Abstract: The effects of water hyacinth infestation on the distribution, abundance and species composition of zooplankton in Awba reservoir were investigated. Samples were collected in each of the sampling areas (water hyacinth infested and open water) by vertical zooplankton hauls using 64 um bolting silk net. The samples were immediately fixed in 4% formalin for preservation. The species composition of the zooplankton from the sampled areas consisted of 3 taxa, comprising of 6 species of Cladocera (Moina sp., Ceriodaphnia sp., Pleuroxus sp., Diaphanosoma sp., Chydorus sp. and Leydigia sp.) 6 species of Rotifers (Asplanchna sp., Trichocerca, Filinia sp., Polyarthra sp., Brachionus sp. and Lecane sp.) and 3 species of Copepoda (Cycloid copepods, Copepodite nauphii and Calanoid copepods. However, the study showed that the Rotifers had higher numerical abundance in the study areas. Water hyacinth infested area had a total density of 95 individuals/L while open water had 215 individuals/L. The study showed that the density of zooplanktons were significantly lower (p< 0.05) in the infested area. Biotic indices such as Margalef`s and Simpson`s indices were lower in the infested area while Shannon-wiener diversity index was significantly lower (p< 0.05) in the water hyacinth infested area. The result suggests that water hyacinth dense mats affected the numerical abundance of the zooplanktons.
INTRODUCTION
Eichhornia crassipes (Mart Solms) popularly known as water hyacinth, is a free floating aquatic weed which belongs to the family Pontedriaceae (Cronk and Fennessy, 2001). This obnoxious aquatic weed has been implicated in fresh and brackish waters in 20 out of 36 States and Federal Capital Territory of Nigeria (Bolorunduro, 2000), thus causing considerable economic losses especially with respect to aquatic fauna which need to be conserved.
Awba reservoir-a repository of domestic waste from the Halls of residence and Staff quarters in University of Ibadan, Nigeria has experienced an explosive growth of this weed. It has been reported that such alien species as water hyacinth alters the ecosystem functions and thus constitute a threat to biological diversity of organisms (aquatic flora and fauna) in such ecosystems and food chains associated with them (Luken and Thieret, 1997). Community composition, distribution and abundance of invertebrates appear to be regulated by water chemistry, substrate type, depth and the presence or absence of aquatic vegetation (Watkin II et al., 1983). Zooplankton constitute a major component of food chain in aquatic ecosystem. They play important role in channeling primary production into fish production (Dejen et al., 2004). Zooplankton studies are necessary since these organisms play important role in the energy transfer from primary producers to organisms of higher trophic levels (Aoyagui and Bonecke, 2004). Therefore variations in terms of species richness of zooplankton may negatively affect the reproductively biology of aquatic fauna. According to Aoyagui and Bonecka (2004) dissolved oxygen, pH, Conductivity and Environmental morphometry have the greatest effect on zooplankton.
The objective of the study therefore is to examine the influence of water hyacinth infestation on the composition, abundance and distribution of zooplankton in Awba reservoir.
MATERIALS AND METHODS
Study area: Awba reservoir is located within the University of Ibadan Campus at an altitude of 185 meters above sea level. It lies between latitude 3°531 E and 17°261 N (Akin-Oriola, 2003). The reservoir has a surface of 6 hectares, maximum depth of 5.5 m and maximum length of 700 m (Omotosho, 1981). The reservoir is marked by two seasons. The Wet and Dry seasons. Rainy (Wet) season occurs between April and October while Dry season is between November and March.
The dominant aquatic plants on the fringes of the reservoir are Pistia stratiotes, Nymphaea lotus, Canna indica, Bahima sp. and Typha australis.
Sampling areas and procedures: The study was carried out between July and November, 2005. The reservoir was divided into 2 sampling areas viz., Water hyacinth infested and Open water areas for the purpose of the study. Sampling for water quality properties and zooplankton were carried out forth nightly between 0700-1100 h on each sampling day. Zooplankton were collected using 64 μm bolting plankton net. Plankton samples concentrated into 1 L container and were immediately preserved in 4% formalin (Jeje, 1990). Zooplankton were identified using the taxonomic keys of Jeje and Fernando (1986), Jeje (1990) and Boney (1983). All results were expressed as number of organisms per litre.
The percentage occurrence and relative numerical abundance of zooplankton were calculated. Biotic indices such as Margalef’s (1949), Shannon-Weiner (1949) and Simpson’s (1945) were used to estimate diversity. Shannon-Weiner diversity values of the sampling areas were further subjected to Student’s t-test. Sorenson’s and Jaccard indices of community similarity were used to determine how zooplankton in both areas are closely related.
RESULTS
Physico-chemical properties: The variations in the Physico-chemical properties of the sampled areas are shown in Table 1. Water temperature and Alkalinity were not significantly different (p>0.05). While dissolved oxygen, pH, turbidity and total suspended solids were significantly different (p<0.05) between infested and Open water areas (Table 1).
Zooplankton composition, density and distribution: A total of 15 species of zooplankton were identified in the study (Table 2). The total number of species and individuals present at the sampled areas (Water hyacinth infested and Open water) are 9 (95) and 15 (225), respectively (Table 3). Analysis of Variance (ANOVA) showed that water hyacinth infested area was significantly lower (p<0.05) than the open water area. Rotifers were the dominant zooplankton group in the infested and open water accounting for 61.1 and 44.1%, respectively; it was followed by Cladocerans 16.9 and 28.1%, respectively, while the least is Copepoda 22.1 and 28%, respectively in the sampled areas as shown in Table 3.
Cladocerans were represented by three families: Moinidae which is made up of only 1 genus each Moina sp. Daphninidae comprising of one specie Ceriodaphnia cornuta, Chydoridae comprising of 1 specie namely Pleuroxus similes which was absent in the infested area; Chydorus sp. and Leydigia sp. was also not represented in the infested area). Cladocerans had a density of 16 org L-1 in the infested area as against open waters 63 org L-1 (Table 3).
Rotiferans were represented by 6 families: Asplanchnidae made up of One genus Asplanchna sp. Trichocerchidae, Synchaetidae and Brachionnidae comprised of a genus and are represented by Trichocerca sp., Polyarthra sp. and Brachionus sp., respectively. These genus was absent in the infested area. Filinidae and lecanidae were made up of one genus each represented by Filinia sp. and lecane sp., respectively. In the water hyacinth infested area Rotiferans were 58 org L-1 while open water had 99 org L-1 (Table 3).
Copepodans comprised of one family: Cyclopidae which is made of 3 species namely Cycloid Copepods, Copepodite and Calanoid copepods. Copepods in water hyacinth infested area accounted for 21 org L-1 while in the Open water, it accounted for 63 org L-1 (Table 3).
Margalef’s species richness, Shannon-Wieners index as well as Simpson’s index was lower in the water hyacinth infested area when compared to open water. The diversity of zooplankton in water hyacinth infested area is significantly lower (p<0.05) in water hyacinth infested area when subjected to students t-test than in open water (Table 4). However, Sorensen’s and Jaccard Coefficients of Community Similarity showed that zooplankton in the sampling area were related.
| Table 1: | Variations in water quality (Mean-standard error) between water hyacinth infested and open water areas in Awba reservoir between July and November, 2005 |
| p indicates significance of One way ANOVA, * = Significant p<0.05, NS = Not Significant p>0.05; TSS = Total Suspended Solids | |
| Table 2: | Distribution of zooplankton species in infested and open water areas of Awba reservoir during the study period |
| Key: + Present, - Absent | |
| Table 3: | Percentage composition and abundance of zooplankton in water hyacinth infested and open water areas of Awba dam during the study period |
| Table 4: | Diversity of zooplankton in the sampling areas of Awba Reservoir |
DISCUSSION
The quest for factors that influence zooplankton diversity is of great interest to Researchers. It has been reported that zooplankton composition and diversity gives an insight into the characteristics and quality of water body (Marson, 1983). The abundance of zooplankton were low in water hyacinth infested area when compared to open water. The reason could be due to the formation of dense mats of Water hyacinth on the water surface, thus a reduction of dissolved oxygen content of the infested area. This is in agreement with Rommens et al. (2003) who reported that low oxygen level inhibited the growth of zooplankton in a water hyacinth infested sub tropical impoundment. The quest for factors that influence zooplankton diversity by researchers is of great interest. According to Aoyagui and Bonecka (2004), Dissolved oxygen, pH, conductivity and environmental morphometry have the greatest effect on zooplankton Therefore the low diversity of zooplankton in the infested areas can also be attributed to dense mats of water hyacinth in the infested area which affects the Physicochemical variables of the area. Zooplankton abundance was low in the infested areas because of high turbidity in the infested area. Egborge (1979) opined that high density of Phytoplankton in a water body associated with less suspended matter boosts zooplankton population as result of transfer in food chain. So the high level of turbidity as a result of heavy load of suspended particles in the infested area will not favour the composition and abundance of zooplankton organisms. Also the ability of Eichhornia crassippes to absorb and utilize nutrients deprives Phytoplankton of them. This leads to a reduced zooplankton population. The Shannon-Weiner zooplankton diversity was significantly lower (p<0.05) in water hyacinth infested area because of non-mixing of autochthonous materials due to dense mats of water hyacinth on the surface of the water body. Evans (2003) reported that the mixing of autochthonous materials in a water body facilitates primary production resulting in zooplankton abundance.
Rotifers had the highest numerical abundance in the sampled areas. It could be due to the fact that rotifers undergo vertical migration, which minimizes competition through niche exploitation and food utilization (Moss, 1988). Moreso, rotifers has the ability to desiccate and the possession of an intra-epidermal curticle which has developed into a thick case or lorica in some species gives extra protection to rotifers and as well as enhances its survival and proliferation under adverse environmental conditions (Ward and Whipple, 1966). Hutchinson (1967) reported that the presence of rotifers for the non vegetated orange lake region may be due to ability to avoid predation, owing to their small size or physiological avoidance of aquatic plants. Among zooplankton, those Cladocera and Copepod taxa which preferred aquatic plants are forms which either feed among them, owing to their large size avoid open water, and use aquatic plants to avoid predation (Green, 1967; Hutchinson, 1967).
The two sampled areas were closely related based on the estimated Sorenson’s (Cs and Jaccard’s (Cj) indices of Community Similarity. The 69% (Cs) and 53% (Cj) are greater than the critical values of 50%.Thses suggests that the higher the joint occurrence (S or j); the higher the similarity. It can be deduced from these results that water hyacinth infestation did not support zooplankton abundance and diversity. Efforts should be intensified towards its management and control.