IV. Discussion
4.1 Water quality aspects, land and water use
The water quality parameters that were analysed in the studied
reservoirs indicate a general trend that is acceptable in comparison with the
WHO guidelines for drinking water (Chapman, 1992), natural levels in freshwater
(Sinkala et al. 2002) and the Zimbabwean effluent standards (ZINWA,
undated) (Table 3.3). In the whole study area, pH was more or less alkaline,
ranging from 7.6 to 8.5- values that are in agreement with the pH of most
natural waters that ranges between 6.0 and 8.5 (Chapman, 1992). Total nitrogen,
total phosphorus, total hardness and electroconductivity were not significantly
different in the National Park as compared to communal lands.
The similarity in the water quality parameters analysed in the
National Park and in the communal lands suggests that ecosystem health, as
defined by water quality, is currently not under serious threat due the land
and water use in the surrounding communal lands. This result was not expected,
based on findings in other areas and the documented impact humans can have on
water quality (Brainwood et al. 2004; Vitousek et al. 1997; Siwela et al. 1996,
Sharma, 2003). However, in the communal areas of this study, the high quality
of water of the reservoirs may be explained by the fact that few or no
significant land and water uses are taking place upstream, of the reservoirs;
all of the farms in the study area were located downstream of the reservoirs
and the human settlement appeared to be located far enough away from the
reservoirs to not constitute a threat. The homesteads that were found close to
reservoir might have not had significant influence, probably due to their
low-density status.
It is widely accepted nowadays that excess fertilization and
manure production create phosphorus surplus that accumulates in soils. Some of
this surplus is transported in soil runoff to aquatic ecosystems (Carpenter
et al., 1998) with respect to rainfall levels. Rainfall levels
contribute to water quality in the sense that little or no runoff result in
little rainfall. This enrichment of water bodies can only happen when farms and
activities
are located upstream of reservoirs. Though the study area
under investigation in this study currently lacks upstream influences, it would
be expected that increased human settlement or any development upstream of the
reservoirs could potentially result in a decrease in water quality in
reservoirs near communal lands.
The study may also document the role vegetative cover can play
in mitigating the impacts of local soil conditions, caused by natural or
anthropogenic phenomena, on water quality in the reservoir. Conductivity
measurements in the soils in both National Park and communal lands were three
to four times higher than measurements in the water (Table 3.1).
The pH was far more acidic in the National Park soils (pH=
5.3) as compared to the waters in the same area that is slightly alkaline
(pH=8). The same trend is found in the communal lands where the soils have a pH
around 7.5 and waters a pH=8.2. This contrast in values suggests that the
surrounding soils have little influence on the water quality of the reservoir
waters that might be attributed to the presence of good vegetation cover around
reservoirs, which constitute a buffer to large transfers of elements. The
presence of riparian vegetation is crucial in retaining some nutrients. This is
confirmed by Carpenter et al.(1998) who state that the maintenance of
vegetated riparian zones or buffer strips may reduce the transport of
phosphorus and nitrogen to reservoirs. It might be suspected that the
degradation of vegetation cover due to human activities on the communal lands
could increase the transport of nutrients to the reservoirs, and alter water
quality in the future.
While it appears that water quality was not directly impacted
by the surrounding soils (values of surrounding soil quality being different
from those of the water quality), it is interesting to note that water colour
may have been influenced by local conditions. Though water quality was found to
be acceptable in the communal lands, a whitish colour of water was present in
almost all of the reservoirs. This colour is very close to a white granite rock
located 250m upstream of Sibasa reservoir and might be the origin of the
colour. Such a strong whitish colour might have an effect on light penetration
in the reservoir and compromise the primary productivity within the water
column. Thus, it may be impacting biota within the reservoirs. This has been
shown by the possible influence
discovered on the quality parameters (pH, electroconductivity,
total nitrogen and hardness) as well as some plankton species (Annex 5).
Sibasa, having the strong whitish colour, might have got a very high
abundance.
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