Abstract: Diatoms as indicator of pollution in Awon reservoir were studied for two years. Two set of water samples were collected, one for physico-chemical characteristics analysis and the other set for biological analysis. Pinnularia lata, P. mesolepta, Navicula rhychocephala, Aulacoseira granulata and Amphora were the most prominent diatoms that served as good indicators of pollution. These organisms predominated during the raining season than during the dry season. Weather conditions ere majorly responsible for the physical and chemical changes in thee reservoir. The wet season of the experiment was characterized by the influx of flood water and the consequent mixing of the reservoir water with attendant increase in transparency and nutrient levels. The dry season of the experiment was characterized by high conductivity and high silica level. The study suggests that the presence of any of the predominated organisms in the water is an indication of water pollution.
INTRODUCTION
Diatoms are the most important phytoplankton group contributing substantially to aquatic productivity of oligo-trophic systems. Anadu et al. (1990) and Khan et al. (1983) demonstrated that water quality affects species composition and abundance. The most common abiotic parameters associated with primary production in lakes and reservoirs are phosphate and nitrate contents which are associated with algal blooms, variations in chlorophyll content and phytoplankton dynamics. Other important abiotic factors in lakes and reservoir productivity studies include dissolved oxygen, alkalinity, carbondioxide, biochemical oxygen demand and dissolved organic matters (Anadu et al., 1990). Ecology of diatoms had been studied (Cox, 1975; Dakshini and Soni, 1982; Lowe, 1974; Patrick, 1964; Patrick, 1973).
In Nigeria, the dominance of diatoms in waters has been documented (Amuda, 1990; Akinyemi, 2000; Imevbore, 1971; Egborge, 1979) and in the coastal waters of the south western Nigeria (Nwankwo, 1986). Presently, there is little or no information on diatom community of Awon reservoir which is a popular reservoir responsible for a significant provision of drinkable water for the human population in the local community.
MATERIALS AND METHODS
Description of the Study Area
The reservoir (Fig. 1) lies between latitude 7°75’N
and longitude 4°E. It was built in 1962. The purpose of the reservoir is
to supply portable water to the people of Oyo community and its environs. The
macrophyte by the reservoir bank comprise mainly of Pistia stratiotes and
few species of Ipomea.
| Fig. 1: | Map of Awon reservoir showing sampling stations |
Collection of Samples
A canoe was used during the period of investigation. Two sets of sample
were collected in 250 mL plastic bottle, one for physico-chemical and the second
set for biological characteristics of the reservoir. The sample for phytoplankton
analysis was preserved in 4% formalin while the sample for physico-chemical
characteristics was deep frozen at -4°C in the laboratory prior to analysis.
Choice of Sampling Stations
Monthly water samples were collected from seven stations created within
the dam site for 2 years.
Physical and Chemical Parameters
In situ surface water temperatures were taken in the field using
an ordinary mercury thermometer. Transparency was measured using a secchi disc
lowered from the surface until it disappeared from view. Nitrate and phosphate
ions were determined using colorimetric method. The Hach colorimetric method
was used for the determination of silica ion.
Biological Analysis
Ten drops of each well-mixed sample were thoroughly investigated under a
Wild binocular microscope with a calibrated eye piece. The density of the diatom
species was computed by calculating average cells per transects. All counts
were recorded as number of organisms (cells, colonies, filaments) mL-1
for the identification of organism, the keys and drawings provided by Patrick
and Reimer (1966) were used.
RESULTS
The results of some physical and chemical characteristics of the Awon reservoir are shown in Table 1 and Fig. 2 and 3 for both stations. Higher temperature values were recorded in the dry season (≥27°C) while lower value (<27°C) were recorded in the wet season and in December (Table 1). The transparency was generally low (<40 cm) during the wet season (May-October) and there was much variation in the reservoir (Table 1, Fig. 2 and 3).
Silica values were higher during the dry season (14 mg L-1) compared to wet season (Table 1). The highest peaks for nitrate values were recorded in the wet months (>2 mg L-1) while the lowest values were recorded in the dry months (Fig. 2) in all stations, phosphate-phosphorus concentration (mg L-1) ranged from 1.01-3.60 mg L-1 in the wet months and 0.03 to 2.31 mg L-1 in the dry months as shown in Fig. 2 and 3.
Diatom Communities
The diatom composition and abundance for some stations in Awon Reservoir
are presented in Table 2 and 3, Fig.
4-11 show the graphical forms of the diatom abundance.
Fifty four diatom species, 52 pinnate and 2 centric forms belonging to 16 genera
were identified. The most abundant species of the pinnate diatoms were Pinnularia
otiensis, P. gibba, P. lata, Navicula menisculus, N.
rhnchocephala, Kurtizing N. pseudogrimmei, N. acuminate, N.
Salinicola, Nitzschia palea, Synedra, ulna, S. acus,
Tabellaria fenestrata, Cymbella ventricosa, Amphora sp.
Diatoms with 40% relative abundance or occurrence were recognized as good indicators of pollution. These diatoms are referred to as pollution tolerant species. The diatoms in this group include P. lata, P. biceps, Navicula rhnchocephala, Aulacoseira granulata, P. mesolepta and Amphora sp. (Table 2 and 3, Fig. 4-11).
| Table 1: | The physical and chemical characteristics of Awon reservoir |
| Table 1: | Continued |
| Fig. 2: | The physical and chemical characteristics of Awon reservoir(station 1) |
| Fig. 3: | The physical and chemical characteristics of Awon reservoir (station 7) |
| Table 2: | Percentage composition of most abundant diatom taxa in station 1 |
| Table 2: | Continued |
| Table 3: | Percentage composition of most abundant diatom taxa in station 7 |
| Table 3: | Continued |
| Fig. 4: | Percentage composition of most abundant diatom taxa in station 1 |
In all the stations sampled, the pollution tolerant organisms were more during the wet months compared to dry months.
| Fig. 5: | Percentage composition of most abundant taxa in station 1 |
| Fig. 6: | Percentage composition of most abundant taxa in station 1 |
| Fig. 7: | Percentage composition of most abundant diatom taxa in station 1 |
| Fig. 8: | Percentage composition of most abundant diatom taxa in station 7 |
| Fig. 9: | Percentage composition of most abundant taxa in station 7 |
| Fig. 10: | Percentage composition of most abundant taxa in station 7 |
| Fig. 11: | Percentage composition of most abundant diatom taxa in station 7 |
DISCUSSION
The raining season between May and October in Nigeria was characterized by low transparency, increased nutrients concentration, increased total suspended solids, higher turbidity and increased flood water condition. This observation tallies with the work of Imevbore (1971) who concluded that the rains bring about low transparency, increased nutrient concentration and high turbidity in dams and reservoirs. The comparatively high temperature of the dry season could be due to high light intensity. The enrichment of the reservoir water in the wet season through flood water resulted in the proliferation of diatoms (Akinyemi, 2000). The higher values of nitrates and phosphates recorded during the wet months may be due to enrichment of the reservoir water from watershed during this period, which resulted in the massive growth of certain species of diatoms (Dakshini and Soni, 1982). The more rapid rate of change in diatom species composition observed during the dry months could be attributed to a more stable environment and availability of organic nutrients. Silica content of the reservoir water was found to be higher during the dry months. This substance could have been brought in during the wet months through floodwater and became concentrated as water volume reduced during the dry months (Anadu et al., 1990). As a result of excess nutrients during the wet season certain species of diatoms formed the dominant organisms in the reservoir. These organisms are called pollution tolerant species. The organisms are Pinnilaria gibba, P. biceps, Navicula rhnchocephala, Aulacoseira granulata, P. mesolepta and Amphora sp. The liveability and substainability of these organisms despite the stressful environmental conditions and highly turbid water qualified them as good indicators of pollution. This agrees with findings of Anadu et al. (1990) and Khan et al. (1993) who pointed out that water quality affects species composition and abundance. The present study suggests that the presence of any of these organisms in water is an indication of water pollution.