International Journal of Aquaculture, 2013, Vol.3, No.4, 11
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http://ija.sophiapublisher.com
14
Kariba from its tributaries during this study period
(
Mhlanga, 2000; Chifamba, 2000). Nitrate plays a
vital role in the biological metabolism of aquatic
organisms notably phytoplankton and macrophytes
(
Boyd, 2003
).
Concentration of nitrate production in
water at any given time is a product of balance
between nitrate productions through the activities of
nitrifying bacteria and nitrate destruction by
autotrophic assimilation and/or bacteria denitrification
(
Gautier et al., 2000).
The results show no significant difference in ammonia
levels between site 1 and site 2, although site 1
generally had high mean ammonia values during the
study period. The mean values of ammonia at the
discharge points are higher than the
maximum
allowable local and international limits.
Thus it can be
concluded that the industries are producing the same
quality of effluent and need the same pollution mitigation
regulatory measures. Elevated concentrations of ammonia
affect the respiratory systems of many animals either
by inhibiting cellular metabolism or by decreasing oxygen
permeability of cell membranes (Lee et al., 2008).
Acute toxicity to fish may cause a loss of equilibrium,
hyper-excitability, an increased breathing rate and
increased cardiac output and oxygen intake, and in
extreme cases convulsions, coma and death (Brown, 1995,
Nhiwatiwa et al., 2011).
Although the mean values of pH, electrical conductivity,
orthophosphate and temperature recorded during the
study were all within the acceptable local and
international range the higher values recorded at the
discharged points relative to the control indicate that
the aquaculture effluent at these points is distorting the
lake water quality. If this discharge continues unabated
and the number of aquaculture enterprises increase
there could be negative implication on the lake water
quality with some parts of the lake becoming
eutrophic although this might be countered by the
large lake volume (Tumbare, 2008).
3
Conclusion
The concentrations of most water parameters
measured in this study were significantly different
compared to the control point showing that the
aquaculture effluent is affecting the lake water quality.
Because Lake Kariba serves as a source of domestic
water supply for drinking, washing, fishing and swimming,
continuous discharge of improperly treated effluent
industries should be stopped as this may cause
localised eutrophication and a change in the trophic
structure. The industrial waste should be thoroughly
monitored and processed according to local aquaculture
effluent standards which are yet to be developed
before its discharge into the lake in order to maintain
the integrity of the Lake Kariba.
4
Materials and Methods
4.1
Study Sites and Designing
The study was conducted in Lake Kariba,
Mashonaland West province of Zimbabwe on the
northern eastern border with Zambia (Figure 1).
Lake Kariba lies at an altitude of 484m above sea
level and between altitudes 16°28
′
S and 18°6
′
S and
longitudes 26°40
′
E and 29°03
′
E.
Lake Kariba is a
warm monomicticmeso-oligotrophic lake with three
distinguishable seasons namely a hot rainy season
(
November-March), cool dry season (May-August)
and a very hot dry season (September-November),
(
Timberlake, 2000).
The lake is
divided into five
geographical and limnologically distinct basins
along its axis, namely Mlibizi (basin 1), Binga
(
basin 2), Sengwa (basin 3), Bumi (basin 4) and
Sanyati (basin 5)
(
Cronberg, 1997
).
The Sanyati
basin, (study basin), is an important spawning
ground for the potadromous fish species. In addition
the nutrient load
from the Sanyati River and its
tributaries contribute significantly to the productivity
of the basin (
Cronberg, 1997
).
4.2
Field Sampling
Monthly water sampling was carried out from
September to December, 2011 in the Sanyati Basin
of Lake Kariba on selected aquaculture farm discharge
points. A control was selected in an un-impacted
area in the lake which was on the leeward upper side
of the discharge points of the crocodile and fish
farms where the effect of wind and down gradient
diffusion of the aquaculture effluent was deemed
minimal. Samples were collected between 09:00 and
12:00
on each sampling day. Systematic distance
sampling was done with sampling beginning in site 1
and terminating in site 3 following acceptable standard
methods and instrumentations (APHA, 2000). At each
station, the surface water temperature, turbidity,
electrical conductivity, pH, total dissolved solids,
percentage oxygen saturation and dissolved oxygen
were measured insitu
.
PH dissolved oxygen and