Some physicochemical parameters like temperature, transparency, dissolved solid, suspended materials, turbidity, conductivity, pH, total alkalinity, dissolved oxygen, nitrate, phosphate and salinity of water samples collected at Lekki Lagoon from November 2010 to September 2012 were evaluated. Fifteen locations were selected for water sampling according to the activities that took place in the lagoon. Physical water quality parameters such as temperature, transparency, total dissolved solids and total suspended solid showed strong seasonal and spatial fluctuation. Parameters like conductivity, pH and total alkalinity had relatively constant values among stations though they varied seasonally. Dissolved oxygen, nitrate and phosphate values were within permissible range for fish. Higher salinity values were recorded in wet seasons than dry seasons suggesting that salinity variation at Lekki Lagoon was due to salt incursion from the adjourning Lagos Lagoon and Mahin Creek during rainy season rather than evaporation factor. Salinity range of 1.70 - 6.00 psu recorded in this study also shows that Lekki Lagoon is tending towards brackish and this may affect the ichthyofauna composition of the lagoon.
Larger percentage of fish being consumed in the country comes from brackish-marine environment [
Lekki Lagoon, located in Lagos and Ogun states of Nigeria covers an area of nearly 247 km2 and lies between longitude 4˚00'E and 4˚15'E and latitude 6˚22'N and 6˚37'N [
Lekki Lagoon is the most important source of affordable animal protein in the form of fish in Epe and Ibeju- Lekki Local Government Areas of Lagos State [
The mean depth of Lekki Lagoon is 3.1 m, and the maximum depth is 6.4m at low tide [
The study area, Lekki Lagoon (
solid, turbidity, conductivity, pH, total alkalinity, dissolved oxygen, nitrate, phosphate and salinity. The analysis was done using a combination of in situ and laboratory method [
The sampling methods were time and space stratified. Time stratification covered two dry seasons and two wet seasons. Space stratification was based on the method of Ajani [
Basic statistical analyses such as means, range, standard deviation, t-test and probability levels were used to test the physicochemical parameter data of Lekki Lagoon. One way and two ways analysis of variance were used to find level of significance on seasonal variation while water quality parameters were subjected to Chi- square test of homogeneity.
Temperature was relatively high during the study period with a mean of 28.73˚C ± 1.83˚C. This may be due to the shallow nature of Lekki Lagoon (3.0 - 6.4 m) according to Kusemiju [
Temperature range recorded for the sampling period was 26.5˚C - 32.1˚C. This is similar to observation of Lawal-Are [
Turner and Millward [
s/n | Zone | Sampling station | Gps coordinates |
---|---|---|---|
1 | A | Ere ise | 06˚34.753'N/004˚01.525'E |
2 | A | Imogo | 06˚32.690'N/004˚04.232'E |
3 | A | Emina | 06˚32.765'N/004˚04.972'E |
4 | B | Origbe | 06˚35.690'N/004˚10.845'E |
5 | B | Iluboye | 06˚34.292'N/004˚07.772'E |
6 | B | Ebute oni | 06˚32.004'N/004˚13.313'E |
7 | C | Aba oyinbo | 06˚30.487'N/004˚05.894'E |
8 | C | Iwopin | 06˚30.614'N/004˚10.259'E |
9 | C | Onigangan | 06˚27.472'N/004˚08.214'E |
10 | D | Ilumofin | 06˚29.478'N/004˚01.009'E |
11 | D | Arala | 06˚29.750'N/004˚05.523'E |
12 | D | Batedo | 06˚26.443'N/004˚06.212'E |
13 | E | Lakoye | 06˚27.621'N/004˚08.743'E |
14 | E | Takunle | 06˚25.757'N/004˚11.234'E |
15 | E | Ise | 06˚25.011'N/004˚12.865'E |
of 0.10 to 0.47 m on the same Lekki Lagoon, but in line with works of Kusemiju [
Total dissolved solid (TDS) and Total suspended solid (TSS) were both higher in the wet season than the dry season. High values of TDS and TSS must have contributed greatly to the low transparency experienced during wet season. Ajani [
In this study, turbidity was lowest in the dry season (6 NTU) and had highest value in the rainy season (22 NTU). Thus, turbidity and transparency had an inverse relationship. This was in line with the work of Babalola [
Water conductivity is a measure of the concentration of dissolved solids in water and it gives an indirect dissolved salt content of fluid. The mean water conductivity for the sampling period was 133.41 ± 15.32 μScm−1 with a range of 94 - 160 μScm−1. The lowest water conductivity (94.00 μScm−1) was recorded in May 2011 at the onset of rainy season in Station 3; while the highest conductivity reading (160 μScm−1) was recorded in September, 2012 at Station 7. Lagoon was relatively rough at this period. A weak positive correlation exist between conductivity and total dissolved solid content of the water (r = 0.22) during the period under review. Seasonal variations in the lagoon water conductivity was significant (P < 0.05), whereas spatial variations was not.
Washington State Department of Ecology [
pH is a measure of the acidity or basicity of an aqueous solution. An aqueous solution with pH value less than 7 is classified as acidic while the one with a pH value greater than 7 is said to be basic or alkaline. The pH varied between 5.00 and 8.00 with a mean of 6.45 ± 0.89 during the sampling period. This range falls within permissible range for fish culture. The lowest water pH was 5.00 and it was recorded in January 2012 at the peak of dry season in Stations 2 and 4; while the highest pH reading (8.00) was recorded in May and July, 2011 at the peak of rainy season. Seasonal pH variation was significant (P < 0.05) whereas there was no significant difference (P > 0.05) in the mean water pH among different Stations. pH is a very important physicochemical parameter that influences fish performance in water. Several fish will stop growing at a lethal pH and eventually die off in extreme cases.
The measurement of the capacity of water to neutralize acids is referred to as alkalinity [
The dissolved oxygen (DO) range for the study period was 4.7 - 8.0 mg/L. This value still falls within acceptable range for optimal performance of fish. DO ranges of 4.9 - 11.2 mg/L, 4.0 - 11.2 mg/L and 5.2 - 11.8 mg/L were reported for Badagry, Lagos and Lekki Lagoons respectively [
Fluctuation in the DO was much in the second sampling year (2011-2012) than the first sampling year (2010-2011). Likewise, DO values for the wet seasons were higher than the dry seasons (
Phosphate and nitrate did not show distinct seasonal variation but did show spatial variation. Highest value of Nitrate 0.39 ± 0.22 mg/L and Phosphate 0.36 ± 0.18 mg/L were recorded in Zone E while Zone A had the lowest Nitrate 0.18 ± 0.15 mg/L and Phosphate 0.15 ± 0.09 mg/L. Nitrate 0.12 ± 0.02 - 0.48 ± 0.15 mg/L and phosphate 0.14 ± 0.09 mg/L - 0.29 ± 0.14 mg/L were recorded in dry and wet seasons respectively. Bulk of the nitrogen might be from agricultural fields or deposition of raw sewage into the lagoon while phosphate addition could be from detergent rich sewage effluents. Phosphate and nitrate showed almost positive relationship in all locations and the relationship was significant.
Salinity range for the study period was between 1.70 and 6.00 psu. The lowest salinity (1.7 psu) was recorded in November 2011 at Station 5 while the highest salinity (6.00 psu) was obtained at Station 13 in July 2011. The highest salinity recorded by Kusemiju [
The seasonal salinity variation during the wet and dry seasons was quite high (
Parameters | 2010-2011 | 2011 | 2011-2012 | 2012 |
---|---|---|---|---|
Dry | Wet | Dry | Wet | |
Temperature (˚C) | 28.33 ± 0.91 | 27.30 ± 1.74 | 29.80 ± 1.32 | 29.47 ± 2.01 |
Transparency (m) | 0.71 ± 0.04 | 0.60 ± 0.90 | 0.70 ± 0.03 | 0.61 ± 0.08 |
Salinity (psu) | 2.53 ± 0.47 | 3.83 ± 0.93 | 2.63 ± 0.63 | 4.43 ± 0.52 |
Dissolved oxygen (mg/L) | 6.33 ± 0.40 | 6.63 ± 0.45 | 5.98 ± 0.65 | 4.43 ± 0.52 |
pH | 5.68 ± 0.38 | 7.08 ± 0.82 | 5.96 ± 0.55 | 7.07 ± 0.65 |
Nitrate (mg/L) | 0.18 ± 0.04 | 0.39 ± 0.15 | 0.20 ± 0.09 | 0.52 ± 0.28 |
Phosphate (mg/L) | 0.16 ± 0.03 | 0.37 ± 0.16 | 0.22 ± 0.10 | 0.29 ± 0.13 |
Total alkalinity (mg/L) | 19.73 ± 0.81 | 17.90 ± 1.13 | 19.58 ± 0.77 | 26.38 ± 9.32 |
Conductivity (μScm−1) | 131.67 ± 7.05 | 126.33 ± 20.50 | 125.23 ± 5.99 | 150.40 ± 5.63 |
Total dissolved solid (mg/L) | 48.82 ± 5.63 | 52.07 ± 8.13 | 46.67 ± 3.07 | 45.66 ± 6.70 |
Total suspended solid (mg/L) | 44.83 ± 4.21 | 50.17 ± 8.07 | 45.07 ± 3.09 | 43.57 ± 7.37 |
Turbidity (m) | 11.83 ± 1.89 | 16.00 ± 3.26 | 11.67 ± 2.37 | 13.50 ± 3.70 |
and 4.43 ± 0.52 psu for years 2010-2011 and 2011-2012 respectively. These suggest that the inflow of water from Lagos Lagoon and Mahin Creek in the rainy season was the main source of increase in salinity level at Lekki Lagoon. Indeed, Lawal-Are [
Salinity as an important ecological factor has been known to have a great effect on the biotic life in aquatic ecosystem because each fish species has a salinity range where they can operate. Blaber [
This study confirms stability in the physical and chemical factors of Lekki Lagoon. The spatial distributions were relatively stable while there was a distinct seasonal variation in most cases. The highest salinity value of 6.00 psu recorded in this study is more than the highest values reported by earlier works which are 0.3 psu, 3.10 psu and 4.70 psu respectively [
There should be regular monitoring and study of the physical and chemical state of the Lekki Lagoon environment in order to provide a basis for an informed decision on sustainable management of the lagoon.
Isaiah O.Opadokun,A. E.Falaye,E. K.Ajani, (2015) Seasonal Variation in Physicochemical Parameters of Lekki Lagoon and the Conservation of Its Ecosystem. Journal of Geoscience and Environment Protection,03,11-17. doi: 10.4236/gep.2015.39002