasonal dynamic of the particle size distribution, pH, redox potential, salinity, conductivity, moisture and organic matter contents of Vridi channel sediments have been followed and compared during two different annual periods, where the climate and the hydromorphology of this estuary have been different. The first annual period, covering the period from April 2014 to March 2015, has been characterized by a climatic regime close to the climatic normal of the study area and, this channel had its former hydromorphology. The second period annual, covering the period from October 2018 to September 2019, has been marked by a disruption of the climatic regime throughout Côte d'Ivoire, and this channel presented its modified hydromorphology. A monthly sampling has been done in this channel during each annual period. All These physical and chemical parameters have been determined by the corresponding AFNOR standards. Results have shown that these superficial sediments, with a predominantly sandy texture over the period from April 2014 to March 2015, have presented a texture predominantly of silt and clays over the period from October 2018 to September 2019. The climate change and the recent hydromorphological modification of this ecosystem had simultaneous very few effects on their pH and redox potential over the study period. On the other hand, they have caused an important increase in their salinity, moisture and organic matter contents and, a decrease in their electrical conductivity over the period from October 2018 to September 2019 relative to those determined in these substrates over the period of April 2014 to March 2015. This fact has certainly affected its biodiversity, especially that of its benthic fauna.

This study focused on the effects of the climate change and the recent modification of Vridi channel on the mobility and distribution of phosphorus (P) in its superficial sediments. To best estimate this fact, the annual mobility and distribution of P were followed and compared before and after this development during two different annual periods, marked differently by the climate change effects. The first annual period has covered the period from April 2014 to March 2015 and this channel presented its former hydromorphology, with hydroclimatic conditions very close to that normal of Abidjan district. The second period annual has covered the period from October 2018 to September 2019 and this channel presented its current hydromorphology and, characterized by a rainfall drop and high ambient air temperatures compared to that normal of Abidjan district. A monthly sampling has been done at 5 cm below the surface sediment in this channel during each annual period. The mobility and distribution of P was assessed in the superficial sediments of this channel according two sequential extraction protocols: the modified Van Eck method and the modified Olsen method. Total phosphorus was assessed according AFNOR NF X31-147. The total phosphorus contents and all P fractions contents of these superficial sediments assessed by these two sequential methods and obtained over the period from April 2014 to March 2015 are higher than those determined over the period from October 2018 to September 2019. Thus, the recent development of this estuary, with as a corollary the modification of the physical and chemical characteristics of its superficial sediments, would have had the effect of clearing P towards the Atlantic Ocean and/or accelerating its sedimentation; hence the reduction of eutrophication risk of this aquatic ecosystem.

The aim of this study is to characterize phosphates adsorption kinetics on the superficial sediments of Vridi channel from its new hydromorphology. So, three scenarios have been carried out taking account the different waters seasons of this estuary by using pH and temperature as relevant parameters. In the first scenario, the experiments carried out at pH = 8 and T = 20°C to simulate phosphates adsorption on these sediments in its great cold season. In the second, the experiments carried out at pH = 8 and T = 25° C to simulate phosphates adsorption on these substrates in its hot season and small cold season. In the third, the experiments carried out at pH = 7 and T = 30°C to simulate phosphates adsorption on these sediments in its flood season. The experiences were carried out according to US EPA/530/SW-87/006-F protocol. KH2PO4 solutions were used as phosphates synthetic solutions. The experimental data were modeling by Lagergeen kinetics model (pseudo-order 1), Blanchard kinetics model (pseudo-order 2), Elovich kinetics model, Weber and Morris kinetics model and, external diffusion kinetics model. The results have showed phosphates adsorption on these sediments increase in the experiments carried out at pH = 8 and T = 20°C to those carried out at pH = 7 and T = 30°C. All experimental data have been well modeling by Blanchard kinetics model. So, these phosphates adsorption kinetics are pseudo-order 2 and the chemisorption is the dominant mechanism. The results obtained by the experimental data modeling by Weber and Morris kinetics model and external diffusion kinetics model showed that this process is not limiting by the molecular diffusion process. The experimental data have been again well modeling by Elovich kinetics model. That has confirmed the chemisorption character of this adsorption, and showed this chemisorption is activated.