The urban tracts of rivers represent an example of altered ecosystem where applied research studies are particularly needed for both management and restoration. Monitoring the changes of water quality in streams and rivers can be performed through studies of the organisms living in it (Fenoglio et al., 2001). Resident biota in a water body are natural monitors of environmental quality and can reveal the effects of episodic as well as cumulative pollution and habitat alteration (Barbour et al., 1996, Rueda et al., 2002).
Miserendino and Pizzolon (1999) and Chessman (2003) had earlier classified the families Lumbriculidae, Tubificidae and Naididae (all oligochaetes) as highly pollution intolerant species indicative of poor water quality and were assigned the lowest sensitivity grade in the biotic index of macrobenthic invertebrates.
River Orogodo is a typical urban stream flowing through Agbor town with about 100,000 inhabitants. This stream is the main drainage system of the town accounting for most of the total run off (Ikomi and Owabor, 1997). The river at the middle reaches is subjected to organic pollution arising from effluent from an abattoir. The effluent of this abattoir comprising of the stomach and intestinal contents of slaughtered animals, ashes of burnt animals and blood stains are discharged regularly into the river without any treatment. Also livestock-derived excreta enter the steam either directly or after draining from their watershed.
Detailed distribution and abundance of Oligochaetes is low in the literature. Certain authors (Miserendino and Pizzolon, 1999, 2000; Ndaruga et al., 2004; Chessman, 2003) had earlier grouped the oligochaetes into families, perhaps owing to difficulty in identifying them into their respective genus and species. From the fore-going it becomes imperative that a study of the groups of oligochaete worms in an organically polluted stream be presented. Consequently, this study was undertaken to present an account of the water quality status and to identify the groups of oligochaetes to species level that are present in the organically polluted segment of the river. A comparative account of the same ecologically characteristics upstream and downstream of the impacted site will be presented with a view to identifying significant changes in the composition, density and distribution of oligochaetes attributable to this effluent.
MATERIALS AND METHODS
Description of the study area: The Orogodo River is a short stream (about
50 km length) located in Delta state of Nigeria. It lies between latitude 50°.10-6°.20N
and longitude 6°.10-6°.26 E (Fig. 1). The stream is
fed principally by ground seepage from aquifer in the thick rainforest zone
of Mbiri and secondarily by precipitation, municipal effluence and surface run
off from the riparian communities.
Map of the study area showing the sampling station (Map of
Nigeria showing the location of study)
It flows through Agbor main town, Owa-Ofie, Ekuma-Abavo, Oyoko in Delta state
and ends in a swamp between Obazagbon-Nugu and the oil rich town of Oben in
Edo state, Southern-Nigeria. Samples of Oligochaete worms were collected monthly
from July 2003 to June 2004 at three sampling stations, the point of discharge
of abattoir effluents, up stream and downstream of this impacted station.
The main features of the stations are described below;
Station 1 is located 15 km from the source. The marginal vegetation consists
of few trees shadows, mainly tall oil palm tree, Elaeis guineensis and
Cocos nucifera L., Emergent vegetation namely Pycreus lanceolatus
Poi and Submerged Vegetation, Ceratophyllum submersum L. The streambed
is predominantly clay and silt. Human activities here include microartisanal
fishery and bathing. The mean water depth is 0.65 m and width 3.5 m.
Station 2 is the impacted station located behind the Agbor town Abattoir. The effluents of the abattoir and livestock-derived excreta including urine and faces are drained directly into the stream here. The nature of the pollutant is described as organic. Some section of the water in this station is conspicuously green with heavy algal growth.
The dominant macrophytes are Nymphae, Lotus, Azolla, Uticularia and Salvinia sp. Also floating duckweed (Lemna minor) are seen close to the river bank. The average water depth is 0.52 m and with 5.8 m.
Station 3 is located downstream of the impacted area, 3 km from the impacted site. The substratum is predominantly clay and silt. It is flanked by Indian bamboo trees (Bambusa sp) and palm trees (Elaeis guineensis), Pandanus sp. and Mitragyna ciliata. Human activities here include bathing, fishing, sacrifices by superstitious believers etc. The average water depth is 1.02 m and width 3.7 m.
Water quality analysis: Sampling for water quality parameters and oligochaetes
were carried out in the three study stations at monthly intervals between July
2003 and June 2004, covering the dry and rainy seasons. Air and water temperature
were recorded with a thermometer, conductivity and pH, were measured using a
portable meter. Dissolved Oxygen, Biochemical Oxygen Demand (BOD), total hardness
and total alkalinity were measured titrametrically after fixing with the necessary
reagents according to methods earlier described by APHA (1985). Other parameters
measured included water velocity, determined using the Pin-pong floatation technique.
Nitrate-nitrogen (NO3-N) and Phosphate-phosphorus) (PO4-P)
were measured spectrophotometrically after reduction with appropriate solutions
Sampling for oligochaetes: Sampling for oligochaetes were carried out as described by Pennak (1978). An Ekman grab sampler was used to collect four random samples of the substratum from a cross section of the river bed at each station. The bottom debris was brought into the laboratory and allowed to stand for a few hours until the worms can be seen at the surface of the debris or crawling up the sides of the container. Masses of filamentous algae were washed out with a stream of water. Small species were observed with the aid of dissecting binoculars. Amphibious and large species were picked up by forceps. Most oligochaetes were indentifed to the species level after sorting using published keys, (Macan, 1959; Clegg, 1969; Hynes 1978; Pennak 1978; Hellawell, 1986) with specific reference to the nature of the gill structure, shape, size of the prostomium and nature of the setae (capilliform, serate, sigmoid, hooked bifurcate, pectinate and biuncinate). Oligochaete abundance was obtained by counting all individuals in a taxon and expressing the results as number m-2.
All statistical methods used in analyzing the water quality parameters and oligochaetes community were adapted from Zar (1984) and Magurran (1988). Taxa richness, diversity and evenness indices were calculated using the computer BASIC programme SP DIVERS (Ludwig and Reynolds, 1988). Jaccards similarity index (Ravera, 2001) was used to compare the sampling locations and to determine which ones were similar in taxa composition and Berger-Parker dominance index (Ravera, 2001) was also evaluated for each sampling station.
Water quality status of River Orogodo: Table 1 summarizes
the mean values of the various parameters monitored at the three selected stations
over a 12-month time span (July 2003-June 2004) along the river.
Air and water temperatures were in the range of 24-31°C in the three stations sampled and were not significantly different (p>0.05).
Mean water depth was also similar in all the stations, except that orthogonal comparison using Duncans multiple range test revealed that station 3 (downstream) of the impacted site showed considerable high depth 1.20±0.24 m as compared with other stations. Generally water depth was considerably higher in the rainy season than the dry season months.
Current velocity was also not significant among the stations (p>0.05). Station 2, however recorded high values of current velocity especially in the rainy season months as a result of the run off of storm water from the town which empties a few kilometers from this station.
Conductivity values were significantly different among the stations sampled (p<0.05) the impacted station (station 2) recorded high conductivity values throughout the study period.
Most of the chemical variables, that is dissolved oxygen, BOD5, Nitrate-nitrogen (NO3-N), Phosphate-phosphorus (PO4-P) all measured in mg L-1, were significantly different among the various stations sampled (p<0.05). Orthogonal comparison using Duncans multiple range test showed that station 2 (impacted site) was the cause of the observed differences in these parameters. Total hardness measured in mg L-1 CaCO3 was also significantly higher (p<0.05) in the impacted station. It had an average of 146±27.24 mg L-1 CaCO3 in the impacted station.
Total alkalinity and pH were the only chemical variables that did not vary significantly among the stations; however station 2 recorded higher levels of total alkalinity and pH as compared with other stations sampled.
Aquatic oligochaetes composition, abundance and distribution in the sampling
stations: The taxa composition, density and distribution of Oligochaetes
in the study area are shown in Table 2. The assemblages fall
into five families namely Aelosomatidae, Naididae, Lumbriculidae, Glossoscolecidae
and Tubificidae. The distribution of these organisms varied from station 1 (up
stream) to downstream as might be expected in view of the different abiotic
characteristics of the stations.
Fourteen taxa of aquatic oligochaetes comprising 4, 801 individuals were recorded
during the entire study. The total number of taxa and individuals present at
upstream (station 1), station 2 and down stream (station 3) were 5 (107), 13
(4,628) and 7 (66), respectively. The most abundant oligochaetes collected from
the three stations distributed along the river belong to the following genera,
Tubifex tubifex (OFM) Nais communis (Piguet),
Dero limnosa (Muller). Less frequent were the genera, Stylaria,
Lumbriculus and Aeolosoma with Aulophorus, Dero digtata,
Chaetogaster, Nais obtusa and Pistina aequiseta
Summary of physical and chemical variables of the sampling
stations of River Orogodo (mean value±standard error)
*Indicates significant difference
The overall composition and distribution of aquatic oligochaetes
in River Orogodo study stations, July 2003-2004 (individuals/m2)
Percentage distribution in composition of oligochaetes at
the study stations
Diversity, richness and dominance indices of oligochaetes
in the study stations of River Orogodo
Jaccards similarity index between pairs of sampling
Of all the individuals collected, station 1, 2 and 3 accounted for 2.23, 96.4
and 1.37%, respectively (Table 3).The overall density was
significantly different at the three stations (ANOVA, p<0.05). An aposteriori
test for multiple comparison showed that the density at station 2 was significantly
higher than those at station 1 and 3 (p<0.05), which were not different from
each other (p>0.05).
The family Naididae contributed 54% of the total oligochaete density. It was
represented by 10 taxa, with Dero limnosa (Leidy) occurring as the most
dominant taxa, closely followed by Nais communis (Piguet) and Stylaria
lacustris (Leidy). Chaetogaster diatrophus (Gruit), Chaetogaster
limnaei (BAER), Nais obtusa (Leidy), Dero digitata (Muller),
Stylaria fossularis (Leidy) and Pistina aequiseta (Bourne) were
the other representative of Naididae family.
Spatial-temporal distribution of oligochaetes density in the
They were only found in station 2 (Impacted station) and completely absent
in station 1 and 3. Only Nais communis (Piguet) and Dero limnosa
(Muller) were present in all the three stations sampled.
The Family Lumbriculidae was represented by one taxa, Lumbriculus variegatus
(Muller), contributing 5.1% of the total oligochaete density. It was present
in the three stations sampled with the impacted station (station 2) recording
the highest abundance (93.1%) of the total Lumbriculus density.
Family Glossoscolecidae was represented by one taxon, which was not further identified into genus. This taxon was completely missing in station 2. It was present in few numbers in station 1 and 3.
Aeolosoma variegatum (VEJD) was the only representative taxa of the family Aeolosomatidae. This species contributed 3.94% of the total oligochaete density. It was absent in station 1 and present in station 2 and 3. Station 2 recorded a very high density of this species (96.8%) as compared to the other stations.
Spatial-temporal dynamics in population density: The spatial-temporal
distribution in abundance of oligochaetes in the study stations is shown in
Fig. 2. In the three stations sampled, the highest densities
were recorded during the dry season months, December for station 1 and 3 and
November for station 1. In station 1 an overall mean density of 13±4.5
organisms was obtained for the dry season (Nov-Feb.). It was 620±316
organisms and 12±10.6 organisms for station 2 and 3, respectively in
the dry season. In the rainy season a mean density of 7±4.0, 269±127
and 3±1.8 organisms were obtained for station 1, 2 and 3, respectively.
Analysis of variance revealed that oligochaetes abundance was significant (p<0.05)
Diversity, dominance and similarity indices: Table
4 shows the diversity and dominance indices calculated for the three sampling
stations. The taxon richness calculated by Margalef index was lowest in station
1 and similar in stations 2 and 3. Shannon diversity (H) was also similar in
stations 2 and 3 and lowest in station 1 there was however no statistical significance
between the stations (p>0.05) sampled. For Evenness, station 1 was more even
(0.949), followed by station 3 (0.908) and station 2 recorded the least evenness
value (0.687). Berger-Parkers dominance index was highest in station 2
and lowest in station 3. Jaccards similarity index was used to compare
the pairs of sampling stations and the result presented in Table
5. All pairs of sampling stations (that is station 1 and 2, station 1 and
3 and stations 2 and 3) were dissimilar; however station 2 and 3 had the closest
The water quality changes observed especially the high BOD5 levels, conductivity, total hardness and low dissolved oxygen values obtained in station 2 portends the abnormality of the water at this station as a result of the impact arising from the untreated abattoir wastes. Nitrate-nitrogen and Phosphate-phosphorus levels of the water obtained from the abattoir discharge station indicate a substantial amount of organic input coming from the abattoir. These values were significantly higher than the levels at the control stations, up stream and downstream of the impacted site. The values of nutrients obtained for this study were very high compared with low titre value reported for similar natural unimpacted stream within southern Nigeria (Ogbeibu and Oribhabor, 2002; Edema et al., 2002).
The relatively high velocity reported in station 2, especially during the rainy season months is attributed to surface run off and storm water especially when it rains.
pH of an aquatic system although not definitive is an indicator of the water quality and the extent of pollution in the water shed (Jonnalagadda and Mhere, 2001). Unpolluted streams normally show a near neutral or slightly alkaline pH. Most of the water samples had a pH of about 7 and 8. Station 2 had slightly higher values of pH which again could be traced to the nature of the effluents which is slightly alkaline.
BOD5 values indicate the extent of organic pollution in aquatic systems, which adversely affect the water quality (Jonnalagadda and Mhere, 2001). In all the water samples BOD5 was less than 4 mg L-1, except in station 2 where BOD5 was in the range of 8 to 14 mg L-1, indicating that the effluents from the abattoir was organic in nature thus reflecting high burden of organic pollution.
Water hardness reported for station 1 and 2 were in the range of 62.4-96.45 mg L-1 CaCO3, which is in the range of soft water by Lind (1979) classification. Station 2 with an average hardness of 146±7.39 mg L-1 CaCO3 is classified as hard water.
The total number of oligochaete taxa reported in this study (14) is high when compared with earlier studies by, Edokpayi et al. (2000), Edema et al. (2002) and Adakole and Annune (2003) which reported 2, 3 and 5 taxa, respectively in Nigerian freshwater streams. The probable reason for this high number of taxa in this study may be due to the organic materials from the abattoir effluents whose substrate is mostly covered by bacteria and sewage fungi which are the main food source for most oligochaetes (Rueda et al., 2002). Again for most authors they did not classify the oligochaetes to species level. Identification was only given to genera or family level. Related studies elsewhere include those of Misenrendino and Pizzolon (2000) that recorded 7 taxa in a fluvial system in Patagonia and Ogbeibu and Oribhabor (2002) recorded 10 taxa in River Ikpoba, Southern Nigeria. The principal taxa in the present study, Dero limnosa, Nais sp., Stylaria sp. and Tubifex sp. have earlier been reported in Nigeria waters (Ogbeibu and Victor, 1989; Edokpayi et al., 2000; Adakole and Annune, 2003; Ikomi et al., 2005).
Station 2 recorded very high abundance of individual oligochaetes as compared to the upstream and downstream stations. The reason for this high density could be that a great majority of oligochaete obtain nutrients by ingesting quantities of the substrate as organic cornponents (Hellawell, 1986) and they are mostly common in the mud and debris substrate of stagnant pools ponds and streams (Mason, 1991), which is the main feature of station 2. Similar studies elsewhere (Solimi et al., 2000, Ravera, 2001; Rueda et al., 2002) revealed that oligochaetes abundance is due to considerable load of organic particles from untreated sewage and livestock effluents.
The overall composition and density of fauna varied both spatially and temporarily in response to physical, chemical and biological factors of the environment.
The dominant family in this study was Naididae with ten representative taxa. It has been proposed by a number of authors (Victor and Dickson, 1985; Ogbeibu and Egborge, 1995; Prygiel et al., 2000), that higher densities of this family in a site is attributed to organic enrichment and abundant masses of filamentous algae. Nais communis and Dero limnosa showed no habitat restriction.
The family Tubificidae was represented by one genus, Tubifex tubifex (OFM). This species was very abundant in station 2 and completely absent in station 1. It occurred only sporadically in station 3. This species has been reported to increase considerably in organic polluted streams (Brinkhurst, 1966) and they possess the ability to thrive in low concentrations of dissolved oxygen (Hellawell, 1986). Tubificids are generally found in sites of high conductivity, fine sand bottom and organic matter enrichment from natural or anthropogenic sources (Miserendino and Pizzolon, 2000), further confirming its high density in the polluted station.
The family Aeolosomatidae was also represented by one genus, Aeolosoma variegatum (VEJD) which was dominant in station 2. This species has the ability to migrate easily through out whole drainage systems and forms a resistant mucus cyst that may be easily transported from place to place (Pennak, 1978).
The family Glossoscolecidae was not further identified into genus. This species was the only oligochaete found in the unpolluted segment of the stream. It occurred in few numbers in station 1 and 3. This family included species that are intolerant of pollution and thus could be used as an indicator of good water quality. Adakole and Anunne (2003) also reported the presence of Glossophonia sp in relatively unpolluted segments of Bindare stream, in Northern Nigeria.
The density and diversity indices vary both spatially and temporally. The pattern of temporal dynamics in the density of the oligochaetes was also affected by the input of the abattoir wastes. Jaccard similarity index lends more evidence that station 2 density was dissimilar from station 1 and 3, respectively.
Generally, the station receiving wastes from the abattoir (Station 2) showed
evidence of environmental degradation, the more sensitive species eliminated,
whereas the more resistant ones increase in population density as an effect
of the decline of competition with the more sensitive benthic species. Again,
the muddy substratum and the relative shallow nature of the stream at this station
(impacted site) must have favoured the abundance of these species. Therefore,
Oligochaetes do reflect an area with low-oxygen concentration and high enrichment.