International Journal of Aquaculture, 2013, Vol.3, No.4, 11
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16
12
inevitably lead to the degradation of the water
quality and will affect living organisms at all trophic
levels especially at the area around the discharge
points. Thus, the thrust of the project was to assess
the quality of water at the aquaculture effluent
discharge points in Lake Kariba.
1
Results
1.1
Conductivity
Conductivity was significantly high at the discharge
points compared to the control point (Analysis of
variance, abbr: ANOVA, p<0.05). There was however,
no significant seasonal (monthly) difference between
the two discharge points (ANOVA, p<0.05). Highest
values of 196.9 mg/L in site 2 (Fish farming) and
160.79
mg/L at site 1 (Crocodile farming) were
recorded during November and October, and these
were within the acceptable limits of EMA and WHO
standards (Table 1).
Table 1 Mean ± SD of the physical and chemical conditions of the three sampling sites in Lake Kariba (Sanyati basin) in September
2011
to January 2012
Site/Parameter
Control
LC
LH
WHO
EMA
Turbidity (NTU)
2.97
±1.81
45.94
±18.78
32.65
±14.38
5.0
5.0
pH
8.42
±0.27
7.76
±0.52
7.44
±0.19
6.5
-
9
6
-
9
Temperature (
℃
)
31.12
±1.94
31.47
±1.84
28.88
±1.62
27.0
35.0
TDS (mg/L)
46.44
±1.28
75.71
±5.37
83.4
±16.45
1000.0
1000.0
Conductivi (µ·S·cm
-1
))
93.30
±1.95
141.02
±20.29
166.5
±34.77
1000.0
1000.0
DO (mg/L)
4.68
±0.62
3.30
±2.09
2.71
±0.75
5.0
5.0
Nitrates (mg/L)
0.43
±0.53
8.91
±12.79
7.69
±5.98
50.0
10.0
Ammonia (mg/L)
0.11
±0.13
2.39
±3.05
1.01
±1.25
0.5
0.5
Ortho-phosphate (mg/L)
0.14
±0.27
0.51
±0.30
0.32
±0.17
0.5
0.5
Note
：
LC=Lake Croc Discharge point (site 1); LH= Lake Harvest discharge points (site 2) and the Control (site 3)
1.2
Dissolved Oxygen
Mean levels of dissolved oxygen at all stations were
lower than the minimum allowable limit (5 mg/L)
for aquatic life during the study period with (Table
1).
However, there was significant difference
(
ANOVA, p<0.05) among the sampled sites but
there was no significant temporal (monthly)
variation (ANOVA, p>0.05).
1.3
Temperature
Mean surface water temperature fluctuated between
28
℃
and 33
℃
during the entire study period. This
range was above the limit of the WHO guideline of
27
℃
although it was within the EMA limit of 35
℃
(
Table 1). No significant differences in temperature
were observed among the three sampling sites and
among sampling periods (ANOVA, p>0.05).
1.4
Total Dissolved Solids
Solids were significantly high (ANOVA, p<0.05) at
the discharge points compared to the control, but no
significant temporal differences were observed
during the study period (ANOVA). The total
dissolved solids at discharge points were also not
significantly different (ANOVA, p>0.05). The
results were within the acceptable range of both the
WHO and EMA guidelines (Table 1).
1.5
Turbidity
Turbidity of the water was significantly high at the
crocodile farm discharge point (Site 1) and fish farm
discharge point (Site 2), with a mean of 45.9NTU
and 32.7NTU respectively compared to 3NTU at the
control point (Site 3) in November (ANOVA, p<0.05).
There was however, no significant difference between
the two discharge points and among sampling periods
(
ANOVA, p>0.05). Turbidity at all the discharge
points exceeded the EMA and WHO maximum
allowable limit of 5 mg/L in all the months during
the study (Table 1).
1.6
PH
The pH ranged from 7.40 in site 1 and site 2 to 8.42
in site 3 the control (Table 1). The range was within
the acceptable limits (EMA 1997; WHO, 2006). Site
1
and site 2 were significantly lower (ANOVA,
p<0.05) in pH than site 3 which was more alkaline.