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 O 2/L) , electrical conductivity (55 - 77 μ S/cm𕒵), 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 h ighest values of Pearson and Spearman correlations w ere 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 (𕒴.781; 𕒴.819) , suspended matter and nitrate (𕒴 . 813; 𕒴.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.
There are many things that people need to survive, one of which is water. The need for water in the day to day activities of man includes for cooking, washing, drinking and for industrial activities [
Kpassa reservoir has a vital value in north-east of Benin as it is the only water resource used for a sustainable water supply of Parakou city and around. Its water quality must then be constantly monitored to prevent its further degradation and promote its sustainable management. In addition since 2000, the reservoir was invaded by aquatic plants. This abnormal situation induced many studies on the water quality parameters related to eutrophication process [
Located at 13.5 km at east of Parakou city (N 09˚17'034; E 002˚43'975) in Borgou department, Kpassa reservoir on Okpara with an area of 190 ha [
The volume of water available in Kpassa reservoir is estimated at 8.2 million m3, either original volume of 9.4 million m3 obtained in 1972 decreased by 1.2 million m3 of sediment estimated in 2014 [
with depth varies between 1.5 and 8.5 m. It is cover on about 90% of its surface by aquatic plants [
Water samples were taken at 30 cm from the surface at eighteen stations each two months during the period from August 2014 to February 2015. Six stations were selected based on their accessibility and the potential pollution sources localization, and each one was then subdivided in three substations to densify the sampling network. Station I (St1) was closer to the raw water pumping point of the national water company; station II (St2) was closer to the East shore of the reservoir where cultural practices was made; stations III (St3) and IV (St4) were located along the West shore of the reservoir near Kpassa village; Station V (St5) was located in the middle of the reservoir and station VI (St6) in upstream of the reservoir.
Water turbidity (Turb), temperature (Temp), pH, Dissolved Oxygen (DO) and Electrical conductivity (Cond) were measured in situ using Water quality meter WQC-24 (TOA DKK Corporation), water transparency (SD) using secchi disk, Total Dissolved Solid (TDS) using conductimeter WTW 3210 SET 1 and suspended matter (Susp) was measured using portable HACH DR890 colorimeter.
The HACH DR2800 spectrophotometer was used to analyze nitrate (λ = 400 nm), nitrite (λ = 507 nm), ammonia (λ = 4655 nm), orthophosphate (λ = 880 nm), total phosphorous (λ = 880 nm), and silica (λ = 815 nm), according to the [
N/P ratio was used to identify the eutrophication limiting element. N and P concentrations was calculated using
The obtained N/P ratio data were compared with the standard [
The trophic state of the reservoir was estimated using [
The overall Carlson’s TSI was then calculated as the average value of TSIChl-a, TSISD and TSITP as follows:
TSI ranges from 0 to 100. TSI values less than 40 correspond to oligotrophic conditions, while between 40 and 50 for mesotrophic and between 50 and 70 for eutrophic. TSI values greater than 70 are associated with hypertrophic conditions.
Statistical analysis was made using the following tests:
• Descriptive statistics to obtain average value, median, standard deviation, minimum, maximum, range, of all measured parameters.
• Pearson and Spearman correlations to determinate the significance of relations between certain parameters.
• ANOVA and Kruskal-Wallis Test to compare sampling locations with each other. The analysis of variance was used after verify the normality of variables (One-Sample Kolmogorov-Smirnov Test) and the homogeneity of variances (Levene Test). When these conditions are not verified, ANOVA was not used but the Kruskal-Wallis Test.
The charts and the statistical tests were generated using SPSS (Statistical Package for the Social Sciences) software version 16.0.
The descriptive statistics of the limnological characteristics of Kpassa reservoir are shown in
The water temperature plays an important role in the solubility of salts and gases. It is one of the most significant parameters which control inborn physical qualities of water [
Parameters | Average value | Median | Standard deviation | Minimum | Maximum | Range |
---|---|---|---|---|---|---|
pH | 6.54 | 6.65 | 0.42 | St1-2; August | St4-1; February | 4.65 - 7.30 |
Temperature (˚C) | 27.51 | 27.63 | 1.55 | St4-3; December | St5-2; February | 25.10 - 29.90 |
Dissolved oxygen (mg/L) | 2.55 | 2.54 | 1.09 | St3-1; October | St2-1; August | 1.00 - 5.79 |
Oxydability (mg O2/L) | 2.79 | 1.28 | 2.95 | St2-3; October | St2-1; August | 0.32 - 10.88 |
Conductivity (µs/cm−1) | 68.13 | 69.15 | 6.32 | St5-3; October | St6-2; February | 55.00 - 77.00 |
TDS (mg/L) | 85.34 | 87.20 | 6.07 | St6-1; October | St3-1; February | 76.00 - 94.00 |
Turbidity (NTU) | 74.10 | 21.50 | 90.42 | St5-3; December | St6-1; August | 15.70 - 274.40 |
Secchi depth (m) | 0.85 | 0.90 | 0.36 | St4-2; October | St3-1; February | 0.24 - 1.55 |
Suspended matter (mg/L) | 24.93 | 9.00 | 29.50 | St6-1; December | St6-1; August | 3.00 - 92.00 |
Total phosphorus (mg/L) | 0.88 | 0.82 | 0.36 | St1-1; February | St6-3; August | 0.25 - 1.90 |
Ortho-phosphate (mg/L) | 0.28 | 0.27 | 0.12 | St1-3; February | St6-3; August | 0.08 - 0.61 |
Nitrate (mg/L) | 2.23 | 2.45 | 1.68 | St1-1; August | St6-1; December | 0.00 - 5.50 |
Nitrite (mg/L) | 0.04 | 0.00 | 0.12 | St1-1; August | St3-1; December | 0.00 - 0.79 |
Ammonia (mg/L) | 0.13 | 0.15 | 0.09 | St6-1; December | St3-1; August | 0.00 - 0.36 |
Chlorophyll a (µg/L) | 429.52 | 50.4 | 729.04 | St3-3; December | St5-3; August | 7.20 - 2334.60 |
Silica (mg/L) | 10.88 | 11.20 | 2.51 | St4-3; August | St6-1; October | 4.34 - 15.67 |
N/P ratio | 11.52 | 11.31 | 9.67 | St2-3; August | St1-3; February | 0.08 - 42.62 |
temperature were characteristic of the dry season but the lowest temperature recorded in December is due to the prevalence of air mass called Harmattan characterized by very cold temperatures. According to [
Dissolved oxygen is an important parameter to assess the waste assimilative capacity of the waters [
Electrical conductivity is the measurement of the ability of a solution to carry electric current. Its ability is dependent upon the presence of ions in solution and its measurement is an excellent indicator of the total dissolved solid in matter [
TDS is causing by dissolved cation and anion species [
Turbidity of water is the presence of suspended and partial dissolved material in water sources [
SD | Turb | Temp | pH | DO | Cond | TDS | Susp | TP | SiO2 | Oxyd | Chl a | N/P ratio | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
SD | 1.000 | −0.820 | 0.179 | 0.749** | −0.146 | 0.749** | 0.729** | −0.819** | 0.768** | 0.042 | −0.086 | −0.465** | −0.480** | 0.322** | −0.529** | −0.535** | 0.749** |
Turb | −0.741** | 1 | −0.214 | −0.607** | 0.334** | −0.565** | −0.521** | 0.793** | −0.766** | −0.293* | 0.168 | 0.517** | 0.522** | −0.449** | 0.665** | 0.663** | −0.776** |
Temp | 0.190 | −0.250* | 1 | 0.041 | −0.240* | 0.033 | 0.048 | 0.074 | 0.041 | −0.507** | 0.518** | −0.021 | −0.033 | 0.038 | −0.149 | −0.062 | −0.015 |
pH | 0.689** | −0.735** | 0.114 | 1 | 0.107 | 0.640** | 0.658** | −0.766** | 0.658** | 0.161 | −0.184 | −0.424** | −0.442** | 0.265* | −0.384** | −0.464** | 0.652** |
DO | −0.224 | 0.637** | −0.245* | −0.241* | 1 | 0.295* | 0.346** | 0.166 | −0.330** | 0.108 | 0.065 | 0.296* | 0.282* | −0.330** | 0.602** | 0.449** | −0.267* |
Cond | 0.664** | −0.220 | −0.052 | 0.383** | 0.314** | 1 | 0.951** | −0.644** | 0.556** | 0.096 | −0.051 | −0.289* | −0.318* | 0.162 | −0.169 | −0.233* | 0.547** |
TDS | 0.726** | −0.276* | 0.071 | 0.434** | 0.271* | 0.967** | 1 | −0.595** | 0.484** | 0.065 | 0.001 | −0.345** | −0.374** | 0.051 | −0.128 | −0.183 | 0.546** |
Susp | −0.781** | 0.982** | −0.174 | −0.759** | 0.555** | −0.303** | −0.351** | 1 | −0.839** | −0.287* | 0.330** | 0.543** | 0.546** | −0.413** | 0.602** | 0.656** | −0.831** |
0.781** | −0.778** | 0.041 | 0.661** | −0.354** | 0.518** | 0.535** | −0.813** | 1 | 0.215 | −0.132 | −0.377** | −0.385** | 0.470** | −0.603** | −0.656** | 0.809** | |
0.110 | −0.184 | −0.343** | 0.195 | 0.074 | 0.127 | 0.094 | −0.199 | 0.180 | 1 | −0.473** | −0.155 | −0.121 | 0.130 | −0.068 | −0.306** | 0.262* | |
−0.101 | 0.184 | 0.486** | −0.131 | 0.106 | −0.142 | −0.063 | 0.223 | −0.171 | −0.380** | 1 | 0.340** | 0.330** | 0.006 | 0.238* | 0.290* | −0.183 | |
−0.494** | 0.619** | −0.094 | −0.427** | 0.380** | −0.237* | −0.311** | 0.630** | −0.424** | −0.225 | 0.348** | 1 | 0.988** | −0.161 | 0.402** | 0.475** | −0.786** | |
TP | −0.492** | 0.581** | −0.099 | −0.428** | 0.334** | −0.270* | −0.343** | 0.599** | −0.418** | −0.196 | 0.331** | 0.989** | 1 | −0.161 | 0.402** | 0.450** | −0,788** |
SiO2 | 0.393** | −0.491** | 0.055 | 0.361** | −0.320** | 0.127 | 0.119 | −0.487** | 0.500** | 0.134 | 0.049 | −0.186 | −0.186 | 1 | −0.366** | −0.321** | 0,376** |
Oxyd | −0.685** | 0.924** | −0.215 | −0.671** | 0.672** | −0.204 | −0.254* | 0.919** | −0.751** | −0.151 | 0.231 | 0.584** | 0.584** | −0.446** | 1 | 0.652** | −0.551** |
Chl a | −0.683** | 0.884** | −0.123 | −0.529** | 0.630** | −0.174 | −0.221* | 0.879** | −0.710** | −0.159 | 0.250* | 0.591** | 0.559** | −0.373** | 0.859** | 1 | −0.625** |
N/P ratio | 0.686** | −0.679** | −0.004 | 0.560** | −0.255* | 0.541** | 0.585** | −0.714** | 0.786** | 0.303** | −0.196 | −0.727** | −0.729** | 0.356** | −0.644** | −0.617** | 1 |
**: Correlation is significant at the 0.01 level; *: Correlation is significant at the 0.05 level.
cause of the turbidity level [
Transparency values express by secchi depth ranged from 0.24 m in October at station St4-2 to 1.55 m in February at station St3-1. High density of phytoplankton and high concentration of suspended matter are the main factors which contribute to reduce water transparency. According to [
Suspended matter concentration fluctuated between 3 mg/L in December (dry season) at station St6-1 and 92 mg/L in August (wet season) at the same station. Suspended matter was essentially due to erosion in the catchment area and to the presence of large quantities of agricultural waste carried in by leaching from the fields [
Agricultural activities are considered as the major provider of nutrients in lakes, especially in the Mediterranean region, where agricultural sector comprises an important economic factor [
Phytoplankton biomass (Chlorophyll a) ranged from 7.20 µg/L in December at station St3-3 to 2334.6 µg/L in August at station St5-3. Nitrogen and phosphorus are generally considered as the main factors for the growth of algae [
Silica values were observed to be high (15.67 mg/L) in the month of October and minimun (4.34 mg/L) in August. The presence of silica throughout the sampling period could be due to the geochemical characteristics of the study area. Silica is an important constituents of different kinds of rock in Kpassa reservoir basin (57% to 74%) [
The N/P ratio ranged from 0.08 to 42.62. Most values were lower than Redfield's ratio of 16/1 [
The spatio-temporal variation of Carlson’s trophic state index (TSI) is shown in
P-probability of One-Sample Kolmogorov-Smirnov test and Levene test are shown in
Parameters | Kolmogorov-Smirnov Test Sig. | Levene Test Sig. |
---|---|---|
pH | 0.001 | 0.128 |
Temperature (˚C) | 0.268 | 0.428 |
Dissolved oxygen (mg/L) | 0.591 | 0.858 |
Oxydability (mg/L) | 0.000 | 0.996 |
Conductivity (µs/cm) | 0.088 | 0.999 |
TDS (mg/L) | 0.091 | 1.000 |
Turbidity (NTU) | 0.000 | 1.000 |
Secchi depth (m) | 0.584 | 0.905 |
Suspended matter (mg/L) | 0.000 | 1.000 |
Total phosphorus (mg/L) | 0.727 | 0.053 |
0.673 | 0.045 | |
0.055 | 0.406 | |
0.000 | 0.000 | |
0.148 | 0.944 | |
SiO2 (mg/L) | 0.938 | 0.124 |
Chla (mg/L) | 0.000 | 0.323 |
N/P ratio | 0.266 | 0.730 |
rences between sampling stations (
The present investigation showed that the physico-chemical properties of Kpassa reservoir were mainly influenced by agricultural non-point source pollution. The second
Parameters | ANOVA Sig. |
---|---|
Temperature (˚C) | 1.000 |
Dissolved oxygen (mg/L) | 0.997 |
Conductivity (µs/cm) | 1.000 |
TDS (mg/L) | 1.000 |
Secchi depth (m) | 1.000 |
Total phosphorus (mg/L) | 0.668 |
0.731 | |
0.962 | |
0.992 | |
SiO2 (mg/L) | 0.669 |
N/P ratio | 0.998 |
Parameters | Kruskal-Wallis Test Sig. |
pH | 1.000 |
Oxydability (mg/L) | 0.960 |
Turbidity (NTU) | 1.000 |
Suspended matter (mg/L) | 1.000 |
0.665 | |
Chla (mg/L) | 0.995 |
most important impact factor was the climatic conditions. Organic matter produced by the macrophyte was found to have a strong impact on dissolved oxygen inducing hypoxic conditions in Kpassa reservoir at the end of wet season. Results of trophic state assessment showed the eutrophic state of Kpassa reservoir in August. In regard to the surface covered by the macrophyte during this period, the eutrophic state of the reservoir was expected. From October to February, the reservoir trophic state fluctuated between mesotrophic and oligotrophic state. Besides the mechanical removal of the plants, other measures of restoration of the reservoir must be implemented for its safeguarding. This study suggests the restriction or the removal of the use of chemical fertilizers in agricultural activities in Kpassa reservoir basin. Differences in the concentration of physico-chemical parameters between sampling stations are not statistically significant. In addition to horizontal sampling, vertical sampling at surface and different depth is advised for future study in Kpassa reservoir.
This research work was supported by PhD grant from the Netherland Program of Reinforcement of Capacities in Post-secondary Teaching (Project NICHE BEN 167). Special thanks to all the members of the Project especially to Pr Marc T. KPODEKON, Coordinator of project NICHE BEN 167 and Pr Euloge K. AGBOSSOU, Director of “Institut National de l’Eau” (INE) for their excellent management. Particular thanks to Pr Moussa BOUKARI, Director of “Laboratoire d’Hydrologie Appliquée” (LHA) for his special support. We are grateful to Dr Dieudonné ZOGO, Director of “Société Nationale des Eaux du Bénin Parakou” (SONEB Parakou) for his valuable contribution. We also thank Dr Lyde TOMETIN, Mohamed BOURE, Akilou SOCOHOU and Gaël SOUNOUVO for field and laboratory assistance.
Boukari, O.T., Mama, D., Abou, Y. and Bawa, M.L. (2016) Physico-Chemical Features of the Kpassa Reservoir, Northern Benin, with Emphasis on Its Trophic State: A Preliminary Study. Journal of Environmental Protection, 7, 2067- 2080. http://dx.doi.org/10.4236/jep.2016.713161