2.2 Data collection
2.2.1 Plankton samples
Zooplankton samples were collected with a zooplankton net of
40 cm diameter and 62um mesh size while phytoplankton samples were collected
using similar nets of 20um mesh size. The samples were collected using a
standardized method presented in Edmondson and Winberg (1971). The concentrated
samples were collected in small 130 ml bottles that were labelled. Four samples
were collected on each reservoir at a horizontal line situated at 10 to 20 m
facing the dam wall for zooplankton and four for phytoplankton. A preservation
solution of 4% formalin was added to the sample bottles of zooplankton and
Lugol solution was added to the bottles containing phytoplankton for fixing
purposes. The samples were then taken to the fish laboratory of the Biological
Sciences Department of the University of Zimbabwe. Taxa were then identified
and counted under an inverted microscope OLYMPUS CK40 and species pictures were
taken using a digital camera NIKON model E995 mounted on the inverted
microscope. The identification of taxa was done using a dichotomic
identification keys presented in Durand and Lévêque (1980)
supplemented by Fernando (2002) and Cander-Lund and Lund (1995).
2.2.2 Physical and chemical data
Transparency was measured on all the selected reservoirs using
a secchi disk.
Transparency of waters is linked to light attenuation in
reservoirs, which impacts the
photosynthetic potential of primary producers
and consequently impact the whole biotic
composition of a reservoir (Hart, 1990). The depth of a
reservoir influences its water quality. Of particular importance is the depth
relative to the surface area and wind intensity because these factors effect
the intensity of mixing in the reservoir (Straskraba and Tundisi, 1999). Water
quality is therefore related to reservoir depth, size and basin morphology.
Therefore, the depth and morphological characteristics of the reservoirs were
described. A stadia rod was used to measure the depth. The slope of the dam was
observed and recorded as steep or gentle.
Soil samples of the substrate of the reservoirs were collected
and brought to the Soil Science and Agricultural Engineering laboratory of the
University of Zimbabwe for analysis of pH, electroconductivity, texture and
colour. A Jenway pH meter Model 3510 (ESSEX) was used for pH analysis of soils.
A 1:1 soil solution (which is deionised water) ratio was used. A conductivity
meter (Ecoscan Con5) was used for electroconductivity analysis. The texture of
soils was analysed by the Boyocous hydrometer method. Rainfall data for
Filabusi and Sibasa areas was collected at Filabusi District Administration
meteorology office while the rainfall data for the National Park was collected
from the Meteorology office in Harare. The colour of soil samples was analysed
using the Munsell soil colour charts (Munsell, 1975).
The vegetation cover surrounding reservoirs was observed and
estimated as abundance scores of 1 to 4 (1 means no vegetation coverage; 2:
poor coverage; 3: good coverage and 4: very good coverage). Presence or absence
of farms upstream of reservoirs was noted as well as the proximity of homestead
upstream and downstream the reservoirs. The activities (anthropogenic) taking
place in the vicinity of the reservoirs were also investigated. A digital
camera CAMEDIA C120 was used to take pictures of the activities in the study
area as well as of the main vegetation cover and soils to facilitate their
identification.
2.2.3 Water samples
Water samples were collected using a Ruttner' s bottle at 0.5
m depth from the surface at the four sampling sites located in face of the
dams' wall. These samples were immediately placed into a cooler box and kept at
low temperatures using ice blocks pending their transportation to a deep
freezer in the laboratory. The following water quality parameters were analysed
after the samples had been brought to the laboratory: Nitrogen, Phosphorus, pH,
electric conductivity (EC) and total hardness. The MuphyRiley Method (ascorbic
acid method) was used for total phosphorus analysis using UV visible
spectrophotometer Spectronic 21 Bausch and Lomb. Total nitrogen was analysed
using the titrimetric method using 0.01 M HCl (Hydrogen chloride). The solution
was made alkaline by MgO (Magnesium oxide) and Dervada alloy.
Electroconductivity and pH were analysed using the equipment described in
section 2.2.2.
Electroconductivity and total hardness are considered because
they might be related to soil composition and exchanges between soil and small
reservoir waters. There is abundant literature that stresses the importance of
phosphorus and nitrogen in the shaping of the structure and abundance of
phytoplankton in reservoirs (Crawley, 2000; Talling & Lemoalle, 1998;
Lemoalle et al. 1981; Pinel-Alloul et al. 1995; Schindler,
1978; Drenner, 1989). Nitrogen and phosphorus are considered because they are
major factors that limit primary production of phytoplankton in reservoirs
(Straskraba and Tundisi, 1999). Nutrient-rich animal excrement deposited along
and within reservoirs constitute a major input of nitrogen and phosphorus in
these areas. Hippos are present in Mpopoma and Chitampa reservoirs. The cattle
found around Denje and Dewa reservoirs might have similar effects, though dung
is mainly in the shoreline of reservoirs and might be transported to the waters
indirectly by rains.
The supply of nitrogen is known to be a key factor controlling
the nature and diversity of plant life, the population of both grazing animals
and their predators, and vital ecological processes such as plant productivity
and the cycling of carbon and soil minerals (Vitousek et al.,
1997).
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