Physico-Chemical Features of the Kpassa Reservoir, Northern Benin, with Emphasis on Its Trophic State: A Preliminary Study

Abstract

This work presents recent data on the physico-chemical properties and the trophic status of Kpassa reservoir at eighteen locations from August 2014 to February 2015. Seventeen physico-chemical parameters were measured and data obtained were statistically analyzed. The descriptive statistics showed their variations (minima-maxima) as follows: pH (4.65 - 7.30), temperature (25.1˚C - 29.9˚C), dissolved oxygen (1 - 5.79 mg/L), oxydability (0.32 - 10.88 mg O2/L), electrical conductivity (55 - 77 μS/cm−1), TDS (76 - 94 mg/L), turbidity (15.70 - 274.40 NTU), transparency (0.24 - 1.55 m), suspended matter (3 - 92 mg/L), total phosphorus (0.25 - 1.90 mg/L), orthophosphate (0.08 - 0.61 mg/L), nitrate (undetected 5.50 mg/L), nitrite (undetected 0.79 mg/L), ammonia (undetected 0.36 mg/L), chlorophyll a (7.20 - 2334.6 μg/L), silica (4.34 - 15.67 mg/L) and N/P ratio (0.08 - 42.62). These parameters were mainly influenced by agricultural activities and climatic conditions. The restriction or the removal of the use of chemical fertilizers in agricultural activities in Kpassa reservoir basin was recommended. The highest values of Pearson and Spearman correlations were observed across TDS and conductivity (0.967; 0.951), turbidity and oxydability (0.924; 0.665), turbidity and chlorophyll a (0.884; 0.663), turbidity and suspended matter (0.982; 0.793), suspended matter and transparency (−0.781; −0.819), suspended matter and nitrate (−0.813; −0.839), suspended matter and oxydability (0.919; 0.602) and suspended matter and chlorophyll a (0.879; 0.656). Carlson’s trophic state index (TSI) values varied between 36 and 66 showing that Kpassa reservoir was eutrophic in August and mesotrophic to oligotrophic during the other months of the sampling period. ANOVA and Kruskal-Wallis test indicated that there were no significant differences between sampling stations.