Abstract: Background and Objective: The study of zooplankton composition, abundance and seasonal variations is helpful in planning and successful fishery management, therefore this study attempts to shed light on the current state of the zooplankton community of this creek given its present poor state of knowledge. Materials and Methods: The zooplankton assemblage structure and distribution in Stubbs Creek, Akwa-Ibom state, Nigeria was studied (September, 2011-August, 2013). Three sampling stations were chosen. Samples were obtained by filtering 100 L of water through standard plankton net with a base collector of 500 mL. The samples were preserved in 5% formalin. Identification and counting of zooplankton was done using Zeis inverted microscope and identification keys. Results: A total of 9 species belonging to 6 taxonomic groups were recorded. The class Maxillopoda was the most abundant in number and species. Maxillopoda made up 50% of the total zooplankton identified, followed by Appendicularia with 22%. Other classes consisted of <10% each of the total zooplankton. Only Maxillopoda had 4 species, the other classes had 1 species each. The low zooplankton diversity observed in this study is common in tropical waters. Calanoida copepod was the most dominant (27.27%) zooplankton taxa, followed by Oikopleura (22.73%). The dominance of copepods in this creek makes it a good breeding ground for most fish fauna. Although there were differences in the abundance of the zooplankton taxa with seasons, they were not statistically different (p>0.05). The highest number of zooplankton taxa (6) was observed in station 1 while the highest number of zooplankton individuals (180) was observed in station 2. Stations 3 recorded the lowest zooplankton taxa of 4. The dominance of copepods may have accounted for the low Margalef species diversity (<1.012) and low Shannon-wiener index (<1.748) recorded during the study. Conclusion: It is agreed by pollution biologist that species diversity declines as pollution effects are more severe. Stubbs creek is environmentally stressed and this requires urgent monitoring of the activities on this creek to maintain a sustainable, productive and healthy ecosystem.
INTRODUCTION
Many rivers in urban and semi urban areas of Nigeria have been used for disposals of both solid wastes and wastewaters, usually untreated and are thus adversely polluted1. This high pollution status threatens and, in many cases, has already altered the ecological balance of most rivers in Nigeria2. Aquatic communities and populations can be considered biological indicators of the contamination level of an environment, where the presence or the absence of species can be an indication of the disturbance of ecosystems3.
Zooplankton are heterotrophic planktonic animals floating in the water, which constitute an important food source for many species of aquatic organism4. Zooplankton is an important link in the energy transfer of organic matter produced by photosynthesis to other trophic levels as well as including indicator species and providing subsidies for present and past ecological processes5. Zooplankton as a biotic component of the aquatic ecosystems plays a key role in cycling of organic materials, helping in regulating algal and microbial productivity through grazing as suspension feeders and predators in the transfer of primary productivity to fish and other consumers6. Due to their large density, shorter life span, drifting nature, high group or species diversity and different tolerance to stress, zooplankton are being used as indicator organisms for the physical, chemical and biological processes in the aquatic ecosystem. They are globally recognized as pollution indicator organisms in the aquatic environment7. Zooplankton studies are of necessity in fisheries, aquaculture and paleolimnological research as they have been known to leave an impression record of geological past8.
According to Kiran et al.9 the study of zooplankton composition, abundance and seasonal variations is helpful in planning and successful fishery management, therefore this study attempts to shed light on the current state of the zooplankton community of this creek given its present poor state of knowledge. Poor attention has been given to zooplankton studies in smaller creeks scattered across the Niger Delta and which contribute significant proportions to the region’s aquatic biodiversity. The aim of the study is to determine the diversity of the zooplankton groups, their abundance and seasonal variations in Stubbs creek, Akwa-Ibom state.
MATERIALS AND METHOD
Study area: Stubbs creek is located within latitude 4.57° and longitude 7.98°. It is a tidal creek. The mangrove of this creek has been overtaken by Nipa palm. Human activities going on within and around this creek include farming, fishing, washing, disposal of excreta, bathing, swimming and timber transportation. Three sampling stations were chosen (Fig. 1). The map (Fig. 1) was produced using ArcGIS 10.3. Stations 1 and 2 were at areas of increased human activities, unlike station 3 which was a relatively calm spot:
| • | Station 1: Located at the point where Stubbs creek empties into Qua-Iboe River (4° 33' 47" N and 7° 59' 7" E). The vegetation along the creek is mainly Nipa palm |
| • | Station 2: Located along Stubbs creek (4° 34' 25" N and 7° 59' 17" E). From this point, farming lands with human settlements could be observed |
| • | Station 3: Located along Stubbs creek (4° 33' 22" N and 7° 59' 9" E). Although close to a bridge, the station was observed to be serene |
Sample collection: Monthly sampling at the 3 stations for 24 months (September, 2011-August, 2013) was carried-out. Samples were obtained by filtering 100 L of water through standard plankton net with a base collector of 500 mL. The samples were preserved in 5% formalin. Identification and counting of zooplankton was done using Zeis inverted microscope and identification keys including10-12. Number of cells of plankter was calculated according to Onuoha13. Rainfall data was collected from the meteorological unit, Department of Geography, University of Uyo, Nigeria.
Statistical analysis: The means, ranges, percentages and one-way analysis of variance (ANOVA) at probability level of p<0.05 of the data generated were determined using SPSS (v. 19 by IBM Corporation, New York, USA) package. Microsoft Excel 2010 (Microsoft Corporation, Washington, USA) was used for graphical illustrations. The community structure (Margalef species diversity, Shannon-wiener index and species evenness) was analyzed using PAST software (v 2.12 by Hammer, Oslo, Norway).
RESULTS
The zooplankton community observed in Stubbs creek during this study is presented in Table 1, 2 and 3. Table 1 shows the number and percentage composition of zooplankton class found in Stubbs creek. Table 2 shows the spatial variation of zooplankton taxa encountered along the creek while Table 3 shows the monthly variation of zooplankton taxa found in the creek.
| Fig. 1: | Map showing sampling stations (1-3) |
Source: Arc GIS 10.3 |
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| Table 1: | Number and percentage composition of zooplankton class recorded at Stubbs creek |
| Table 2: | Spatial variation of zooplankton taxa (No. mL–1) encountered along Stubbs creek |
| Fig. 2: | Rainfall data during the period of study |
Source: Meteorological unit, Department of Geography, University of Uyo, Nigeria |
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| Table 3: | Monthly variation of zooplankton class encountered along Stubbs creek |
The class Maxillopoda was the most abundant in number and species. Maxillopoda made up 50% of the total zooplankton identified, followed by Appendicularia with 22%. Other classes consisted of less than 10% each of the total zooplankton. Only Maxillopoda had 4 species, the other classes had one species each. Calanoida copepod was the most dominant (27.27%) zooplankton taxa, followed by Oikopleura (22.73%). The least abundant zooplankton taxa were Branchionus plicatilis, Barnacle nauplius and Crustacean nauplius larvae each making up 4.55% of the total zooplankton composition. Maxillopoda was relatively most abundant in all the stations. None of the zooplankton taxa was observed in all the stations except Oikopleura. Branchionus plicatilis, Barnacle nauplius and Crustacean nauplius larvae had very low frequencies, only appearing in one station each. The presence of low zooplankton diversity reflects the health of the ecosystem.
Figure 2 shows a graphical presentation of rainfall for the period of study and Fig. 3 is a graphical presentation of the seasonal variations of the zooplankton class in Stubbs creek during the study.
| Fig. 3: | Class seasonal variation of zooplankton in Stubbs creek |
| Fig. 4: | Relationship between number of individuals and number of taxa of zooplankton in Stubbs creek |
| Fig. 5: | Relationships between the diversity indices of macro-invertebrates in Stubbs creek |
Figure 4 gives a graphical presentation of the relationship between number of individuals and number of taxa of zooplankton in Stubbs creek for the study period. The rainfall data obtained from the Meteorological unit, Department of Geography, University of Uyo, Nigeria, showed a 7 months’ wet season period, which stretched from April-October and a dry season extending from November-March. Malacostraca, Spirotrichea and Appendicularia were more frequent in the wet season than dry season while Monogononta, Hydrozoa and Maxillopoda were more frequent in the dry season than wet season. Monogononta was only encountered in the dry season while Spirotrichea was only encountered in the wet season. Although there were differences in the abundance of the zooplankton taxa with seasons, they were not statistically different (p>0.05). The highest number of zooplankton taxa (6) was observed in station 1 while the highest number of zooplankton individuals (180) was observed in station 2. Stations 3 recorded the lowest zooplankton taxa of 4. Figure 5 shows the relationships between the diversity indices of macro-invertebrates sampled from Stubbs creek.
DISCUSSION
Nine zooplankton taxa comprising of 6 classes were identified in this study. This result is similar to 10 species of zooplankton by Yakubu et al.14 from Nun River but <24 species reported by Zabbey et al.15 from Imo River and the 66 species reported by Ekwu and Sikoki7 in the lower Cross River estuary all in the Niger Delta. The difference in the number of zooplankton species in this study and other studies may be attributed to the natural conditions of water bodies and time of sampling. FAO16 had earlier reported that distributions of zooplankton vary from place to place and year to year due to the dynamic nature of aquatic systems. Carney17 also reported that most zooplankton migrate upward from deeper strata as darkness approaches and return to the deeper areas at dawn. Furthermore, Welcomme18 and Wetzel19 attributed zooplankton abundance to differences in flow, turbidity, dissolved oxygen concentration and conductivity.
Zooplankton species type, number and distribution in any particular aquatic habitat usually create clues on the prevailing physical and chemical conditions of that habitat20. Therefore, the interaction between various environmental variables can either favour the growth or mortality of zooplankton, both spatially and seasonally21. As reported by Abowei and Sikoki22, zooplankton have been underlined as bio-indicators of aquatic environmental perturbation which might be because of their easy identification during their period of high density and high sensitivity to aquatic environmental change compared to other aquatic fauna. Also, the diversity, abundance and seasonality of different zooplankton groups in aquatic ecosystem affect different biotic components therein, making them have significant potential in assessing aquatic ecosystem health. In terms of composition, Calanoida copepod was the most dominant zooplankton taxa, followed by Oikopleura. The least abundant zooplankton were Branchionus plicatilis, Barnacle nauplius and Crustacean nauplius. The creek can be a good breeding ground for most fish fauna because of the dominance of copepods as claimed by Olomukoro and Oronsaye23. The dominance of copepods in this study in terms of abundance indicates pollution according to Ruivo24 and is similar to reports by Onyema et al.25, Davies26, Raji and Saidu27 and Ikhuoriah et al.28 but in contrast to Adeyemi29 and Abdul et al.30, who reported the dominance of rotifer. The high population densities of rotifers have been attributed to their parthenogenetic reproductive pattern and short developmental rates under favourable conditions31. Branchionus plicatus was the only rotifer observed in this study. It is common in tropical waters32. Dirican et al.33 reported the prevalence of rotifer species including Brachionus and Keratella as indicators of eutrophic condition in aquatic systems. Similarly, Rao and Durve34 and Padmanabha and Belagali35 confirmed that the occurrence of species like Filinia longiseta, Brachionus forficula and Brachionus angularis and high level of some ostracods and cladoceras species such as Bosmina, Moina and Macrothrix indicate high level organic pollution as a result of high organic matter deposit.
According to Ikomi and Anyanwu36, the presence of a species will depend on its environmental tolerance but the resources available to it will determine its abundance. If competition or predation is reduced or the food supply or suitable habitat increased, the species will become more abundant. Margalef species diversity (d) ranged from 0.6266-1.012 across the stations with the highest at station 1 and lowest at station 3 while Shannon-Wiener index ranged from 1.242-1.748 with the highest at station 1 and lowest at station 3. Species Evenness (j) index was highest (0.9755) at station 1 and least in station 3 (0.8953). The higher diversity indices observed in station 1 compared to other stations showed the abundance of zooplankton in the region where Stubbs creek empties into Qua Iboe river. The diversity indices, including Shannon-Weiner Index (1.242-1.748) and Margalef species diversity (0.6266-1.012) reported in this study is lower than that reported by Varadharajan and Soundarapandian37 (5.5201-5688, 5.176-5.567) for South East coast of India, Ikhuoriah et al.28 (1.592-2.508, 1.496-3.196)for River Ossiomo, Nigeria and Abdul et al.30 (2.954-3.108, 3.091-3.192) for a South-west estuary, Nigeria. However, Species Evenness Index (0.8953-0.9755) for the present study is higher than report of Ikhuoriah et al.28 (0.5461-0.6916) and Abdul et al.30 (0.731-0.829). According to Onyema et al.25, the dominance of copepods may have accounted for the low Margalef species diversity (<1.012) and low Shannon-wiener index (<1.748) recorded during the study. Also the low species diversity values might be associated with environment under stress. It is agreed by pollution biologist that species diversity declines as pollution effects are more severe26. As suggested by Varadharajan and Soundarapandian37, the diversity of zooplankton species in aquatic ecosystems is linked to its abundance.
CONCLUSION
A total of 9 species belonging to 6 taxonomic groups were recorded. The low zooplankton diversity observed in this study is common in tropical waters. The class Maxillopoda was the most abundant in number and species. Maxillopoda made up 50% of the total zooplankton identified. Although there were differences in the abundance of the zooplankton class with seasons, they were not statistically different (p>0.05). Calanoida copepod was the most dominant (27.27%) zooplankton taxa. The dominance of copepods in this creek makes it a good breeding ground for most fish fauna and also may have accounted for the low Margalef species diversity (<1.012) and low Shannon-wiener index (<1.748) recorded during the study. It is agreed by pollution biologist that species diversity declines as pollution effects are more severe.
SIGNIFICANCE STATEMENT
The study shows variation in composition, abundance of the zooplankton encountered in Stubbs Creek, Nigeria. The study further confirmed that zooplankton organisms are unique indicators for pollution status and productivity of aquatic ecosystem. The occurrence of Branchionus plicatilis suggests the creek to be highly organically polluted as a result of high organic matter deposit. The prevalence of copepods from the present study places Stubbs creek as a favourable breeding ground for fish assemblages. Stubbs creek is environmentally stressed and this requires urgent monitoring of the activities on this creek to maintain a sustainable, productive and healthy ecosystem.
ACKNOWLEDGMENT
The authors would like to acknowledge the assistance of Mr. Bassey U. Bassey during sampling.