E. R. Akpan
Institute of Oceanography, University of Calabar,
J. O. Offem
Department of Chemistry, University of Calabar,
A. E. Nya
Institute of Oceanography, University of Calabar,
Monthly measurements of physico-chemical variables: temperature, pH, transparency, salinity, dissolved oxygen and biochemical oxygen demand were undertaken in the Great Kwa River from November, 1987 to May, 1989. Water temperature ranged from 25.8oC to 34oC. Temperatures were significantly higher during the dry season than the wet season (P<0.05, n=9) but there were no significant spatial and vertical variations. pH ranged from 5 to 6.75. Dry season values were significantly higher than wet season measurements. Spatial trends were characterized by a significant decrease towards the sea during the wet season and an opposite trend during the dry season. There was no significant vertical variation in pH. Water transparency varied from 0.1 m to 1.2 m. Dry season values were significantly higher than wet season but there was no significant spatial variation. Salinity ranged from 0.01 to 0.81 ppt. Dry season values were significantly higher than wet season. There was no significant spatial variation in salinity during the wet season, but during the dry season salinity increased significantly towards the sea. Dissolved oxygen ranged from 4 to 8.4 mg/l. DO showed significant seasonal variation characterized by higher dry season values. There was no significant spatial variation but surface values were higher than bottom values during both seasons. Biochemical oxygen demand (BOD5) varied from 0.13 to 3.55 mg/l. Dry season values were significantly lower than wet season. BOD increased significantly upstream during the wet season but during the dry season there w as no significant spatial variation.
Key Words: Great Kwa River Nigeria, Physicochemical measurements, water quality.
Rivers serve multiple uses most of them being critical to human settlement and survival. Such uses include, portable water abstraction, fisheries exploitation, transportation, irrigation, animal husbandry, industrial water abstraction and recreation etc. Most rivers have, however, become sinks for municipal and industrial wastes. Depending on the quality and quantity of waste input , the physical, chemical and biological balance of receiving waters may be significantly modified resulting in pollution and its associated consequences.
The Great Kwa River is one of the major tributaries of the Cross River Estuary. It takes its rise from the Oban Hills in Nigeria, flows southwards and discharges into the Cross River Estuary around latitude 4o45’N and longitudes 8o20’E (Fig. 1). The lower reaches of the River drains the eastern coast of the Calabar Municipality (the Capital of Cross River State of Nigeria). The lower Great Kwa is characterized by semi-diurnal tides and extensive mud flats.
Apart from artisanal fisheries which targets mainly the Macrobrachium, human activities within the Great Kwa catchment is limited to small scale farming and aquaculture. With increasing population pressure associated with the Export free zone status of Calabar, human settlement and industrial layouts are expanding rapidly into the freshwater and mangrove swamps of the Great Kwa. Presently, the Calabar Municipality has no waste treatment facilities. Municipal wastes, including those from cottage industries are disposed off in scattered surface dump sites and open drains. During the characteristic torrential rains, most of the wastes are washed into the Great Kwa and Calabar Rivers. Expected future developments will put increasing pressure on the self-purification capacity of the Rivers with negative consequences on most water uses.
The present study was conducted to obtain baseline data and information on the quality of the Great Kwa for use in future quality and environmental impact studies.
Data collection and analysis
Sampling was conducted once monthly from November, 1987 to May, 1989. Samples were collected during spring tide at low tides from both surface and bottom depths using Nansen reversible sampler.
Water temperature was measured using mercury thermometer. pH was determined with SCHOTT electronic pH meter. Transparency was measured with Secchi disk. Salinity was obtained from chlorinity titration. Dissolved oxygen (DO) and biochemical oxygen demand (BOD5) were determined by the Winkler’s method. BOD was measured as the difference in DO concentration before and after incubation for 5 days at 20oC.
Results of physicochemical measurements in the Great Kwa River are summarized in table 1.
Figure 2 shows the spatio-seasonal trends in the physicochemical parameters. Although the highest temperature during the study occurred in the wet season, temperatures were significantly higher during the dry season than the wet season (P<0.05). There were no significant spatial and vertical variations in temperature. Similarly, pH values were significantly higher during the dry season than wet season. pH, however, displayed significant spatial variation characterized by a decrease towards the sea during the wet season and an opposite trend during the dry season. During the dry season, pH correlated positively with salinity (r = 0.6919, P = 0.05).
Transparency was significantly higher during the dry season than wet season but there was no significant spatial variation. During the wet season, transparency correlated positively with salinity and negatively with BOD.
Salinity was significantly higher during the dry season than wet season. Salinity did not show any significant spatial variation during the wet season but during the dry season values increased significantly towards the sea. There was no significant vertical variation.
Dissolved oxygen was significantly higher at bottom levels than surface and dry season had higher values than the wet season. During the dry season, DO showed positive correlations with transparency and temperature.
BOD was significantly higher during the wet than the dry season. There was no significant spatial variation during the dry season but during the wet season it increased significantly upstream. BOD showed negative correlations to transparency during both wet and dry seasons.
The temperature range observed, is usual in unpolluted tropical rivers (Raymondt, 1980). Similar temperature ranges have been reported elsewhere within the Cross River (Moses, 1979; Nawa, 1982; RPI, 1985; IOC, 1986; Etim & Akpan, 1991) and some other rivers of southern and western Nigeria (Ajayi & Osibanjo, 1981; RPI, 1985). The observed seasonal variation is directly attributed to the climate of the study area which is usually characterized by a hot dry season and cold wet season (Akpan, 1999). In the Calabar River, a close relationship between surface water temperatures and ambient air temperature was reported (Akpan, 2000). The observed vertical homogeneity is attributed to the shallow nature of the river (<6 m depth) and tidal mixing. Spatial homogeneity indicates the absence of heated effluent discharges usually associated with thermal power stations.
The observed pH levels are usual for unpolluted tropical rivers (Prati et al., 1971).The acidic nature of the pH values is attributable to geological and biochemical factors within the river catchment. According to Rodier (1975) water originating from terrains poor in lime or silica usually have pH values close to 7 or slightly lower. The geology of the lower Great Kwa basin is characterized by shales and sand stone. Such geological formations are poor in silica (Meybeck, 1980), which may partly account for the low values. The lower wet season values compared to dry season is attributed to dilution of ionic concentrations by rain water of low pH. The pH values of rain can be as low as 5.6 due mainly to dissolved CO2 (Nebel, 1987). Similar changes in seasonal values of pH attributed to rainfall, has been reported in temperate rivers (Cloern & Nichols, 1985). Additionally, during the wet season there is an increased input of humic materials from the associated swamps and creeks which contributed to observed low pH values. This is supported by the observed significant decrease in pH towards the sea during the wet season. Ajayi & Osibanjo (1981), reported low pH values in rivers of western Nigeria and attributed it to input of decaying organic matter and humic materials from the swamp forests. During the dry season, pH correlated positively with salinity, indicating the influence of salt water intrusion.
The observed transparency levels are usual in tidal rivers of southern Nigeria (RPI, 1985). According to Kausch (1990) high turbidities (i.e. low transparencies) is a characteristic of most estuaries with most having photic depths less than 1 m. The observed low transparencies particularly during the wet season is attributed to the input of silt and other turbid materials via surface run-off. In the Vaal River Estuary of South Africa, the presence of suspended sediments has been reported as the major source of high turbidities (Grobler et al., 1981). In the Great Kwa, a positive correlation was obtained between salinity and transparency during the wet season, indicating that rainfall-related inputs (mainly from surface run-off) is responsible for the low transparencies.
The range of salinities observed is low and usual for estuaries with high riverine discharge. Similar levels have been reported in the Cross River (Moses, 1979; Löwenberg & Künzel, 1992). The generally lower values compared to reports from the Cross River, is attributed to the limitation of sampling to only the low tide which excludes the influence of seawater intrusion. The low wet season compared to dry season values is attributed to high freshwater discharge. High freshwater discharge during the wet season is also responsible for the absence of spatial variation. During the dry season rainfall recedes and discharge from most creeks ceases so that marine influence and salt water intrusion dominate leading to significant increases in salinity towards the sea. Salinity changes in estuaries are mainly controlled by freshwater discharge and precipitation (Cronin et al., 1962; Haertel et al., 1969). In the Cross River, the wet season is marked by large increases in freshwater discharge (Etim & Akpan, 1991; Löwenberg & Künzel, 1992). Löwenberg & Künzel (1992) reported an increase from 879 m3s-1 during the wet season to 2,533 m3s-1 during the dry season in the Cross River and noted that salinities in the mid Cross River Estuary could change from 0.5 ppt during the wet season to 12 ppt during the dry season. The observed absence of vertical stratification may be attributed to the generally shallow nature of the Great Kwa as well as to limitation of sampling to low tide.
The range of DO concentrations observed, is similar to those reported for the Cross River. Moses (1979) obtained DO values of 3 to 3.5 mg/l for the upper Cross River during the wet season. Löwenberg & Künzel (1992) reported DO saturation values between 54 and 114% for the Cross River Estuary with the higher values occurring in the dry season. The observed high dry season and surface concentrations of DO compared to wet season and bottom levels is attributed mainly to algal productivity. In the Blue Nile, high dry season DO values was attributed to phytoplankton productivity (Talling & Rzoska, 1967). The generally shallow photic depths (low transparency) observed in the river, greatly limits phytoplankton productivity resulting in significantly lower levels of DO at the bottom. In addition, atmospheric exchange associated with wind shear can also contribute to higher surface DO levels. BOD was found to increase significantly during the wet compared to the dry season. High wet season BOD levels are ascribed to input of decaying organic matter from swamps during rains. During the dry season, when input from swamps via rain run-off secedes there was no significant spatial variation in BOD but during the wet season values decreased significantly towards the sea indicating the importance of riverine inputs. Ajayi & Osibanjo (1981) reported similar findings in rivers of western Nigeria and attributed it to import of decaying organic matter and humus from swamps into the rivers. The observed high wet season BOD levels is mainly responsible for the lower wet season compared to dry season DO levels. Generally, the range of BOD levels obtained is below values indicative of pollution (Ubong & Gobo, 2001).
We wish to thank the administration of the Institute of Oceanography, University of Calabar, for the provision of research boat and facilities.
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