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Assessing the impact of anthropogenic influences on the sediment quality of Owalla Reservoir, Southwest, Nigeria

Abstract

The study aims to investigate the impact of anthropogenic influences within the catchment areas on the sediment physico-chemical quality of Owalla Reservoir in Osun State, Nigeria with the primary aim of creating baseline information on the aspect of limnology. Twenty sampling stations were established along the reservoir representing, its three different sections and two zones. The sampling was conducted for a period of two years to cover both seasons of the year in the area. The sediment samples were collected using a Van-Veen grab of 0.04 m2 area (0.2m × 0.2m) from the waterbed at each sampling locations; labelled, kept in cooler at 4°C and transported to the laboratory for the analysis. The reservoir sediment was mainly clayey-sand in texture, while the particle size distribution was in this order: sand (73.1%) > clay (17.4%) > silt (9.4%). The exchangeable cations order of dominance was in this order: Mg2+ > Ca2+ > H+ > Al3+ > K+ >Na+ and anions in this order: PO34- > SO42- > NO3-> Cl-. The highest mean values for water depth, transparency, air temperature, water temperature and sediment temperature (5.28 ± 0.40 m, 1.60 ± 0.05 m, 31.90 ± 0.29°C, 30.6 ± 0.18°C and 27.6 ± 0.23°C) were recorded during the dry season compared with the rainy season. The results obtained from this study, shows that Owalla Reservoir is fairly clean and not contaminated with toxic pollutants. The sediment pH is within acidic range, and enriched with nutrients due to the anthropogenic activities around this reservoir catchment areas. However, if the organic enrichments and siltation in this reservoir is not controlled, it could lead to the future eutrophication of this waterbody, which can cause water quality degradation, alter the aquatic biota and natural ageing process of this reservoir making it unsuitable for its purposes.

Introduction

Increase in demand for potable drinking water around globes is evident. This can be attributed to loss storage capacities that occur as a result of sediment influx into the Reservoirs or/ Lakes, which can also affect the water quality [1]. The average rate of storage capacity loss per year ranged from 0.5 to 1% of the total water storage capacity worldwide [1]. Sedimentation is one of major threats to reservoirs’ sustainability across the globe. This is because it reduces the original design storage capacity and its uses as a parameter in assessing the life expectancy of any reservoir as well as its water management system [2]. Sediments comprise of all detrital, inorganic, or organic particles that eventually settle at the bottom of any waterbody, which play a crucial role in an aquatic environment [3]. Sediments are typically a matrix of materials that can be very heterogeneous in terms of their physical, chemical, and biological features, and they can operate as both a sink /or nutrients source and/or other contaminants in waterbodies [4]. According to Boyd [5], sediment is made up of organic matter, weathered elements, and it varies from place to place depending on the parent materials, climate, and biological activity. Sediment is the primary location for microbial decomposition of organic detritus and biogeochemical recycling of nutrients, as well as chemical elements deposition [6].

Sediment is eroded from the landscape, transported by river systems, and eventually deposited in a lakes or Reservoirs [7]. Toxic chemicals can become attached, or absorbed, to sediment particles and then transported to and deposited in other areas. The sediment trapped in the reservoirs or lakes is not inevitable but not in all reservoir, because of their design which allow flow of sediment out of the reservoir, using a range of proven techniques that are applicable to different conditions. The rapid inflow of sediment or other detritus from river systems, deposited and sinks in reservoirs are subject to a natural ageing process by increase/rapid growth of hydrophyte infestation, eutrophication and siltation which can lessen its lifetime and can interrupt its primary purpose. It can alter the water quality of a reservoir, affect the distribution and diversity of aquatic organisms especially bottom dweller organisms and also depriving downstream reaches of sediments that is essential to maintain its morphology, ecology, as well as replenishing vital land at the coast and to support the riparian ecosystem [8]. Sediment transport pose a number of problems for watershed management and also impairs the flow regulation operation of a reservoir, it reduces storage capacity and adversely affect reservoirs’ functions such as water supply, flood control or navigation and power production. Sediment particles can carry agricultural and industrial toxic compounds and if these released in the habitat, can be harmful to an aquatic biota from primary producer to consumers. Ranged from phytoplankton to fish which come across all taxonomic groups, trophic positions and biological organization levels [9]. The change in sediment chemistry affect the aquatic organisms especially bottom dweller and these impacts vary substantially among family, species and life stages [10]. On fish, direct impacts range on a timescale from very short to long term effects and grade along a severity scale distinguishing behavioural, sub-lethal, para-lethal and lethal effects [11]. Sediment decreases the penetration of light in water, that could affect fish schooling practices, and their survival.

Sediment in high concentrations irritates the gills of fish, and can also destroy the protective mucous covering the eyes and scales of fish, making them more susceptible to infection and diseases. Sediment in high can dislodge aquatic plants and organisms. Settling sediments can bury and suffocate fish eggs. Behavioural effects such as avoidance of the sediment flume are the first signals of impairment in fishes [12]. Salmonids are more sensitive than other families, their larvae and early juveniles are considered more sensitive than adults [13]. However, if the disturbance intensifies, a variety of physiological and metabolic responses, linked with oxygen acquisition, growth, or stress control can occur [14]. Any forcing variable, climatic change or human in origin, will be reflected in the response of the lake through an influx of greater quantities of sediment or through a change in water quality. Breaking down of organic content in sediment by microorganisms can lead to oxygen depletion and gases (methane, hydrogen sulphide and other gases) releases can cause water quality degradation, which has overtime threatens the sustainability of water supply in African at large [15, 16]. Regrettably, sediment management approaches are not been put into consideration during construction of many Reservoir/Dam in Nigeria, which invariably prevented an opportunity to sustain the reservoir function in future, and to minimize downstream impacts of sediment starvation [4].

Sediment is a natural part of aquatic habitats and any changes in its characteristics can affect the physical, chemical and biological integrity of aquatic ecosystem [17]. Sediment plays a major role in the transport and fate of pollutants, yet they are often neglected as one of major factor in water quality assessment. The association of toxic chemicals deposited in sediment is a globe issue due to its impact on aquatic organisms. By studying the sediment characteristics of waterbody, scientists can determine the sources and evaluate the impact of pollutants on aquatic ecosystem and water quality. This study will provide useful information on the trend of pollution levels from anthropogenic activities on the sediment quality of Owalla reservoir, the geographical and temporal fluctuations in its physico-chemical properties, as well as details about the impact of sedimentation damages on this reservoir.

Materials and methods

Study area

The Owalla Reservoir is one of the largest impounded waterbodies in southwest, Nigeria. It is located at Oba-Oke close to Okinni in Osogbo, Osun State and it lies between Latitudes of 07° 53.5’N to 07° 59.0’N and Longitudes of 004° 31.5’E to 004° 35.0’E with an average elevation of 336 ± 8m above sea level. The reservoir was impounded from Erinle River in 1989 by Ede Power Plant by the Water Corporation of Nigeria [18]. The reservoir was created to supply drinking potable water to people living in about 7 cities in Osun State (Osogbo, Ede, Ife, Gbongan, Erin-Osun, Ilobu and Ifon) as well as other towns and rural communities in Osun central, Osun West and Ife area in Osun state [19]. The reservoir is bordered by four (4) Local Government namely: Odo Otin, Ifelodun, Irepodun, and Orolu Local Government in Osun State at its northern, eastern, western, and southern portions, respectively. The major riparian communities surrounded this reservoir are Ilie, Oba, Bara, Onipakiti, Kuti, Igbokiti, Idiroko, Eko-Ende, and Ore, etc. mostly peasant farmers, fishermen or petty traders in agricultural goods [20]. The reservoir has a maximum width of 3.5 kilometres and a maximum length of 12 kilometres from the dam wall to the upstream (i.e., the point where the main river, Erinle, enters the reservoir). The dam wall is approximately 677 m long and 27.5 m high. Its surface is around 14.5 km2, and its volume is approximately 94 106 m3 [20].

The study area is of lowland tropical rainforest flora makes up the landscape [21, 22], the soils are ferruginous tropical red soil (laterites), with little erosion and soil deterioration [23], typical tropical climate and is surrounded by the basement complex of southwest Nigeria [20]. The major sources of anthropogenic activities around this catchment basin come from domestic and agricultural waste. Also, widespread and persistent practice of rotational bush farming, couple with the widespread cultivation of cash crops such as cocoa, kolanuts, platin around this waterbody has led to the destruction original vegetation [18]. At the upstream of the reservoir, the major human activities include: farmers, fishermen, washing and bathing, cattle graze, traders of agricultural products and pisciculture or fish farming is pronounced in this region of the reservoir which involves commercial breeding of fish, in fish tanks or artificial enclosures such as fish ponds [24, 25]. At Mid-basin of the reservoir, the major human activities include: farming, fishing and washing activities, construction of bridge linking Eko-Eden and Oba was on going in this area, and there was means water transportation by canoe to cross to the neighbouring town. At the downstream there is lesser human activities because it is deepest part except fishing activities [2426]. The reservoir has great potential to become a major tourists’ attraction, in the state due possibilities of engaging in various water sports and activities. Neighbouring communities in the area, commute via canoes to get their children to school and other domestic activities.

Sampling stations and program.

Twenty (20) sampling stations were established on Owalla reservoir representing the entire three portions of the reservoir (upstream, mid-basin, and downstream) and two zones (littoral and open water) as presented in Table 1. Bottom sediments samples was collected at interval of every three months for period of two year to cover both season (April to October represent Rainy season while November to March represent Dry season) using a plank boat propelled by an outboard motor.

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Table 1. Grid co-ordinates and morphometric characteristics of the sampling stations.

https://doi.org/10.1371/journal.pwat.0000135.t001

Sample collections/field determination and laboratory analysis.

The bottom sediment samples were grabbed 4–5 times using Van-Veen grab of 0.04 m2 area (0.2m × 0.2m) from the waterbed at each sampling locations. The sediment samples collected were kept in a well-labelled glass container or polythene bag which was then stored in cooler at 4 ֯C to halt biological activity and to prevent any chemical transformation in the sediments and transported to the laboratory for further analysis [27]. To ensure accurate results and reliability of the data gathered acceptable standard methods and procedures that is adequate and proper quality control and quality assurance (QA/QC) measures were adopted both at the field site and during the laboratory analysis.

Sediment analysis was based on air-dried samples sterile trays at room temperature (25 ֯C) and chemical analyses were carried out using laboratory manuals and guides by Golterman et al. [28], APHA [29]. Sediment samples were gently crushed with mortar and pestle in the laboratory. Particle size analysis was determined using the hydrometer method with sodium hexametaphosphate {Na6(PO3)6} as dispersing agent [30] and the textural composition was determined based on the percentage occurrence of the three main sediment particles (sand, silt and clay) using the triangular graph method [31]. Hydrogen-ion concentration (pH) was determine using a glass electrode pH meter (PCE-PHD-1 pH meter) oxidation-reduction potential in 1: 2 sediment-water suspensions were also assessed after 30 minutes of equilibration [32]. The exchangeable cations (Na+, K+, Mg2+, and Ca2+) of the sediments were extracted into solution with 1N (pH 7.0) ammonium acetate solution, NH4OAC (I.I.T.A., 1999). Na+ and K+ concentrations were measured using a flame analyser at wavelengths of 295 nm, 383 nm respectively, while Mg2+ and Ca2+ were measured using Atomic Absorption Spectrophotometer-(AAS) (Perkin-Elmer Model 403) respectively [33].

Exchangeable acidity (H+ and Al3+) contents were extracted into an aqueous solution using 1.0 M KCl, and their concentrations were determined by titrating the filtered extract with 0.01 M NaOH and HCl solutions while using phenolphthalein indicator [34]. Chloride was determined by titration method based on sediment-to-water suspension of 1:5 filtered and titrated with 5% potassium chromate (K2CrO4) solution and 0.014N silver nitrate solution [29]. Sulphate (SO42-) was extracted with 1N calcium acetate Ca(OAc)2, from the sediment, and the concentration was then quantified spectrophotometrically (Turbidometric technique) using the violet filter at 420 nm wavelength [29]. The sediment samples were digested for total nitrogen (digestion mixture contains Selenium powder, Lithium sulphate and Hydrogen peroxide) with the addition of reagent comprising (sodium salicylate, sodium citrate, sodium tartarate, sodium nitroprusside, sodium hypochlorite, water, and sodium hydroxide) for colour development. After that, a spectrophotometer was used to measure the absorbance at 665 nm [35]. Phosphorus was determined by the Bray N0-1 method using 4.4 ml of the digestion mixture (0.03N NH4F + 0.025N HCl) on 0.2 g of sediment, and then the phosphorus concentrations were then measured using a colorimeter at 660 nm [30]. Chromic-acid digestion; potassium dichromate (10 ml of 1N K2Cr2O7) acidified with conc. sulphuric acid, or chromic acid as an oxidant, was used to digest a known quantity (between 0.1 and 0.5 g) of air-dried sediment for a full 24 hours, and the organic carbon contents were determined by titration using; Barium-diphenylamine-sulfonate (BDAS), an indicator, and ferrous ammonium sulphate (FeNH4S04.7H20) (FAS), a titrant [36].

Statistical analysis.

The data obtained from this study was subjected to normality and homoscedasticity test by Log transformation to restoring equal variance among the sampling station since the sample size differ between stations. Data were analysed by computing the descriptive and inferential analysis. Two-way analysis of variance (ANOVA) was used to test significant variations among the stations while student t-test was used to test seasonal variation. Principal component Analysis (PCA) was used to check the interrelationship between the sediment physico-chemical parameters among station and season using Microsoft Excel, Paleontological Statistics (PAST Version 3.17) and R-software.

Results

The results of sediment chemistry analysis obtained from Owalla reservoir revealed the order of dominance pattern for major exchangeable ions in this order. Cations: Mg2+ > Ca2+ > H+ > Al3+ > K+ > Na+ and anions: PO43- > SO42- > NO3- > Cl-. The range of depth recorded in this study varies between 0.98 to 21.0 m with mean value of 5.17 ± 0.29 m while the overall mean values of transparency, air temperature, water temperature and sediment temperature recorded during the periods of study were 1.56 ± 0.04 m; 36.0°C; 33.0°C and 34.0°C, respectively (Table 2). The highest overall mean of sand (73.1 ± 1.66%); Silt (9.4 ± 0.77%) and clay (17.4 ± 1.07%) fractions were recorded for the textural composition. The overall mean values of pH (H2O), pH (CaCl2), ORP (H2O) and ORP (CaCl2) were 5.27 ± 0.06; 4.80 ± 0.05; 85.26 ± 3.54mV and 111.47 ± 2.85mV. The conductivity values observed during this study ranged from: 64.3 to 2630 μScm-1 (284.98 ± 45.65 μScm-1) while the overall mean concentrations for aluminium, hydrogen ion, sodium, potassium, calcium, magnesium, chloride, sulphate, phosphorous, total nitrogen and organic matter were 0.28 ± 0.01 cmol/kg; 0.04 ± 0.20 cmol/kg; 0.18 ± 0.01 cmol/kg; 0.28 ± 0.02 cmol/kg; 1.45 ± 0.10 cmol/kg and 2.14 ± 0.09 cmol/kg, 31.46 ± 1.05 μgg-1; 172.56 ± 18.31 μgg-1; 97.78 ± 7.36 μgg-1; 0.13 ± 0.003% and 3.20 ± 0.20%, respectively. The overall mean concentration of nitrate and phosphate recorded during this study ranged from 35.44 to 9746 μgg-1and 19.293 to 1368.85 μgg-1 with mean values of 5354.76 ± 144.02 μgg-1 and 342.71 ± 26.69μgg-1 (Table 2). Nitrate have the highest median value of 5316 while aluminium had the highest skewness and kurtosis values (3.20 and 14.13). Silt and sulphate had the highest coefficient of variation (CV) value of > 100% while clay, conductivity, ORP (H2O), aluminium, hydrogen ion, sodium, potassium, calcium, magnesium, CEC, total nitrogen, phosphate and organic matter were in range of: 50% > CV < 100%. Transparency, air temperature, water temperature, sediment temperature, sand, pH (H2O), pH (CaCl2), ORP (CaCl2), base saturation, chloride, nitrate and phosphorus recorded CV of < 50% as presented in Table 2.

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Table 2. Overall descriptive statistics of sediment physico-chemical parameters of Owalla Reservoir.

https://doi.org/10.1371/journal.pwat.0000135.t002

Spatially, the highest mean depth recorded during this study was observed in the downstream station (8.12 ± 1.17 m) while the lowest mean depth was observed at upstream station and there was high significant difference (p < 0.001) across all the station as presented in Table 3. The highest mean of silt, magnesium, base saturated, phosphorus and organic matter (12.89 ± 2.15%, 2.37 ± 0.19 μgg-1, 85.30 ± 1.34%, 110.43 ±14.79 μgg-1 and 3.52 ± 0.40%) was recorded at upstream station. The highest mean values of transparency, percentage sand and pH (Cacl2) (1.88 ± 0.08 m, 80.90 ± 2.52% and 4.99 ± 0.13) was observed at the downstream station while air temperature, water temperature and CEC values (30.50 ± 0.31 ֯C, 30.10 ± 0.15 ֯C and 4.98 ± 0.27) were at Mid-basin station (Table 3). The maximum mean values of depth, conductivity, ORP (H2O), ORP (CaCl2), potassium, sulphate, nitrate, phosphate, sodium and total nitrogen (7.19 ± 0.55m, 828.30 ± 91.70 μScm-1, 115.10 ± 4.87, 136,00 ± 3.69, 0.35 ± 0.03 cmol/kg, 284.92 ± 39.97 μgg-1, 6067.52 ± 192.87 μgg-1, 436.93 ± 49.14 μgg-1, 0.21 ± 0.02 μgg-1 and 0.14 ± 0.00% was recorded in open-water while higher mean values of sediment temperature, pH (H2O), pH (Cacl2) and hydrogen ion (28.00 ± 0.15 ֯C, 5.57 ± 0.07, 5.05 ± 0.05 and 0.35 ± 0.02) was recorded at the littoral zone (Table 4). The Principal component analysis (PCA) show a strong positive relationship between pH (H2O), sand fraction (%), and depth while a negative correlation between chloride. aluminium, air temperature, conductivity, sulphate, chloride, total nitrogen, hydrogen ion, base saturation and potassium in the downstream station while parameters like clay fraction (%), nitrate, CEC, phosphate, phosphorus, transparency correlate with others at upstream station (Fig 1. Principal Component Analysis (PCA) showing the relationship between sampling stations and sediment physico-chemical parameters of Owalla Reservoir). Only transparency show a positive correlation at the Mid-basin station while pH (CaCl2), ORP (CaCl2), ORP (H2O), organic matter, percentage silt, water temperature, calcium and sodium show a negative correlation with each other as shown in Fig 1. Principal Component Analysis (PCA) showing the relationship between sampling stations and sediment physico-chemical parameters of Owalla Reservoir.

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Fig 1. Principal Component Analysis (PCA) showing the relationship between sampling stations and sediment physico-chemical parameters of Owalla Reservoir.

Where: ST = Sediment temperature, K = Potassium ion, Base Sat. = Base saturation, TN = Total Nitrogen, H ion = Hydrogen ion, Cl = Chloride, AT = Air temperature, EC = Conductivity, D = Depth, SD = Sand, CL = Clay, TRN = Transparency, OM = Organic Matter, Mg = Magnesium ion, SL = Silt, WT = Water temperature, Ca = Calcium ion, Na = Sodium ion.

https://doi.org/10.1371/journal.pwat.0000135.g001

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Table 3. Spatio-temporal variation of sediment physico-chemical parameters of Owalla Reservoir.

https://doi.org/10.1371/journal.pwat.0000135.t003

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Table 4. Spatial variation (breadth) in sediment physico-chemical parameters from the major axis of Owalla Reservoir.

https://doi.org/10.1371/journal.pwat.0000135.t004

Seasonally, the highest mean values of depth, transparency, air temperature, water temperature and sediment temperature (5.28 ± 0.40 m; 1.60 ± 0.05 m; 31.90 ± 0.29°C; 30.6 ± 0.18°C and 27.6 ± 0.23°C) were recorded during the dry season compared with the rainy season and there was a high significant difference (p < 0.001) in air and water temperature between the seasons (Table 3). The sediment physico-chemical parameters like sand fraction (%), pH (H2O), pH (CaCl2), base saturation and organic matter (77.10 ± 1.80%, 5.33 ± 0.09, 4.87 ± 0.07, 82.34 ± 1.19% and 3.27 ± 0.30%) were higher during the dry season compared to rainy season while silt, clay, conductivity, ORP (H2O), ORP (CaCl2), aluminium, hydrogen ion, potassium, magnesium, CEC, chloride, sulphate, nitrate, phosphate, phosphorus and total nitrogen (10.90 ± 1.30%, 19.90 ± 1.69%, 643.30 ± 67.41 μScm-1, 89.00 ± 4.92, 115.20 ± 3.86, 0.30±0.02 cmol/kg; 0.41 ± 0.02 cmol/kg; 0.03 ± 0.02 cmol/kg; 2.18 ± 0.14 cmol/kg; 4.82 ± 0.27 cmol/kg; 179.13 ± 22.62μgg-1; 5543.04 ± 211.75 μgg-1; 375.34 ± 39.55μgg-1;106.78 ± 10.91μgg-1 and 0.13 ± 0.003%) were higher in the rainy season than in the dry season (Table 3). There was positive correlation between pH (CaCl2), CEC, phosphate, conductivity, sediment temperature and silt during the rainy season while potassium, nitrate, sulphate, ORP (H2O), clay, ORP (CaCl2), aluminium, hydrogen ion, transparency, base saturation and sodium shown a negative relationship during rainy season. There is strong positive correlation between water temperature and depth during dry season while air temperature, sand, pH (H2O), magnesium, calcium and total nitrogen in the dry season (Fig 2. Principal Component Analysis (PCA) showing the relationship between season and sediment physico-chemical parameters of Owalla Reservoir). Textural types shown by some ternary plot are: Sand-84 (55.3%); Clayey-sand-42 (27.6%); Silty-sand-6 (3.9%); Sandy-clay-5 (3.3%); Sandy-mud-5 (3.3%); Clayey-mud-3 (2.0%); Silty-mud-3 (2.0%); Silty-clay-2 (1.3%); Clayey-silt-1 (0.66%) and Sandy-silt-1 (0.66%) each of the total sediment textural type as presented in Fig 3. Ternary diagram showing the sediment textural composition of Owalla Reservoir. The major dominant ions in sediment analysed from Owella Reservoir are Magnesium (Cations) and Sulphate (Anions) but contradict the result from trilinear/piper plot were Calcium and chloride are most dominant from the diagram (Fig 4. Trilinear diagrams showing the major Cations and Anions order of dominance in the sediment of Owalla Reservoir). The water is alkaline earth exceed alkali metals and strong acids exceed weak types (Fig 5. Modified Trilinear diagrams showing the alkaline/alkalie of Owalla Reservoir).

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Fig 2. Principal Component Analysis (PCA) showing the relationship between season and sediment physico-chemical parameters of Owalla Reservoir.

Where: ST = Sediment temperature, K = Potassium ion, Base Sat. = Base saturation, TN = Total Nitrogen, H = Hydrogen ion, Cl = Chloride, AT = Air temperature, EC = Conductivity, D = Depth, SD = Sand, CL = Clay, TRN = Transparency, OM = Organic Matter, Mg = Magnesium ion, SL = Silt, WT = Water Temperature, Ca = Calcium ion, Na = Sodium ion.

https://doi.org/10.1371/journal.pwat.0000135.g002

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Fig 4. Trilinear diagrams showing the major Cations and Anions order of dominance in the sediment of Owalla Reservoir.

https://doi.org/10.1371/journal.pwat.0000135.g004

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Fig 5. Modified trilinear diagrams showing the alkaline/alkalie of Owalla Reservoir.

https://doi.org/10.1371/journal.pwat.0000135.g005

Discussion

In aquatic ecosystems, bottom sediments are crucial as they provide habitat for a variety of macro- and epibenthic species. The aquatic organism health and the entire waterbody could be significantly affected when bottom sediment is contaminated. Based on sediment physico-chemical parameter results revealed high coefficient variation (CVs) values for some parameters such as silt, sulphate, phosphate, total nitrogen, organic matter, calcium etc. could be likely due to high impact of non-natural sources (biological and anthropogenic) on this waterbody compared to natural sources which is low [37]. The CVs value of pH (in H2O) and pH (CaCl2) recorded during this study were less 50%, which showed that environment variables can influence the pH value either acidity or alkalinity while concentrations of the other parameters can be affected at any given time by factors such as residence time, rate of input along with the concentration, interactions between the parameters, and the current physical ambient circumstances. The overall mean of sediment texture (% sand (73.1%), % clay (17.4%), and % silt (9.4%)) could be as result of runoffs from parent materials weathering or soil geology composition from nearby catchment area that contribute to high percentage of sand observed in this study. The result obtained similar to earlier investigation on sediment composition in Nigeria waterbody by Ezekiel et al. [38] who reported a slightly different results of sand % (88.6%), clay (7.09%) and silt (4.31%) in Sombreiro River. The overall pH (H2O) and pH (CaCl2) mean concentrations (5.27 ± 0.06 and 4.89 ± 0.05) recorded from Owalla sediment indicate that there is large intake of organic matter from anthropogenic activities around this waterbody. During the breaking down of organic matter in the sediment by microorganisms’ releases gases like methane, hydrogen sulphide, carbon dioxide etc. that lower the sediment pH and leading to depletion oxygen [39]. Similar finding was reported by Aliu et al, [40] who recorded lower sediment pH in Opa reservoir and Asibor and Adeniyi [41] in Asejire Reservoir but contrast the report of Umesi [42] who observed the mean acidic pH ranges of 2.70 ± 0.10 to 5.5 ± 0.31 in Rumueme Creek, Port Harcourt. Typically, human activities and natural factors have a significant impact on the sediment physico-chemical quality of any water body. The ranged of organic matter content (0.69 to 14.10%) recorded in this study indicate that Owalla sediment is organically rich in organic matter because it greater than 1% [43].

Spatially, the high mean air, water and sediment temperature values were recorded at Mid-basin station while the low values obtained at downstream station. The results obtained could be due to sampling time and collection which usually took place in the afternoon, when solar radiation and heat absorption are high could have significant difference on temperature at Mid-basin compared with other stations which is presume related to this result. The low mean sediment temperature recorded at downstream station could be relative to depth and less anthropogenic activities around this station. In comparison to the mid-basin and upstream (lotic) basins, the percentage mean sand was higher at the downstream (lentic) basin might be because of sand transported from upstream and Mid-basin finally end-up at downstream and it static which is located close to the littoral zone. The lower pH (H2O) and pH (CaCl2) means values recorded at upstream station (5.09 ± 0.10 and 4.62 ± 0.08) were lower compared with the Mid-basin and downstream stations which indicate the impact of high anthropogenic activities at upstream station. This indicate there is high input of organic content from farmland and domestic waste discharges into this waterbody by the villager which could increase the anaerobic processes in breaking down of organic matter at bottom sediment which produce gases that contribute acidic pH range recorded at this study. The finding contradicts the report of Ogbeibu et al. [44], that no uniformity in the grouping of the mean potential of hydrogen ions in respect to either the lotic or the lentic systems in Benin River. The existence of bottom dweller aquatic environment could be threatened by this pH mean concentration range recorded.

The mean of organic matter concentration recorded at all station were higher which could be as a result of anthropogenic activities around this waterbody. The result of this finding was significantly higher the work of Adeyemo et al. [45] who report the organic matter range values of > 0.31–1.14% in the bed sediment from waterbody in Ibadan city and was above amount previously reported from bed sediment in Benin River [44]. Extreme organic matter accumulate in the sediment may be harmful to bottom dweller organisms and have great impact on the water such as oxygen depletion [46]. The high conductivity value (642.00 ± 125.92 μScm-1) was observed at downstream sediment sample might be due to downstream serve as a reservoir where various ion and other by products of heavy from human activity transported from upstream and Mid-basin end-up and settle because static waterbody. The result obtained in this study was similar to report of Hyland et al. [46], who recorded high conductivity value in bottom sediment in Warri River but contradicts to low conductivity values recorded by Ogbeibu et al. [44] in Benin River and Olomukoro and Egborge, [47] in Adoni, Niger Delta. The sediment’s nutrient contents were very similar to those recorded from waterbody in Nigeria. The average phosphorus and phosphate concentrations appear to be higher upstream than other stations which indicate impact of human activities such as agricultural runoff that contains majorly nitrogen compounds and phosphorus from fertilizers, pesticides, salts and poultry wastes at this station could contribute to high phosphate content recorded. These illustrate the nutrients’ non-point source input and the strong impact of biological and artificial activities on the nutrients. Regarding the spatial patterns in the reservoir regime, there was no discernible fluctuation in the mean concentration of chemical properties such as sulphate, chloride, and total nitrogen and they were within the range reported by Kolo et al. [48] for the Lake Chad Basin in Borno State, Nigeria. The sodium and potassium concentrations were higher at Mid-basin which due to water retention times and sink-like characteristics of lentic environments.

Seasonally, all the sediment physico-chemical parameters examined in Owalla reservoir shown seasonal significantly variation. The mean values of air, water, and sediment temperature (31.90 ± 0.29°C, 30.60 ± 0.18°C, and 27.60 ± 0.23°C respectively) recorded in this study was higher in the dry season than during rainy season could be as a result of high heat generate during the dry season of the year, the heat from the water is significantly absorbed by lake sediment, and the heat is transferred back to the water back in the rainy season [6]. The highest mean values of clay and silt was recorded during the rainy season, while mean values of sand was higher in the dry season could be due to light soil washing off from surround farms which was expose during farm cultivation by runoffs during the rainy season. The unusually low water volume during the dry season compared to the rainy season along with overland runoff occurring predominantly during the rainy season could cause high silt and clay recorded in this study, similar to the report of Ong et al. [49]. This result could be attributed to surface runoffs during the rainy season that contain substantial amounts of clay and silt, whereas the higher percentage of sand during the dry season as a result of heavy sand sedimentation and water concentration.

The mean pH concentrations recorded in both seasons fell within acidic range while the oxidation of FeS to H2SO4 may be the cause of the comparatively low pH value during the wet season [50]. Metal adsorbed by sediment compete with hydrogen ions at low pH, which make them to remobilize in water column or due to redox changes in sediments and water column [51]. Water dilution during rainy season as a result of surface run-offs could increase ions concentration in the waterbody which might also increase electrical conductivity in the rainy season (643.30 ± 67.41 μScm-1) but lower in the dry season (526.60 ± 61.29 μScm-1). The high concentrations nutrient parameters such as phosphorus, nitrate, and phosphate (106.78 ± 10.91 gg-1, 5543.04 ± 211.45 gg-1 and 375.34 ± 39.55 gg-1) was observed during the rainy season compared with the dry season may be attributed to high organic matter from the top layer, and get into waterbody through runoffs and sink to bed sediment. The discharge of domestic wastes into the waterway could also contribute high limiting nutrients as well as phosphate and nitrogen fertilizers used by farmer in their farms find their way into water sink to sediment [52, 53].

The low total nitrogen concentration recorded during the dry season may be attributed to the low organic matter from runoffs into the waterbody. High total nitrogen value observed during the rainy season maybe due to riparian cultivation using nitrogenous fertilizers or/ nitrogenous organic or inorganic debris being washed into the reservoir from catchment basins during the rainy season [54]. The high organic matter concentration observed during the rainy season (4.31 ± 0.28%) compared to the dry season could be due to the rapid decomposition of foliage and other vegetative remains in waterbody [55]. The introducing of leached or eroded materials that high organic constituent transported from terrestrial biota (allochthonous sources) and primary production within aquatic ecosystems (autochthonous sources) get absorbed by sediment bed which could contribute to high organic content recorded during the rainy season. The concentration of exchangeable ions (Al3+, H+, Na+, K+, Ca2+, Mg2+, Cl- and SO42-) were significantly higher in rainy season than dry season maybe as a result of weathering of minerals and deposition of those minerals in sediment during the rainy season may influence high concentrations of exchangeable ions during rainy season [54].

The highest mean sediment temperature was recorded at littoral zone of the reservoir compared with the open water zone. This could be due to type of adjacent land forms in the basin appear to be a significant factor in the spatial variation in temperatures within the sediments at similar water depths, as shoreline (littoral) sediments of deep lakes have been found to be significantly warmer than open-water sediment. The mean sand % observed at littoral zone was higher compared to open water zone, this could be due to high levels of siltation (sand and gravel) entering the water bodies via surface runoff, which tend to settle quickly in shallow water areas whereas finer particles (silt and clay) tend to remain in suspension until they reach the more tranquil deeper water [56]. The distribution of grain sizes is determined by depth, where clay and silt sizes rose with depth, sand fraction fell in the same pattern [6]. The sediment pH concentration was lower at open-water zone than the littoral zone and conductivity and oxidation-reduction potential (ORP) mean values were significantly higher in the open water zone than the littoral zone. The decomposition of organic matter releases organic acids which react with bed sediment and causes decrease in sediment pH of a reservoir [57]. The major exchangeable ions were higher in the open water than in the littoral region maybe due to weathering of parent rock which is high with ions from surround this waterbody entering through runoffs. The mean of silt and sand percentage was higher at open water zone compared with littoral region in this study maybe due to According finding that fraction of exchangeable ions changed when particle size changes and vice versa depending on the adsorption and cationic exchange processes [58].

In general, the sample points in the piper diagram were grouped into 6 areas based on the Trilinear/or piper plot diagram that was constructed for the Preambular area utilizing the analytical data received from the sediment physcio-chemical analysis: There are six different types: Mg2+-NO3PO43 type; (2) Na+-Cl- type; (3) Ca2+-Mg2+-Cl- type; (4) Ca2+-Na+- NO3-PO43 type; (5) Ca2+-Cl- type and (6) Na-NO3-PO43- type. Analysis of Owalla sediment showed that it belongs to Ca2+-Cl- types. Trilinear or Piper plot analysis of the sediment types indicates that the contribution from the weathering of pyroxenes and amphibole in the hard rocks is clearly discernible. Sulphate was the most prevalent cation and magnesium was the most prevalent anion in the Owalla sediment maybe due to topographic area and rock weathering which can contribute to high value of calcium and magnesium that enter into waterbody through runoffs. Alkaline Earth out performed alkali metals in this study’s sediment type, while strong acid anions outperformed weak acid anions [59]. The six fields as mentioned by Chadha (1999) is given in below. (1) Alkaline earths exceed alkali metals; (2) Alkali metals exceed alkaline earths; (3) Weak acidic anions exceed strong acidic anions (4) Strong acidic anions exceed weak acidic anions (5) Alkaline earths and weak acidic anions exceed both alkali metals and strong acidic anions, respectively. The concentrations of exchangeable cations (Al3+, H+, Na+, K+, Ca2+ and Mg2+), anions/nutrient compounds (Cl- and SO42-, P and N) and organic matter generally showed a marked similarity in their pattern of variations. Their mean values were mostly higher in the rainy season (although, not significantly higher) than in the dry season except for the Cl- content was with significantly higher in the rainy season than in the dry season. On the whole, Na+ and K+ were significantly higher in the rainy season than in the dry season for sediment from the upstream basin of the reservoir. The higher concentrations of exchangeable cations and anions during the rainy season according to Sawant et al. [54] may be due to weathering of minerals and their eventual deposition in sediments during the rainy season.

The organic matter content of sediments in the present study varied from 0.69 to 14.10% with mean of 3.20 ± 0.20%. The range of organic matter suggested that the sediments varied from low to being organic in nature and on the average could be regarded as moderate in organic matter content. Sediments with organic matter values exceeding 1% are usually regarded as organically rich sediments [43]. Thus, the average organic carbon concentration recorded in the present study (1.86%) shows the sediment to be organically rich. USEPA [60] used the following levels for total organic carbon to class impact on sediments: ≤ 1%, Organic carbon = Low Impact; 1–3% Organic carbon = Intermediate Impact; > 3% Organic carbon = High Impact.

From the above; the organic carbon concentrations in sediments from Owalla Reservoir were in the intermediate impact level with regard to likely effect on benthic communities. The carbon, nitrogen and phosphorus (C: N: P) ratio of sediments from the reservoir was approximately 100: 1: 0.1. Sediment C: N: P ratio has been used as an indicator of the origin of organic matter (OM) in waterbodies, and used to determine whether the sediment is composed mainly by autochthonous or allochthonous matter [61]. High ratio could be an indication that the source(s) are mainly from allochthonous inputs while low ratio is indicative of autochthonous organic pollution. The ratio of total organic carbon to total nitrogen (C:N ratio) has also been used as an indicator of the source of organic matter in sediments (i.e., terrestrial or aquatic); When the source of OC is mainly terrestrial, the ratio of OC/TN is greater than 15 (C/N > 15), when the source of OC is mainly aquatic, the ratio of OC/TN is lower than 10 (C/N < 10) [57]. In this study, the C:N ratio is greater than 15, which is an indication that the source of organic matter was most probably of terrestrial sources. Furthermore, C: N: P ratio could also be used to determine the nature of the allochthonous organic pollution; for instance, domestic/agricultural effluents from poultry wastes have been found to contain high level of phosphorus and compound of phosphorus. The C: N: P molar ratios were also found to be acceptable indicators for distinguishing between sandy sediments and silt/clay sediments, the former characterized by low ratios and the latter by high ratios; and also, being associated with agriculture runoff and urban settlements [62].

The higher organic matter recorded in the rainy season than in the dry season may also be attributed to the relatively high supply of organic matter from the reservoir surroundings especially from the abundant vegetation, agricultural run-off, vegetative run-off and wastes etc. According to Saravanakumar et al. [55] organic matter in lake sediments is derived from primary production within the aquatic ecosystem (autochthonous sources) and also from terrestrial biota (allochthonous sources) by transport of leached and eroded materials into the lake which could usually be an increased phenomenon in the rainy season period. Higher value of nitrogen in the rainy season period could be as a result of nitrogenous fertilizers by riparian cultivators, and also through more decomposition and immobilization processes of nitrogenous organic/inorganic matter washed down the reservoir from the catchment basin during the period of rainfall; while that of phosphorus could be due to discharges containing fertilizers, pesticides, insecticides, herbicides used by cultivators around that are washed down into the reservoir in the rainy season [54].

Conclusion

Based on the results of the present study, it could be inferred that Owalla Reservoir is fairly clean and not contaminated with most of the investigated physico-chemical characteristics of the sediment. However, the sediment is getting enriched organically as the average organic carbon concentration (1.86%) recorded in the present study was high. The carbon, nitrogen and phosphorus (C: N: P) ratio of sediments from the reservoir which was approximately 100: 1: 0.1 while the C:N ratio which was greater the 15, showed that the origin of the organic matter (OM) in the reservoir’s sediment is mainly of; from anthropogenic activities (allochthonous inputs) around the reservoir’s catchment areas. High concentrations of some parameters (organic content, phosphate, phosphorus, total nitrogen, silt, clay and a decrease in pH) upstream; (nitrate, calcium, sodium and aluminium) at the mid-basin and (conductivity and sulphate) downstream reveal the effect of human activities around these sections of the reservoir. The high influx of runoffs during the rainy season which eventually sink into the sediment bed also contributes to the high concentrations of most parameters in the rainy season and also a possible sediment quality alteration.

Recommendations

Consistent nutrients (aquatic production limiting factor) enrichments from anthropogenic activities around the catchment areas of the reservoir could lead to the growth of hydrophyte and as well cause eutrophication of the reservoir if not checked and could in turn lead to water quality degradation, affect aquatic biota especially bottom dweller organisms and the natural ageing process of this reservoir making it unsuitable for most of its purposes of creation or construction.

The findings also have important implications for the development of effective potable water management and strategies for the control of point and diffuse-source pollution within the waterbody. Further research should conduct on the influence of pollution levels in sediment beds from anthropogenic activities on the developmental stages of aquatic organisms (larvae, early juveniles are less pollution tolerant and more sensitive than adults). However, if bottom sediment intensifies disturbance, it affects the physiological and metabolic responses of organisms and if possible uses eDNA metabarcoding to study organisms present before and after the sediment has been contaminated by human activities.

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