Hydromorphological and physicochemical characterization of water properties in the lagoon Porto-Novo (Benin Republic)

Abstract

Porto-Novo lagoon is surface water attached to a municipality with a high human population density. Human populations living around it are depending upon various uses with negative repercussions on its quality. The aim of this work is therefore to study the hydrological, morphometric and physicochemical parameters of this hydrosystem towards its characterization. The study was conducted from June 2015 to February 2016 including so the duringfour seasons in the year in West Africa. Probes were sampled between the water surface and 0.50 m depth at 15 different stations, taking into account the most anthropized zones. Samples taken in plastic bottles were transported to the laboratory andanalyzed using appropriate techniques. 16 physicochemical parameters were investigated. Principal Component Analysis (PCA) and hierarchical classification method (simple and Euclidean) served to characterize and classify the 15 stations under consideration. Results show that the lagoon of Porto-Novo is a shallow surface (1.68 m on average) without apparent thermal stratification. Water turnover time is inferior to an hour (18.04 mn) for an average outlet flow rate which correspond to1.94.108 m3/hour. The physicochemical data allowed to diagnose the eutrophication phenomenon and to characterize the eutrophic state of the water fluid of this lagoon. This eutrophication was caused by an excessive supply of compounds in particular those rich in nitrogen and phosphorus, and was manifested by an increased development of phytoplankton organisms and aquatic macrophytes. These nutrients are originally from human activities on the watershed as well as from industry, hotel complexes and other punctual sources of pollution linked to this lagoon. Classification tests permitted to group the 15 stations, and S6, S4 and S3appeared to be characterized by high organic loads, whereas S5, S8, S9 and S14 indicated high degrees of mineralization.