PLANKTON DIVERSITY AS AN INDICATOR OF WATER QUALITY IN
SMALL RESERVOIRS IN THE LIMPOPO BASIN, ZIMBABWE
Lefranc BUSANE Basima1,* , Aidan SENZANJE2, Brian MARSHALL3, Katherine SHICK4
1
Dept of Biology/Hydrobiology, Centre Universitaire de Bukavu. Bukavu, D.R.CONGO
PO Box 570 Bukavu-DRCongo
P.O. Box 435 Cyangugu-Rwanda
2
Dept of Soil Science and Agricultural Engineering, University of Zimbabwe, P.O. Box MP 167,
Mount Pleasant, Harare, ZIMBABWE
3
Dept of Biology, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare,
ZIMBABWE
4
P.O. Box 56 Wise River MT, 59762, USA
*: Corresponding author: PO. Box 435 Cyangugu-Rwanda. Tel: +243-97-735-697
Email addresses:
frankbasima@hotmail.com, frankbusane@yahoo.fr
ABSTRACT
This paper reports on a study carried out from February to April 2005 in the southern
part of Zimbabwe in the Mzingwane catchment, Limpopo basin to investigate the impacts
of land and water use on the water quality and ecosystem health of eight small man-made
reservoirs. Four reservoirs were located in communal lands while the remaining were
located in the National Park considered pristine. Plankton community structure was
identified in terms of abundance and diversity as an indirect assessment of water quality
and ecosystem health. In addition, phosphorus, nitrogen, pH, transparency, electric
conductivity and hardness were analysed. The results obtained indicate that the
communal lands’ areas have not gone through major land and water use changes that
impact on the quality of reservoirs since no significant difference was obtained between
communal lands and National Park in terms of plankton community (P>0.05). However,
a 2 significant difference was found between the phytoplankton groups abundance
(P<0.01). Though the highest phytoplankton abundance was observed in April,
February showed the highest diversity. The zooplankton community was less diverse and
less abundant and did not show any seasonality pattern. Phosphorus (0.0220.037 mg/l)
and nitrogen (0.1010.027 mg/l) had similar trends in the study area during the study
period. Transparency of water was very low (ca. 27 cm secchi depth) in 75% of the
reservoirs with communal lands’ reservoirs having a whitish colour with a likely effect of
reducing light penetration and therefore photosynthesis. The paper concludes by
acknowledging that communal lands have not gone through major land and water use
changes likely to effect its water quality and compromise its ecosystem health. Water
managers are urged to continuously monitor the changes in land and water uses around
these multipurpose reservoirs in order to prevent possible detrimental land and water
uses.
Key words: plankton community, small man-made reservoir, water quality, land use, water
resources management
INTRODUCTION
Most of Zimbabwe’s rural population lives in areas where the mean annual rainfall is
below 800 mm. This rainfall is highly seasonal, falling between November and March,
and varies from year to year with frequent droughts. Consequently, about 11000
reservoirs have been constructed throughout the country, of which about 8000 have
capacities of less than 0.1 x 106 m3 (Marshall and Maes, 1994). However these small
reservoirs have not been studied much although they support a number of activities in
many parts of the country.
Because of this lack of knowledge, it is important that these small reservoirs are better
understood, especially in regard to changes in water quality as a result of human
activities. This is presently a global issue because the deterioration of water quality has
disturbed ecosystem functioning and led to the contamination and pollution of ground
and surface waters in many places (Ongley, 1996).
This paper describes the water quality and plankton diversity and abundance in relation to
land use in eight small reservoirs in a semi-arid region of Zimbabwe.
It tests the
hypothesis that land use has an adverse effect on water quality in these reservoirs. Four of
these reservoirs were located in a national park and four in a communal land with a
relatively dense population of peasant farmers should exhibit differences in physicochemical characteristics because of differences in land use. These differences should then
affect the plankton communities in the dams.
2
Study area
The study was carried out in February and April 2005 in the southern part of Zimbabwe
in the Mzingwane and Tuli catchments (Limpopo basin) (Figure 1). Four reservoirs
(Sibasa, Makoshe, Denje and Dewa) were located in the Insiza communal lands while
four others (Chitampa, Mpopoma, Maleme and Mesilume) were located in the Matopos
National Park, 34 km south of Bulawayo.
Fig.1 Map of the study area (right) in the Mzingwane catchment (direction of the arrow),
Limpopo basin (left)
Nature Park
(Matopos)
Communal lands
(Insiza district)
UPPER MZINGWANE
Umzin
gwane
li
Tu
Shas
N
han
Insiz a
i
Shashe
M
zi
ne
we
Bubi
SHASHE
MW ENEZI
Shashe
LOWER MZINGWANE
Rivers.shp
Boundary.shp
Limpopo
0
100 Kilometers
These reservoirs were relatively small with a capacity < 3 x 106 m3 and a maximum depth
of eight metres. Those in the communal lands supply water to the local people and their
livestock, and are used for gardening, brick making and fishing. Those in the national
park supply water to wildlife and are used for recreational purposes such as angling.
METHODS
Soil samples were taken from the catchment area of each dam to give an indication of the
effects of land use: these were analysed for colour, texture using the buoyant hydrometer
method, and pH and electrical conductivity using the appropriate meters.
3
Water and plankton samples were collected from a depth of 0.5 m at four stations in each
reservoir, set out in a line facing the dam wall. Water samples were collected with a
Ruttner bottle and placed in a cooler box before being brought to the laboratory for
analysis. Total phosphorus, total nitrogen, hardness, pH and electrical conductivity were
determined. Total phosphorus and total nitrogen were respectively determined by the
Muphy-Riley method, titrimetric method using 0.01 M HCl. Color, texture, pH and
electric conductivity were analysed on soil samples collected around the small reservoirs.
These parameters were selected because they are easily indicative of the influence of
surrounding land uses to adjacent water bodies.
Plankton samples were collected with 20 and 62-µm plankton nets for phytoplankton and
zooplankton, respectively (Edmondson and Winburg, 1971). Phytoplankton samples were
preserved in Lugol’s solution while zooplankton samples were preserved in 4% formalin.
The samples were later analysed under an inverted microscope and the species using
Durand and Lévêque (1980), Canter-Lund and Lund (1995), and Fernando (2002).
RESULTS
Soil and water quality
The characteristics of the dams’ surrounding soils are presented in Table 1. The pH of the
soils in the national park was slightly acidic 5-6.6 with an average of 5.5 while communal
lands has alkaline soils ranging from 7.5 to 7.8. No significant difference was found
4
between the means of electric conductivity between the national park and communal
lands.
Table 1. Soil characteristics in the catchment areas of the eight reservoirs.
Area
Reservoir
Wet Colour
Dry Colour
Water colour
pH
EC
National
Park
Maleme
2.5Y3/2 (Very dark
grayish brown)
Light dark
(clear)
5
315
Mezilume
2.5Y3/2 (Very dark
grayish brown)
Dark grayish
(coffee)
5.2
400
4
Mpopoma
2.5Y3/2 (Very dark
grayish brown)
10Yr4/2(Dark
grayish brown)
2.5Y4/2 (Dark
grayish brown)
2.5Y5/2
(Grayish
brown)
2.5Y5/2
(Grayish
brown)
2.5Y6/2 (Light
brownish gray)
10Yr6/2 (light
brownish Gray)
2.5Y6/4 (light
yellowish
brown)
10 Yr 5/2
(Grayish
brown)
10 Yr6/4 (light
yellowish
brown)
2.5Yr6/0 (Gray)
Vegeta
tion
Score*
3
Light dark
(clear)
Brownish
5.1
730
4
6.6
354
2
Whitish
(milky)
7.5
558
3
Gray whitish
brown
7.5
456
3
Gray whitish
brown
7.8
330
2
Dark grayish
7.5
230
2
Chitampa
Communa
l lands
Sibasa
Dewa
10Yr3/3 (Dark
brown)
Denje
10Yr3/3 (Dark
brown)
Makoshe
2.5Yr3/0 (Very
dark gray)
The water in the reservoirs was slightly alkaline (pH = 7.6-8.5), with low conductivity (<
200 S cm-1 while hardness ranged from 23 to 104 mg l-1 (Table 2).
Table 2. Water quality characteristics of the eight reservoirs. Sib = Sibasa, Dew = Dewa,
Denj = Denje, Mak = Makoshe, Mal = Maleme, Mes = Mesilume, Mpo = Mpopoma, Chi
= Chintampa, EC = conductivity, TP = total phosphorus.
Variable
Communal lands
Sib
pH
Dew
National Park
Denj
Mak
Malem
Mezil
Mpop
Chita
FEB
7.6
8.1
7.7
7.7
8.5
7.9
8.0
7.6
APR
8.2
8.5
8.4
8.4
8.4
7.6
8.3
7.7
EC (S/cm) FEB
107
200
200
157
147
76
109
93
APR
111
250
183
198
138
102
110
109
Secchi depth (m)
0.2
0.3
0.2
0.3
0.5
1.7
2
0.1
Total Hardness (g/L)
42
104
58
84
47
23
36
36
Total Nitrogen (g/L)
94
87
84
99
8
12
11
14
7
11
6
6
4
3
3
7
5
TP (g/L) FEB
APR
8
17
38
5
4
59
2
34
There were some differences in water quality between the two sets of reservoirs (Table
2). Both were slightly alkaline (mean pH = 8.1 and 8.0 in communal lands and national
park, respectively) and with low conductivity (mean = 176 and 100 S cm-1,
respectively). Mean transparency in the national park reservoirs was about four times
higher (1.1 m) than in those in the communal lands (0.3 m). The water in the communal
land reservoirs was harder than in the national parks (mean hardness = 72.0 and 35.5 mg
l-1, respectively) and had a higher concentration of total nitrogen (mean = 91.0 and 11.3
g l-1, respectively). Although there was no significant difference in the mean
concentrations of total phosphorus (12.25 g l-1 in communal lands and 14.50 g l-1 in
the national park) these values were rather distorted by two high concentrations of
phosphorus in Mesilume and Chitampa dams in February. On the whole phosphorus
concentrations tended to be a little higher in the communal lands.
Phytoplankton abundance and diversity
The reservoirs’ flora identified and counted in February and April 2005 is presented in
Table 3. Hydrodictyon was the most abundant and accounted for an average of 30% of
the overall April phytoplankton samples. However, Hydrodictyon abundance was very
low in February at 0.1% of the phytoplankton sample. Hydrodictyon was followed by
Anabaena (20%), Peridinium (16%) and Melosira (12%) in April samples. February
samples showed an abundance of Melosira (19%) followed by Ceratium (17%) and
Pinnularia (12%). The phytoplankton taxa were much more abundant in April than in
February constituting 84% against 16% (February) (Table 3). However, February’s
6
phytoplankton taxa within major groups were much more diverse that April’s.
Chlorophytes had the highest number of taxa (29 in February and 20 in April).
Chlorophytes were followed in diversity by bacillariophytes or diatoms (17 taxa in
February and 12 in April). Though a difference has been noticed between February and
April samples in terms of abundance, no student’s t-test statistical significant difference
in phytoplankton species composition was found.
The total phytoplankton identified was far more diverse in February than in April (38
species against 22 in Communal lands) and (49 species against 32 in the National Park),
with a significant difference in their means using a student’s t-test (t=33, df=1, P=0.02).
These figures show as well that the National Park was more diverse as compared to
communal lands. This is confirmed by the significant difference found using the student’s
for the diversity in the National Park compared to the communal lands (t=-21, df=1,
P=0.03).
The highest Simpson’s index of diversity was obtained by the National Park in February
samples (0.91) followed by communal lands in April (0.77), communal lands in February
(0.76) and lastly National Park in April (0.5).
However, April samples showed more abundance than February (37,813 individuals per
litre against 16,386 in the National Park and 52,047 against 12,227 in the communal
lands). Abundance in February samples in communal lands was dominated by
Dinophytes (50%) followed by Bacillariophytes (27%), while in the National Park
Bacillariophytes (51%) dominated followed by Chlorophytes (41%). The Dinophytes
were not found in the National Park. April samples were dominated in terms of
abundance in the communal lands by Dinophytes (38.9%) followed by Cyanophytes
7
(34.4%) and Bacillariophytes. Differences in total phytoplankton abundance are shown
on Fig.2 and Fig.3.
8
Table 3. Composition and abundance (No. l-1) of the phytoplankton in eight reservoirs
located on communal lands and National Park in rural Zimbabwe
February 2005
Class
Chlorophyta
Bacillariophyta
Cyanophyta
Canophyta
Euglenophyta
Fungi
Dinophyta
Xanthophyta
Cryptophyta
Chrysophyta
Total
Taxa
Volvox
Amscottia
Sphaerocystis
Dictyosphaerium
Micractinium
Scenedesmus
Staurodesmus
Ankistrodesmus
Pediastrum
Sorastrum
Haematococcus
cladophora
Staurastrum
Unidentified2
Unidentified1
Unidentitfied3
Cosmarium
Euastrum
Sphaerozoma
Xanthidium
Actinastrum
Arthrodesmus
Hydrodictyon
Closterium
Selenastrum
Spondylosium
Onynchonema
Pleurotaenium
Micrasterias
Indet1
Onynchonema
Penium
Zygnema
Spirogyra
Cylindrocystis
Navicula
Surirella
Melosira
Achnantes
Asterionella
Cymatopleura
Rhopalodia
Oscillatoria
Gomphosphaerium
Gomphonema
Fragilaria
Rhizosolenia
Synedra
Pinnularia
Stephanodiscus
Amphiprora
Cymbella
Gyrosigma
Coelosphaerium
Microcystis
Microchaete
Merismopedia
Anabaena
Nostoc
Trachelomonas
Phacus
Euglena
Astasia
Rhizosiphon
Chytridium
Sporangium
Ceratium
Peridinium
Ophiocytium
Cryptomonas
Dinobryon
Communal lands National Park Abundance
128
0
128
304
616
921
166
154
320
47
0
47
54
26
80
14
31
46
13
173
186
5
0
5
31
73
105
2
0
2
0
647
647
4
12
16
10
1252
1262
0
1161
1161
0
864
864
0
263
263
8
441
449
21
94
116
0
78
78
0
93
93
0
462
462
0
38
38
0
27
27
4
2
5
0
13
13
0
73
73
0
26
26
0
28
28
32
96
128
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
142
825
968
53
5
58
3055
2310
5365
0
61
61
0
3
3
0
53
53
0
60
60
35
28
62
0
28
28
6
0
6
4
0
4
38
125
162
0
1430
1430
0
3397
3397
0
2
2
0
4
4
2
0
2
0
0
0
800
306
1106
153
101
254
4
0
4
4
0
4
48
33
80
93
0
93
0
21
21
116
427
543
115
7
122
150
0
150
0
2
2
24
96
120
441
308
749
4945
0
4945
1202
2
1204
3
0
3
0
11
11
0
0
0
12276
16386
28662
April 2005
Communal
lands
0
21
176
0
0
98
5
10
2562
0
0
0
1371
0
0
0
13
0
0
0
0
0
0
132
0
0
0
0
0
0
0
0
0
0
0
574
47
7923
0
0
0
0
0
0
0
0
147
78
5
0
0
0
18
0
0
0
0
17916
0
0
536
163
0
0
0
16
6151
14084
0
0
0
52046
National Park Abundance
0
0
0
21
122
298
0
0
0
0
5
104
248
254
78
88
28
2590
0
0
0
0
0
0
2264
3636
0
0
0
0
0
0
186
199
0
0
93
93
0
0
0
0
26
26
27221
27221
23
155
0
0
0
0
0
0
1082
1082
52
52
233
233
62
62
16
16
383
383
16
16
10
10
717
1291
0
47
2585
10508
127
127
0
0
0
0
0
0
0
0
0
0
0
0
31
31
414
562
72
150
427
432
0
0
0
0
194
194
0
18
0
0
0
0
0
0
0
0
0
17916
0
0
0
0
585
1120
18
181
0
0
0
0
0
0
0
16
18
6169
5
14089
0
0
0
0
471
471
37812
89859
9
10000
35000
Tot Communal lands
Tot National Park
Tot Communal Lands
Tot National Park
30000
8000
Number per liter
Number per liter
25000
6000
4000
20000
15000
10000
2000
5000
0
0
ChloroBacillarioCyano Cano Eugleno Fungi
Dino Xantho Crypto
Fig. 2 Abundance of phytoplankton major groups/February 2005
Chloro Bacilario Chryso
Cyano
Dino
Eugleno
Fungi
Fig. 3 Abundance of phytoplankton major groups/ April 2005
There was a significant difference in abundance between February and April
phytoplankton samples in the communal lands using the t-test (t=-2.06; P=0.05; df=14)
but no significant difference was found in the National Park (t=-0.86; P=0.39; df=14).
However, there was high significant difference in phytoplankton abundance between the
communal lands and the National park using a chi-square test (P<0.01).
Zooplankton diversity and abundance
Crustaceans and rotifers represented the zooplankton community with few individuals
belonging to the Ostracods group. The zooplankton community was dominated in
February by Copepods with Cyclops having 28.6 % followed by their youngsters
(nauplii) with 15.2%, the rotifer Keratella (14.2%) and Copepod calanoids nauplii (13%).
Communal lands had the highest zooplankton abundance in both February and April
samples with respectively 63% and 57%. Cyclops also dominated in April with 27 % of
the total abundance followed its nauplii, a cladoceran species, the rotifers Keratella and
Brachionus (ca. 10% for each of them).
Differences were noticed in zooplankton community structure figures between February
and April and between communal lands and the National Park (Fig. 4 and Fig. 5). Highest
abundances were found in communal lands with total abundance being 63.3% in
10
February and 56.9% in April against 36.7% in February and 43.1% in the National Park.
There was variability among the major groups in the two sampling periods. February’s
abundance was dominated by copepods with 60% of the total abundance (Cyclops 44%
and Calanoids 16%) followed by Rotifers (24%) and cladocerans (16%) with communal
lands having 63% of the total abundance. April’s abundance was also dominated by
Calanoids (49% in total with Cyclops having 38% and Calanoids 11%) followed by
rotifers (28%) and cladocerans (22%). Communal lands dominated the abundance in
April again with 57%. Zooplankton diversity did not show any difference between the
communal lands and the National Park, all having 13 species found in both February and
April.
160
40
140
120
30
100
Number in Litre
Number per litre
Communal land2
National Park2
Communal lands
National Park
80
60
20
40
10
20
0
Cladocera
Cyclopoda
Calanoida
Rotifera
Fig. 4. Zooplankton abundance in the study area in February 2005
0
Cladocera
Cyclopoda
Calanoida
Rotifera
Fig. 5. Zooplankton abundance in the study area in April 2005
DISCUSSION
Water quality aspects, and land use
The few water quality parameters that were analysed in the studied reservoirs and
presented in Table 1 indicate a general trend that is acceptable in comparison with the
WHO guidelines for drinking water (Chapman, 1992) and natural levels in freshwater
(Sinkala et al. 2002). For instance in the whole study area, pH was more or less alkaline
11
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 are in the normal and acceptable ranges and did not
show any affinity (significance) to 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 relatively
good 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. This is indicative of good planning by
settlers.
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.
12
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. 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 watercolour 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 is a major difference with the
national Park reservoirs’ water, which was mostly clear. This colour was very close to a
13
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. Sibasa, having the strong whitish colour,
might have got a very high abundance unlike the results obtained in the study.
Plankton community composition, diversity and abundance in relation to land and
water use
Seasonal variation in plankton diversity and abundance
This study found that the diversity and abundance of plankton species varied seasonally.
While this study failed to conclusively support the overall variation with statistical
significance, it is believed that rainfall patterns were responsible for the noted seasonal
variation. The two sampling periods fell within the span of the normal rainy season that
extends from November to April, though this was basically a dry season.. More
conclusive evidence for seasonal variation in plankton diversity and abundance may be
reached if future sampling-sessions took place both within the normal rainy and dry
seasons.
Results suggest here that rainfall variability can significantly impact the diversity and
abundance of plankton communities. This is an important concept, as rainfall patterns in
this area have been erratic in the past 10-15 years, and have the potential to continue
following non-normal trends due to climate variability or climate change in the future.
Rainfall patterns are considered here in the sense that their intensity might induce the
14
transport of sediments from upstream of reservoirs to the reservoirs. These sediments, if
transported are likely to affect the reservoirs water quality, their biotic composition and
ecosystems health.
Apart from this seasonal variation due probably to rainfall patterns, no difference was
noted between the communal lands and the National Park related to land and water use.
The reason, therefore, for a tendency to acknowledge the good status of the health of the
environment in the communal lands. This shows similarities between reservoirs in
communal lands to the pristine-considered reservoirs in the National Park. However, care
should be taken to applaud this finding since a more detailed investigation including all
the seasons need to be done in order to be sure of the behaviour of these ecosystems. As
Cander-Lund and Lund (1995) confirm that like humans need a regular check-up at the
hospital, the health of aquatic ecosystems need to be monitored through the observation
of their plankton composition- a regular monitoring might also be interesting to get on
track with the evolution of the status of these ecosystems.
Plankton composition
The significant difference found in phytoplankton abundance samples (and highlighted in
Fig. 2 and Fig. 3) using the chi-square method might be due to natural differences
between the National Park and the communal lands. The most abundant phytoplankton
taxon found during this investigation was Hydrodictyon spp. in April samples. Its
abundance might be due to the fact that the species is known to break into pieces
(Cander-Lund and Lund, 1995). It was found, however, in April samples that the taxa
15
was widely distributed. Hydrodictyon was rarely observed in February samples.
Anabaena sp., a blue-green algae (Cyanophyta), was the second most abundant species
observed in April though it was rare in the February samples. Though Anabaena sp. is
always associated with algal blooms, its abundance was not high enough to create an
algal bloom. Literature shows, in fact, that Anabaena sp. can be found in non-polluted
waters (Cander-Lund and Lund, 1995). However, the presence of this species, and others
that prefer similar ecological conditions, in areas where they are not expected to normally
occur might be a sign of the enrichment of waters by nutrients, a term referred to as
eutrophication. The current aquatic community structure would likely change with the
onset of eutrophication, perhaps altering water quality and rendering the reservoirs
unsuitable habitat for a variety of plankton species and unsuitable for human uses as they
currently stand. One particular risk of the cyanophytes group is the fact that most of the
species (including Anabaena sp.) contain toxic substances that can lead to fish kills
wherever their blooms occurs, especially in hyper-eutrophic ecosystems. They have
Nitrogen-fixing sites (heterocysts) on their organisms and are therefore able to fix
nitrogen; which means that they can proliferate rapidly. Anabaena is, particularly, known
to produce neurotoxins that affect the human central nervous system and hepatotoxins
that affect human liver (Chipfunde, L., Zimbabwe National Water Authority, personal
communication, 2005).
Ceratium, a dinophytes that is likewise known to produce toxic substances and red water
blooms, was found in both samples of February and April. Some species are rich in plants
nutrients such as phosphates and nitrates (Cander-Lund and Lund, 1995). Cander-Lund
16
and Lund (1995) states that even in such lakes it is often accompanied by cyanophytes.
The results obtained in this work have shown the presence of Ceratium as well as
cyanophytes, though the water bodies were nutrient-poor (oligotrophic). Ceratium and
Peridinium (another dinophytes) increased in abundance in April samples due probably
to favourable conditions to their proliferations such as an increase in total nitrogen levels.
This cannot be confirmed since total nitrogen was not analysed for February samples. An
increase in nutrient levels in the study area would enhance a high productivity level of
dinophytes and cyanophytes, leading to algal blooms, which would compromise health of
the ecosystems as they currently stand. It is therefore crucial to keep the water bodies
under observation.
The species that could cause algal blooms like Anabaena were mostly present in the
communal lands. Ceratium and Peridinium have also been found in high abundance in
the communal lands as compared to the National Park. These taxa are known to be
proliferating in nutrient rich waters (Cander-Lund and Lund, 1995); Ceratium being able
to exploit organic and inorganic nutrients and gain competitive advantage over purely
photosynthetic species (Smalley and Coats, 2002). Because these nutrient enrichment
indicative species are abundant in the communal lands, an argument would be made that
communal land sites should be monitored for an influx of nutrients that could spur them
into an algal bloom.
The lower diversity and abundance found in this study for the zooplankton community
might be explained by the presence of planktivorous fishes and most probably low light
penetration (low transparency, especially in the communal lands). Though fish abundance
17
was not part of this study, it was noted that fishes were present in all of the reservoirs.
Humans were observed actively fishing on the reservoirs. Planktivorous organisms have
preferences for specific food items (Wetzel, 1983). Large planktons are the preferred
food item, as they contain the most energetic reward to balance the energy loss the fish
has most incurred when hunting. In the case of the studied reservoirs, large planktons
were composed of big cladocerans like some species of Daphnia and Calanoids. The idea
of active hunting on large zooplankton can easily be derived where large Calanoids are
scarce while their juveniles are abundant. Large Cyclopoids and cladocerans were also
rarely found in the samples and most of the time when they were found they were only
carcasses. So there seems to be a good zooplankton productivity, which is very well
regulated by high predation by fish. This is in accord with Hrbàćek et al. (1958) in
Wetzel (1983) who shows that the size of the zooplankton community is regulated by the
presence of fish predators. The zooplankton community structure found is also in
agreement with Arcifa et al. (1986) who concluded that plankton proliferation is greatly
affected by the predator-prey relationships in reservoirs.
Management implications
The results of this study can be used to guide future management of these and similar
man-made reservoirs in rural Zimbabwe. Reservoirs on communal lands had similar
water quality as found in the National Park, which is attributed to the lack of upstream
development surrounding these particular reservoirs. However, it is believed that if
human populations alter the current use of these water bodies and develop upstream
areas, water quality will suffer. Therefore, it would be stressed that upstream
18
development, particularly development that would result in an influx of nitrogen and
phosphorus, be limited in these areas. This is particularly important as the reservoirs
contained phytoplankton that would proliferate into toxic algal blooms with the influx of
those particular nutrients. Such blooms would compromise the quality of water for both
human use and the health of the current aquatic community.
Secondly, this study found that local soil conditions were very different from water
conditions. This result can be attributed to the presence of a healthy vegetative cover
layer surrounding the reservoirs. Such a vegetative layer acts as a buffer to influxes of
elements, and helps to maintain stable and healthy water conditions. Reservoir managers
should maintain a healthy vegetative buffer around the water body to assist in mitigating
any future changes in local conditions.
CONCLUSION
This paper investigated the impacts of land and water use activities on the water quality
of small man-made reservoirs located in communal lands in Zimbabwe in comparison
with pristine considered environment of the National Park.
Water quality throughout the study area was at acceptable levels, and did not significantly
differ between National Park and communal lands. This finding was contrary to
expectations, and indicates that water conditions may be better in areas of human
influence than currently thought. This pattern is attributed to limited upstream
development, a condition that should be maintained to ensure the integrity of the aquatic
ecosystems. High levels of vegetative cover were also thought to mitigate the impacts of
19
local conditions on water bodies, and should be preserved to protect these systems from
future change.
The diversity and abundance of plankton communities in the study area were influenced
by an informal sub-seasonal rainfall patterns. Species composition of both phyto- and
zooplankton were similar to expectations based on other ponds and lakes. However,
species of phytoplankton were found that could potentially develop into toxic algal
blooms with changes in water quality. Zooplankton species abundance was at a lower
level than expected, possibly due to the presence of planktivorous fish in the reservoirs.
Water managers are urged to note that investigations are carried out to provide
information on health of the water bodies and allow them to develop strategies for better
management of catchment and water resources. It is very important to maintain the
current status of the ecosystems investigated by prohibiting any upstream development
likely to affect the water bodies and conduct environmental impacts assessment of any
major development plan. It is also important to investigate the status of small man-made
reservoirs located in the other areas of the Mzingwane catchment in order to assess and
evaluate the planning of settlers and for better management of water resources in the area.
ACKNOWLEDGMENTS
SRP CP40 is acknowledged for funding the fieldwork and sample analysis. WATERNET
partially sponsored the work through the Department of Civil Engineering of the
University of Zimbabwe. The authors are grateful to Prof. Jean-Berckmans B. Muhigwa
Centre Universitaire de Bukavu, DRCongo) for the help provided in statistical analysis,
20
Chattra Mani Sharma (NORAGRIC/Norway) for reviewing the manuscript and Elisabeth
Munyoro (Biological Sciences, University of Zimbabwe) for the sampling equipments
provision and preparation and Eng. Z. Hoko for the logistics.
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