Author(s): Hamil Vodougnon, Djiman Lederoun, Gildas Amoussou, Donald Adjibogoun and Philippe Lalèyè

Abstract: Ecologic stress resulting from human activities in the Lake Nokoué and Porto-Novo Lagoon complex was investigated from July 2015 to June 2016 at six stations of which three were located in each waterbody. The Lake Nokoué stations were Ganvié, Center of the lake and Dantokpa. According to the Porto-Novo Lagoon, the stations were Aguégués, Djassin and Agbokou. In that regard, water samples were analyzed for heavy metals as well as for Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD-5). The ecological stress Indices (ABC, DAP and SEP) have allowed estimating, both spacially and temporally. The average BOD-5 and COD values of, respectively, 55 ± 47.7 and 407 ± 182.6 mg/L observed on Lake Nokoué are greater than the recommended thresholds in the continental waters of Benin. The average mercury content of the Porto-Novo Lagoon was found to be 1.7 ± 1 µg/L which is also greater than the WHO recommendation of 0.5 µg/L. The same applies to lead whose average concentration exceeds the norm in that waterbody. Overall, 62 species belonging to 49 genera and 35 families were identified in that lagoon complex. In the Porto-Novo Lagoon, the fish of the Agbokou area are dealing with a mild stress level. On the Lake Nokoué, fish of the Dantokpa area and those of the central part of the lake, closest to the ocean, are facing an elevated ecologic stress. In terms of temporal variations, the rainy season induces strong variations in the ecologic stress in Lake Nokoué, as opposed to the dry season which do not induce any ecologic stress in the complex. Generally speaking, fish shoals in the Nokoué Lake are faced with a higher stress level than their Porto-Novo Lagoon counterparts.

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