Water Quality Monitoring in Kpassa Reservoir (Northen Benin): Physico-Chemical Parameters, Trophic State and N: P Ratio

Abstract

The study was carried out to assess the physico-chemical parameters, the trophic state and the N:P ratio of the Kpassa reservoir. The reservoir was studied at six sampling sites over the period going from August 2014 to May 2016. Water transparency was determined in situ with the secchi disk SDMO QUINIOU model F043SE. Chlorophyll a, NO 3-, NO 2-, NH 4 + , PO 4 3and total phosphorus were analyzed using the standard methods. Measures were made with the HACH DR2800 spectrophotometer. The trophic state was assessed using the Carlson's trophic state index (TSI). The N:P ratio was evaluated using NO 3-, NO 2-, NH 4 + and PO 4 3concentrations. The physico-chemical parameters investigated were: transparency (0.25-1.92m), chlorophyll a (8.2-233.46µg/L), nitrate (undetected-4.73mg/L), nitrite (undetected-0.23mg/L), ammonia (undetected-0.21mg/L), orthophosphate (0.03-1.04mg/L) and total phosphorus (0.1-3.2mg/L). The trophic state index (TSI) values oscillate between 38 and 59. The majority of the values are characteristic of mesotrophic conditions. Kpassa reservoir is eutrophic only in August 2014 at the whole studied stations. N:P ratio values vary between 0.22 and 60.46 showing that eutrophication in Kpassa reservoir is limited by phosphorus and nitrogen. Some physico-chemical parameters values are upper than the WHO and Ifremer safe limit making the reservoir water unfit for human consumption at the concerned sites and during the concerned period. It is recommended that the national company of Benin water (SONEB) sensitize the population on the rational use of the fertilizers and the adoption of water control technics in the farm. Washing and crockery activities should be also forbidden along the reservoir river. The trophic state of the reservoir seems improved compared to previous studies results. According to the N: P ratio values, measurements to be implemented for eutrophication management in Kpassa reservoir should emphasize on the control of the flow of nitrogen and phosphorus compounds.