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Full text of "MACROINVERTEBRATE FAUNA OF A TROPICAL SOUTHERN RESERVOIR, EKITI STATE, NIGERIA."

Continental J. Biological Sciences 4 (1): 30 - 40, 201 1 ISSN: 2141-4122 

© Wilolud Journals, 201 1 http://www.wiloludjournal.com 

Printed in Nigeria 



MACROINVERTEBRATE FAUNA OF A TROPICAL SOUTHERN RESERVOIR, EKITI STATE, 

NIGERIA. 

J.B. Edward 1 , and A.A.A. Ugwumba 2 
1 Department of Zoology, University of Ado Ekiti, P.M.B. 5363, Ado Ekiti Ekiti State, Nigeria. 



2 



Department of Zoology, University of Ibadan, Oyo State, Nigeria. 



ABSTRACT 

A survey of the macroinvertebrate fauna of Egbe Reservoir, Ekiti State, Nigeria was carried out. 
Sampling of surface waters and macroinvertebrates was carried out twice in a month from September 
2004 to December 2006. Physico-chemical parameters determined include pH, conductivity, alkalinity, 
dissolved oxygen and biochemical oxygen demand (BOD) using APHA methods. Macroinvertebrates 
were collected by kick sampling and with Van veen grab. Data analysis was done using descriptive 
statistics, Duncan multiple range, pearson correlation, paired t tests and diversity indices. Eighteen taxa 
of macroinvertebrates in two Phyla of Mollusca and Arthropoda were identified. Gastropods had the 
highest numerical abundance (41.8 %), diversity (d= 0.61, H=1.56) and evenness (J=0.87). Odonata 
and Ephemeroptera (Insecta) had the lowest diversity (d=0.00, H= 0.00 and d= 0.14, H= 0.13) and 
numerical abundance (0.4% and 6.3%, respectively). The gastropod, Melanoides tuberculata, which is 
the most abundant macroinvertebrate is an indicator of polluted water. This suggests that the reservoir 
may be tending towards organic pollution. This is further confirmed by the low abundance of 
Ephemeroptera and Odonata which are indicators of clean water. Measures should be taken to prevent 
the reservoir from further deterioration and eventual eutrophication. 

KEYWORDS: Macroinvertebrates, Egbe Reservoir, Pollution. 

INTRODUCTION 

Macroinvertebrates are important at the food web of aquatic ecosystems. Benthic dipteran larvae e.g. 
Chironomus species and mollusks are consumed in great quantities by many fishes and so play an important role 
in the ecology of the aquatic ecosystems as shown by a lot of works on food and feeding habits of some 
freshwater fishes (Imevbore and Bakare, 1970; Ogari and Dadzie, 1987; Fagade and Olaniyan, 1973 and 
Ugwumba and Ikusemiju, 1994). They have being used to assess the biological productivity of lakes and rivers 
(Mehmet et. al., 2002). Freshwaters are being polluted and facing various limnological problems due to 
increasing anthropogenic activities. Benthic invertebrates are used in determining and observing eutrophication, 
pollution and water quality, and many of them have been accepted as biomonitor species. Their diversity and 
abundance is a reflection of the quality of water and its sediments. 

There are many studies in various parts of the world on the benthic macroinvertebrates of freshwater bodies. 
These include the works of Petr (1970) on the bottom fauna of the rapids of Black Volta River in Ghana, 
Johnson and Brinkhurst (1976) on Quinte Lake, Ontario, Canada; Darlington (1977) on Lake George in Uganda. 
In Nigeria, Victor and Dickson (1985) worked on macroinvertebrates of a perturbed stream in southern Nigeria; 
Adebisi (1989) studied the planktonic and benthic organisms of Olupona fish farm, Ibadan, Oyo State; Oke 
(1998) worked on the limnology and macrobenthos of Owena Reservoir in Ondo State; Ogbeibu and Oribharbor 
(2002) on the upper reaches of Ikpoba River; Victor and Ogbeibu (1985a and b); Victor and Dickson (1985) and 
Ogbeibu and Victor (1989) on some southern steams. Presently, no studies have been conducted on the general 
benthic fauna of any stream or river in Ekiti State. Yet, this area lies within the tropical rainforest expected to 
have a high diversity of aquatic organisms. This present study was therefore carried out to provide some 
baseline information on the composition, seasonal abundance and distribution of the benthic fauna of Egbe 
Reservoir, Ekiti State. 

Study Area 

Egbe Reservoir is the major source of domestic water supply to the people of Gbonyin Local Government Area 

of Ekiti State and parts of Ondo State, mainly the Akoko areas (Fig. 1). The reservoir also serves as a source of 

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J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 



irrigation of farmed lands as well as a means of livelihood for the local fishermen who depend mainly on the 
fishery resources of the reservoir. 

The reservoir was formed as a result of damming the River Osse, a major river that takes its source from Kwara 
State and runs through Ekiti to Ondo States. The river was dammed at Egbe Ekiti, and was constructed in 1975 
and commissioned in 1989. The transverse survey is 272.5 hectares and the depth of the reservoir is 56.4m at its 
deepest point . The reservoir is located on an undulating plane and lies between latitudes 7°36 N and 7 39 North 
and longitude 5°32'E and 5 36' East of the Equator as shown in Figure 1. It is surrounded by highlands from 
which runoffs also feed the reservoir during raining season. The bottom substratum of the reservoir is rocky. 

Four sampling stations were chosen in the reservoir. Station 1 (Dam station) is about 200m from the water 
treatment house of the reservoir. Vegetation around this station is mainly grasses and various species of birds 
including cattle egrets frequent this site, perching on the rocks. Activity around here includes only cattle 
grazing. Station 2 (Rocky station) is 399m from station 1. The area has thick vegetation comprising mainly of 
palm trees. 

The major activities around here are farming and fishing. Station 3 (Bridge station) is located along the road, 
close to a bridge and is 450m to station 2. This area also has thick vegetation cover and the major activities 
around here include farming, fishing and car washing and laundering. Station 4 (River Osse inlet station) is 
about 300m from the previous site and is bounded by vegetation. Water birds also frequent this site and 
activities around this place include farming, fishing, cattle grazing, bathing and laundering. Virtually all the 
available landmass around the reservoir and its environs is essentially used for agricultural activities. Some of 
the local fishermen also build their huts close to the reservoir. These areas also have high density of animal 
confinement, rocks, and coastal plain sands. 

The Reservoir is surrounded on each side by a stretch of thick forest made up of trees like Chlorophora excelsa 
(African teak, "Iroko"), Terminalia Superba (Limba "Afara") and Senna occidentalis - coffee. Floating plants 
include Ceratophyllum 



5 '^OQ 5^323CT 5'53'uj 




1 Kilometers 



Fig 1 : Map of Egbe Reservoir showing Sampling stations(1,2,3,and 4). 



Figure 1: Egbe Ekiti showing the location of the reservoir and the sampling stations 

submersum, Mormodica balsamina (Balsaam pear), Commelina diffusa waterside Commelina), Cyperus 
articulatus (Sedge) and Pistia stratioles. Weeds at the bank of the Reservoir include Chromolaena odorata 
(Siam weed), Aspilia africana (Bush marigold), and Cynodon_plectostachynm. 

The Reservoir also supports a commercial fishery of about twenty-five fishermen fishing daily with traps, sets 
and cast nets, hook and line. However, only nine of these fishermen were available during this period of study. 
Fish species in the Reservoir include Tilapia zillii, Oreochromis niloticus, Channa obscurus, Clarias 
gariepinus, C. anguillaris, Hepsetus odoe, Sarotherodon melanopteron, and Mormyrus sp. 



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J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 



MATERIALS AND METHODS 

Macroinvertebrate Sampling and Identification 

Kick sampling method was used to collect the macroinvertebrates of the river bank sediments. This method was 

carried out by using a stick to agitate lm 2 area of the substratum. The dislodged macroinvertebrates that moved 

downstream were then collected with a 150um mesh size net held against the water current with the mouth 

facing upstream. Three replicate samples were collected at each sampled station. 

In the laboratory, the sediments collected were washed through graduated sieves of 0.5mm, 1mm, 2mm, and 
3mm. The washed sediments with macroinvertebrates were poured into a white tray and sorted out. Sorting of 
the macroinvertebrates in the sediment sample was enhanced by staining the washed sediment samples with 
Rose Bengal solution. The macroinvertebrates collected from each station were then sorted into different 
taxonomic groups using a x 10 scanning lens. Each taxonomic group was placed in a specimen bottle containing 
4% formalin and properly labeled. The macroinvertebrates were later identified to genus or species level where 
possible with a compound microscope using identification guides of Pennak (1978), APHA / AWWA / WPCF 
(1992) and Yoloye (1994). Each identified taxon was counted and the number of individuals recorded per unit 



RESULTS 

The macroinvertebrates identified in Egbe Reservoir are listed in Table 1 showed the checklist and data on the 
numerical abundance and percentage composition by number of macrobenthic invertebrates from the various 
sampling stations for the overall as well as the dry and rainy season of the study period. 

Eighteen (18) macroinvertebrate genera were identified and they were of two phyla of Mollusca and 
Arthropoda. The Mollusca were all of one class of gastropoda (pond snails). Arthropoda were represented by 
two classes, Arachnida represented by one genera of Hydracarina and Insecta, represented by four orders of 
Odonata, Diptera, Hemiptera and Ephemeroptera. 

The macroinvertebrates of the reservoir was dominated by the gastropoda with five species and one genus. They 
constituted 41.8% of the total number of macroinvertebrates (Fig 2). Amongst them, Melanoides tuberculata 
had the highest percentage composition (14.2%) followed by Bulinus globossus (8.9%). The least abundant 
gastropod was Gabiella sp. which made up 1.2% of the total number of macroinvertebrate. 

Amongst the Arthropods, the Order Diptera is the most dominant Insect, comprising of four genera which made 
up 31.8% of the total macroinvertebrate abundance by number (Fig. 2). In this order, Anopheles larvae made up 
the highest percentage composition 25.3% of the total number of macroinvertebrate (Table 1). The least 
abundant insect in this order was Tipula sp. with 0.8% of the total macroinvertebrate population. 

The order Hemiptera was next in abundance which was also made up of one species and three genera. It 
constituted 19.3% of the total number of macroinvertebrate (Fig. 2). Notonecta sp. was the most abundant 
making up 18.1% of the total macroinvertebrate by number. The least abundant was Sigara sp. constituting 
only 0.2% of the total macroinvertebrates (Table 1). 

Ephemeroptera were represented by one species Ephmerella excrucians and one genus Caenis sp. They 
constituted 6.3% of the total number of macroinvertebrate (see Fig. 2). Odonata was represented by only one 
species of Macromia magnified which had 
the least percentage composition, 0.3% of the total macroinvertebrate by number. 

The Class Arachnida was represented by only one genus Hydracarina of the order Acarina. It comprised the 
least percentage composition, 0.3% of the total number of macroinvertebrate of the reservoir. 



32 



J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 



Table 1: Checklist, Numerical and percentage abundance of macroinvertebrates in Egbe Reservoir 



Macroinvertebrate 


Dry Season 




Rainy Season 




Overall 




Gastropoda (Pond snails) 


Abund. 


% 


Abund. 


% 


Abund. 


% 


Melanoides tuberculata 


1485 


11.8 


3659 


15.4 


5144 


14.2 


Bulinus globossus 


1155 


9.2 


2094 


8.8 


3249 


8.9 


Lymnaea natalensis 


1008 


8.0 


2119 


8.9 


3127 


8.6 


Biomphalaria pfeifferi 


496 


3.9 


1895 


8.0 


2391 


6.5 


Physa waterlotti 


148 


1.2 


711 


3.0 


859 


2.4 


Gabiella sp. 


234 


1.9 


201 


0.8 


435 


1.2 


Total 


4526 


36.0 


10679 


44.9 


15205 


41.8 


Diptera 














Anopheles larvae 


3191 


25.4 


5997 


25.2 


9188 


25.3 


Palpomyia sp. 


306 


2.4 


1441 


6.1 


1747 


4.8 


Symbiocladius sp. 








323 


1.4 


323 


0.9 


Tipula sp. 


103 


0.8 


180 


0.8 


283 


0.8 


Total 


3600 


28.6 


7941 


33.5 


11541 


31.8 


Hemiptera 














Notonecta sp. 


2893 


23.0 


3960 


0.3 


6583 


18.1 


Lethocerus americanus 


83 


0.7 


148 


0.7 


231 


0.6 


Gerris sp. 


46 


0.5 


89 


15.4 


135 


0.4 


Sigara sp. 


30 


0.2 


33 


0.1 


63 


0.2 


Total 


3052 


24.4 


3960 


16.6 


7012 


19.3 


Ephemeroptera 














Ephemerella excrucians 


1211 


9.6 


1008 


4.2 


2219 


6.1 


Caenis sp. 


47 


0.4 


53 


0.2 


100 


0.2 


Total 


1258 


10.0 


1061 


4.4 


2319 


6.3 


Odonata 














Macromia magnifica 


75 


0.6 


64 


0.3 


139 


0.4 


Total 


75 


0.6 


64 


0.3 


139 


0.4 


Arachnida 














Hydracarina sp. 


56 


0.4 


71 


0.3 


127 


0.4 


Total 


56 


0.4 


71 


0.3 


127 


0.4 


Grand Total Abundance 


12567 


100 


23776 


100 


36343 


100 



19.3 




41.8 



□ Gastropoda 
m Ephemeroptera 



□ Diptera 
m Odonata 



ra Hemiptera 
□ Arachnida 



Figure 2: Relative abundance by number of macroinvertebrates in Egbe Reservoir during 
(Sept. 2005Dec. 2006). 



the period of study 



33 



J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 



Seasonal distribution of Macroinvertebrates 

In the dry season gastropods still had the highest percentage composition by number 36.0% of the total number 
of macroinvertebrates (Fig. 3). Melanoides tuberculata was also the most abundant gastropod (11.8%), followed 
by Bulinus globossus (9.2%), and the least abundant was Physa waterlottii (1.9%) (Table 1). During the wet 
season, gastropods were higher in abundance constituting (44.9%) of the total number of macroinvertebrates 
(see Fig. 4.), with Melanoides tuberculata having the highest percentage composition, 15.4% by number of the 
total macroinvertebrate abundance. Next in abundance was Lymnaea natalensis which made up 8.9% of the total 
number of macroinvertebrates. The least abundant gastropod was Gabiella sp. comprising only 0.8% of the total 
number of macroinvertebrates (Table 1). 

The Diptera made up 28.6% and 33.5% of the total number of macroinvertebrates during the dry and rainy 
seasons respectively (Figures 3 and 4). Anopheles larvae had the highest percentage composition (25.4% and 
25.2%) during both dry and wet seasons respectively, while Tipula sp. also recorded the least abundance of 
0.8% during both seasons (Table 1). 

Hemiptera was higher in abundance during dry season constituting 24.4% of the total number of 
macroinvertebrates and 16.6% during rainy seasons (Figures 3 and 4). In this Order, Notonecta sp. was the most 
abundant making up 23.0% of the total number of macroinvertebrates during dry season and 15.5% during the 
rainy season (see Table 1). The least abundant Hemiptera was Sigara sp. constituting only 0.2% and 0.1% of 
the total number of macroinvertebrates during dry and rainy seasons respectively (Table 1). 

Ephemerella excrucians an Ephemeroptera had higher percentage composition by number during both dry and 
wet seasons (9.6% and 4.4%) than Caenis sp. (0.4% and 0.2%, respectively,Table 1). The Odonata Macromia 
magnifica constituted the least percentage composition by number (0.6% and 0.3%) of Insecta during both dry 
and rainy seasons respectively. The Class Arachnida recorded the least percentage composition by number of 
macroinvertebrates (0.4% and 0.3%) during both dry and wet seasons respectively (see Table 1). 



24.4 




28.6 



□ Gastropoda 
E3 Ephemeroptera 



□ Diptera 
■ Odonata 



OH Hemiptera 
□ Arachnida 



Figure 3: Relative abundance by number of macroinvertebrates in Egbe Reservoir during dry season 



34 



J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 




□ Gastropoda 
■ Ephemeroptera 



Diptera 
■ Odonata 



Hemiptera 
□ Arachnida 



Figure 4: : Relative abundance by number of macroinvertebrates in Egbe Reservoir during rainy season 

Spatially, gastropods were most abundant (43.7% by number) at the Rocky station and least (39.4% by 
number) at the Dam station. The Diptera had their highest (33.9%) composition by number also at the Rocky 
station and its least (28.1%) at the Inlet station. Hemiptera was highest in abundance numerically (23.4%) at the 
Inlet station and lowest (17.1%) at the Rocky station. Ephemeroptera had its highest numerical abundance 
(8.0%) at the Dam station and the least (4.9%) at the Rocky station, while Odonata recorded 0.7% by number at 
the Bridge station and 0.2% at the Rocky station. Arachnida was the least abundant macroinvertebrate in all the 
sampled stations with 0.5% by number at the Dam station and 0.2% by number at the Inlet station (Fig. 5). 

During the seasons, gastropods were also the most abundant macroinvertebrate numerically. They constituted 
the highest numerical percentage (40.0%) at the Bridge station and the least (32.9%) at the Inlet station during 
the dry season, while they had 46.5% by number at the Rocky station and the least 42.4% at the Dam station 
during the rainy season (Fig. 5). Diptera were next in abundance numerically. They recorded the highest 
composition by number (30.5%) at the Inlet station and the least 27.8% at the Dam station during the dry 
season. In the rainy season, their abundance was higher (36.6%) at the Dam station and least (26.7%) at the inlet 
station. Hemiptera was most abundant numerically (26.4%) at the Inlet station and least (21.5%) at the Bridge 
station during the dry season, while in the rainy season, it recorded 21.9% at the Inlet station and 13.5% at the 
Rocky station (Fig. 5). 

Ephemeroptera were higher in abundance during the dry season in all the sampled stations. Its highest numerical 
abundance (12.8%) was recorded at the Dam station and the least (8.3%) at the Rocky station. During the rainy 
season, Ephemeroptera had 5.6% by number at the Bridge station and 3.5% both at the Rocky and Inlet stations. 
Odonata were also more in abundance in the dry season than in the rainy season, having its highest composition 
by number 0.9% at the Bridge station and the least 0.1% at the Rocky station, while in the rainy season, it 
recorded 0.6% by number also at the Bridge station and 0.1% at the Dam station. Arachnida was the least 
abundant macroinvertebrate during both seasons. It recorded 0.8% abundance by number at the Bridge station 
and 0.2% by number both at the Rocky and Inlet stations during the dry season. In the rainy season, it had 0.5% 
by number at the Dam station and 0.2% both at the Bridge and Inlet stations (Fig. 5). 



35 



J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 



50 n 

45 

40 
o 35 H 
"§ 30 

P 

S 25 

S3 

a 20 - 

CD 
O 

£ 15 -I 

10 





D Dam station 
Rocky station 
B Brigde station 
□ Inlet station 



Gastropoda Diptera Hemiptera Ephemeroptera 

Macroinvertebrates 



Odonata 



Arachnida 



Figure 5: Percentage abundance by number of macroinvertebrates in the four stations of Egbe 
the period of study (Sept. 2005- Dec. 2006). 



Reservoir during 



Macroinvertebrate Diversity 

Tables 2-4 showed the macroinvertebrate diversity indices. Gastropoda generally had the highest diversity (d = 
0.61; H = 1.56) and its equitability value was also high (J = 0.87) for the overall sample (Table 24). The next 
most diverse taxa were the Diptera (d = 0.38; H = 0.60) and Hemiptera (d= 0.41; H= 0.28). However, the 
Diptera was more evenly distributed (J= 0.43) than Hemiptera (J= 0.21). 

During the seasons, a similar trend as above was also noticed. Gastropoda also exhibited the highest diversity 
during both rainy (d = 0.64; H = 156) and dry seasons (d = 0.72; H = 1.52). Its equitability values were also 
highest in both seasons, J = 0.87 and J = 0.85 for rainy and dry seasons respectively (Tables 3 and 4). Diptera 
were also the next most diverse and evenly distributed (d = 0.40; H = 0.66 and J = 0.48 for rainy and d = 0.72; H 
= 1.52 and J = 0.85 for the dry seasons). 

The pattern of diversity and distribution observed above was also evident among the five locations of the 
reservoir. That is, gastropods still had the highest diversity and distribution, in the two seasons too, and were 
followed by the dipterans (see Table 3 and 4). 

DISCUSSION 

The macroinvertebrate fauna composition of Egbe Reservoir is characterized by low taxa number. This is not 
unusual in tropical waters; for instance, in Lake George, Uganda, the bottom fauna was poor in species 
(Darlington, 1977). Victor and Dickson (1985) and Umeozor (1996) also observed a similarly low taxa number 
and diversity in Ikpoba River and Calabar River, Nigeria respectively. Edokpayi et al. (2000), Ogbeibu (2001), 
Adakole and Annune (2003) also reported low taxa number in some tropical streams and rivers. They ascribed 
this low species diversity to some physico-chemical conditions of water like fast flow, high pH, low dissolved 
oxygen and low conductivity. These factors probably caused disruption of life cycle, reproductive cycle, food 
chain and migrations or imposed physiological stress on even the tolerant macroinvertebrate (Adakole and 
Annune, 2003). 

However, in the case of Egbe Reservior, the low taxa number observed could in addition be due to factors other 
than physico-chemical conditions of the water such as habitat preference, resource partitioning, and food 
availability. For instance, Odum (1971) had reported that diversity tends to be low in physically controlled 
systems such as ponds and dams. This could be the case here in Egbe Reservoir because the artisanal fishermen 
regularly clear the waterways of macrophytes for smooth sailing of their boats. 



36 



J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 

Table 2: Diversity index values of macroinvertebrate during the period of study 

Taxa D H J 

1.56 0.87 

0.60 0.43 

0.28 0.21 

016 023 

D= Margalefs diversity index, H= Shanon-Wiener's index, J= Equitability measure 

Table 3: Diversity index values of macroinvertebrate during the dry and rainy seasons 



Gastropoda 


0.61 


Diptera 


0.38 


Hemiptera 


0.41 


Ephemeroptera 


0.16 







Dry Season 






Rainy Season 


Taxa 


D 


H 


J 


D 


H J 


Gastropoda 
Diptera 
Hemiptera 
Ephemeroptera 


0.72 
0.29 
0.46 
0.14 


1.52 
0.40 
0.25 
0.13 


0.85 
0.37 
0.18 
0.18 


0.64 
0.40 
0.41 
0.18 


1.56 0.87 
0.66 0.48 
0.30 0.23 
0.20 0.26 



D= Margalefs diversity index, H= Shanon-Wiener's index, J= Equitability measure 

The aquatic macrophytes serves as habitats for most macroinvertebrates especially gastropods and their eggs. 
Influx of agrochemicals, bathing and laundry activities could also impact negatively on these aquatic fauna. 
These activities cause pollution of the water body and may affect species diversity of macroinvertebrates of the 
reservoir. Umeozor (1996) in his study on the new Calabar River reported that the important factors governing 
the occurrence and distribution of macroinvertebrates are the physico-chemical qualities of the water and the 
nature of immediate substrates. Any severe alterations of these factors will substantially affect the 
macroinvertebrate community. Bishop (1973) also reported that pollution in its total effect tends to disrupt the 
natural complexity and stability of an ecosystem and lead to a less stable state in which diversity is reduced and 
the self-regulating buffering capacity against change is diminished. It may therefore be implied in this study that 
as the waters of the reservoir is observed to be slightly polluted, this may have influenced the overall 
distribution of the benthic fauna. 

The higher abundance of macroinvertebrates in the rainy season is contrary to the reports of Umeozor (1996) on 
the macroinvertebrate abundance in the New Calabar River; Zabbey (2002) on the benthic macroinvertebrates of 
Woji Creek off Bonny River, and Zikoki and Zabbey (2006) on the benthic community of the middle reaches of 
Imo River. These authors recorded a higher abundance of macroinvertebrates in the dry season than rainy 
season. They explained that during the rains, sedimentary particles become unstable causing dislodgement of 
the benthic animals. However, the gastropods and insecta that dominated the invertebrate population of Egbe 
Reservoir have been documented to be relatively tolerant of physical and chemical variations in the 
environment, and are present in a broad range of environment irrespective of the seasons (Ormerod, 1988). 

The most important factors which influence the abundance and distribution of macroinvertebrates apart from 
physical and chemical qualities of water include habitat area, immediate substrate, trophic structure, resource 
partitioning and predation (Bishop, 1973; Ogbeibu and Victor, 1989, Ogbeibu and Egborge, 1995). Another 
important feature of freshwater systems that may account for the higher abundance recorded during the rainy 
season is that predators other than fish and aquatic invertebrates exploit invertebrate prey better in the dry 
season than they do in the rainy season. This is because in the dry season, the water level becomes reduced and 
clearer, the substrate also stabilizes and the population builds up, thereby making available for the fish and avian 
predators more prey items. Predatory losses to such carnivorous endotherms as birds can also be particularly 
high (Cooper et al, 1991 Ormerod and Tyler, 1988). Different species of these water-loving birds were present 
around Egbe Reservoir. This factor may have contributed to the observed reduced abundance during the dry 
season. 

Habitat preference among the invertebrates may be one of the chief factors responsible for the decrease in 
abundance with sampling stations. The substrate around the Dam station is rocky. It is located close to the base 
of a rock; and the bottom substrate is composed of coarsed sand and gravels; vegetation around here is mainly 
aquatic macrophytes like Ceratophyllum and Nymphea. Aquatic birds and cattle also frequent this site. 
Vegetation around the other stations are also aquatic macrophytes and often thick, though with varying bottom 



37 



J.B. Edward, and A.A.A. Ugwumba: Continental J. Biological Sciences 4 (1): 30 - 40, 2011 



substrates. 

Other factors which may influence the abundance and distribution of invertebrates include the possible role of 
competition and resource partitioning, physical tolerances, and the trophic structure. These factors, coupled 
with habitat differences observed in this study probably acted singly or in combination to influence the 
variations in abundance of macroinvertebrates. 

The low values of diversity indices may indicate decimating impact of impoundment on the benthic 
communities. Impoundment of water bodies have been known to have profound effect on the distribution and 
abundance of the resident organisms in Tropical waters (Egborge, 1977; 1979b; 1979c; 1981 and Ogbeibu et al., 
2002). The low values may also indicate pollutional stress which is further confirmed by the low eveness (J) 
values of all the sampled stations for Odonata and Ephemeroptera which are clean water insects. Abundance of 
macroinvertebrates in Egbe Reservoir revealed a distinct pattern of dominance: a lotic system which is 
characterized by single species dominance. This depicts a simple homogenous environment favouring the 
dominance of a single group, thus making the system less robust and fragile (Ogbeibu and Egborge, 1995). 

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Received for Publication: 04/04/201 1 
Accepted for Publication: 28/05/201 1 

Corresponding author 

J.B. Edward 

Department of Zoology, University of Ado Ekiti, P.M.B. 5363, Ado Ekiti Ekiti State, Nigeria 



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