Ecological Status of Opa Reservoir , Obafemi Awolowo University , Ile Ife , based on the Abundance and Diversity of its Planktonic Flora

A study investigating the spatial and temporal distribution, composition and abundance of plankton in Opa reservoir, Obafemi Awolowo University, Ile-Ife, Southwest Nigeria, was conducted over a period of an annual cycle. The study was undertaken with a view of providing a more recent catalogue of planktonic flora and possibly an update of the reservoir’s trophic status. Quantitative net planktons were collected monthly from both the surface and bottom levels at three sampling stations established at the dam site (lacustrine), mid-lake (transition) and upper inflow (riverine) parts of the reservoir. The divisions recorded were Bacillariophyta > Cyanophyta > Chlorophyta > Euglenophyta > Myzozoa > Ochrophyta = Charophyta > Cryptophyta in order of abundance. Vertically, the highest occurrence of species was recorded at the lacustrine bottom station (71 species), while the least occurrence was observed in the transition bottom station (51 species). A total of sixteen plankton species showed significant seasonal variation in abundance during this study period, while only seven species had significant spatial variation (p ≤ 0.05). Higher abundance was observed during the rainy season (170,797,350 Org/m from seventy-two species) than dry season (5,138,400 Org/m from forty-nine species). Notable bio-indicator plankton species recorded were Anabaena circinalis, Anabaena flos-aquae, Microcystis sp., Aphanocapsa litoralis and Microcystis aeruginosa. Some other pollution indicator species recorded were Synedra ulna, Oscillatoria agardhii, Phacus sp., Surirella sp., Closterium sp., Aphanocapsa sp. and Euglena sp. Hence, Opa reservoir is very rich in Bacillariophyta (diatoms), followed by Cyanophyta (bluegreen) and Chlorophyta (green algae), which are known to characterize eutrophic lakes.


Introduction
By the virtue of the position of phytoplankton at the base of the aquatic food web, they stand as the most important factor of production in the aquatic ecosystem (Moshood, 2009).Various ecological changes such as presence, absence, replacement or addition of species can also be monitored using the phytoplanktonic community as a potential tool (Codd, 1995).Therefore, the presence of phytoplankton in reservoirs goes a long way in determining the sustainability and productivity of most aquatic habitats.The growth significance and sustainability of any ecosystem is largely accounted for by the diversities of phytoplankton and their abundance.Both factors are equally related and do change as their interaction is influenced by the environment and population processes (Benedict and Gabriel, 2012).
Phytoplankton are known to be very important in estimation of the potential fish yield (Hecky and Kling, 1981), productivity (Park et al., 2003), water quality (Walsh et al., 2001), energy flow (Simciv, 2005), trophic status (Reynolds, 1999), and water management (Beyruth, 2000).Phytoplankton such as Microcytis sp., Anabaena sp., Oscillatoria sp. are known indicators of pollution while the presence and abundance of Chlorophyceae are indicative of the environment's suitability for fish production (Olasehinde and Abeke, 2012).The suitability of microalgal components as bio-indicators of the water condition is because they confer more tolerance than many other biotas used for monitoring environmental changes (Nwankwo and Akinsoji, 1992).
season reduces significantly, whereas in the rainy season, there is increased volume of water inflow resulting from floods leading to high turbidity and a general immersion of the vegetation on the shoreline.This seasonal fluctuation in the water discharge into the reservoir directly affects its water level.
Three sampling stations A, B and C were established on the reservoir denoting the lacustrine, transition and the riverine area of the reservoir along the horizontal axis respectively (Fig. 1).The station A is located at the dam-site just beside the wall, an area assumed to be the deepest part of the lake, station B is the middle of the lake, while the C station is towards the inflow into the lake.A permanent buoy (rubber float) was used to demarcate each of the three sampling stations for ease of subsequent recognition.The distances between the stations and the grid coordinate of each station was taken and recorded using the Global Positioning System (GPS) handheld receiver.

Sample collection
Water samples were collected monthly from both the surface and bottom levels at the three sampling stations on the reservoir for a period of one year for phytoplankton analysis between October 2012 and November 2013.An improvised water sampler of 2.5 L capacity was used to take bottom water samples at required depths.Net plankton was sampled by pouring 20 litres of water through plankton net of 50 μm mesh size and the net planktonic contents was poured into a 30 ml sampling bottle and preserved with few drops of 5% formaldehyde and a drop of Lugol's solution for examination and identification.The preserved subsample containing plankton was examined in the laboratory using OMAX binocular light compound photo and their scaled pictures taken.
Planktonic population abundance was estimated based on the count records of the final concentrate volume of the Adesakin et al. (2017) reported direct discharge of untreated municipal/industrial waste as well as run off from agricultural areas into Opa reservoir, with resultant significant effects on the reservoir's physicochemical parameters both temporally and spatially and this may possibly inflict a level of risk to the inhabiting aquatic biota.This, coupled with the fact that the last published record of plankton research carried out on Opa reservoir was that of Rotifers only by Akinbuwa and Adeniyi (1996), lead to the present study.The study seeks to determine the taxonomic composition, diversity and abundance of phytoplanktonic organisms of Opa reservoir with respect to spatial and temporal distribution, as well as to assess the water quality and trophic status of the reservoir with a view to determining the effects of the discharges.

Plant material
The study site, Opa reservoir (Fig. 1), is located between longitude 004 °31'40''E to 004 °32'45''E, and latitude 07 °30'N to 07 °31'N, within the Obafemi Awolowo University community, Ile-Ife, Southwest Nigeria (Fawole and Arawomo, 2000).The reservoir was established in 1978 by the impoundment of River Opa which sources from Oke-Opa, a set of hills on the Eastern side of the Ife/Ilesha road, Ile-Ife, Osun state (Akinbuwa and Adeniyi, 1996).A number of rivers, including Amuta, Esinmirin, Obudu and Opa unite to form the Opa River.The reservoir has a catchment basin of about 116 km 2 (Akinbuwa and Adeniyi, 1996).Its total surface area is 0.95 km 2 , while the maximum capacity is about 675,000 m 3 with depth of 0.95 m and 6.4 m at littoral zone and open water respectively (Fawole and Arawomo, 2000).The dam wall made of gravel is about 0.28 km long and about 15 m from the foundation to the crest (Akinbuwa and Adeniyi, 1991).As expected of tropical shallow reservoirs, the water volume during the dry sub-sample with respect to the original volume of water filtered with plankton net and the result was then expressed in organisms per cubic metre of the original water sample.

Data analysis
Data collected were subjected to various descriptive and inferential analyses such as the means and standard deviations which gave the depiction of planktonic species abundance with respect to season and location.Analysis of variance was used to compare mean abundance of identified planktonic species, while correlation was used to show the relationship between different planktonic groups.Moreover, Principal Component Analysis (PCA) was used to reduce all interactions into components that also showed the relationship between recorded plankton species as applicable using SPSS Version 21 software (SPSS, 2012).Plankton community structure was determined using Species diversity indices (Shannon and Weaver, 1949), Dominance (Magurran, 2004), Species equitability or evenness (Pielou, 1966) and Species richness (Margalef, 1951;Menhinick, 1964).

Spatial variation
Phytoplankton total abundance ranged from 15,855,150 Org/m 3 at transition (surface) to 53,956,350 Org/m 3 at the lacustrine zone (surafce) of the reservoir.The recorded abundance as compared with zooplankton abundance recorded during the study period showed an average of 112 times (5.71-175.04times) higher phytoplankton than the zooplankton recorded (Table 2).

Diversity
The species richness recorded in the rainy season was higher than that of the dry season.Simpson's index shows a higher diversity in the dry season than wet and this agrees with the Hill's second diversity, which measured the number of very abundant species to be higher in the dry season than the wet season.Similarly, Shannon's index supports a slightly increase in the number of species and more evenness of distribution in the dry season than the wet.This is more revealed by a higher number of abundant species as Hill's first diversity index (Table 5).
Spatially, however, the maximum richness occurred in the lacustrine surface station, while the lowest occurred in the riverine surface.
The highest diversity occurred in the lacustrine bottom as revealed by both Simpson's and Shannon's indices.The station with the most evenly distributed species is the riverine bottom (Table 5).

Species association
Principal Component Analysis (PCA) based on correlation analysis, was used to reduce the component factors to those with most influence (Table 6).Twentyeight factors were found to have an Eigen value greater than 1, that is the strongest correlation between the components and the original set of flexible quantities accounting for a cumulative variance of 87.70 but only five were selected.
Component 1 the highest total variance of 7.56% and maximum Eigen value of 6.28 showed strongest loading (0.881) for abundance of Bacillaria paradoxa.Other species that had strong positive loadings within component 1 were Anabaena flos-aquae, Peridinium sp., Cosmarium depressum and C. subcrenatum.Seventeen species showed positive correlation within the first component with seven of them having high or moderate loading.Component 2 showed positive correlation for twenty species recorded, which was the highest number of positive correlations recorded (

Discussion
The study found that Bacillariophyta, Chlorophyta and Cyanophyta dominated the net phytoplankton of Opa Reservoir, which is in agreement with records of other studies on African tropical reservoirs, especially Nigerian reservoirs (Adeniyi, 1978;Bwalla et al., 2010;Edward and Ugwumba, 2010;Offem et al., 2011;and Atobatele, 2013).The record of the family Bacillariophyceae as most abundant is similar to the record of Abowei et al. (2012) (Koluama area) as well as Ogamba et al. (2004)  The maximum phytoplanktonic abundance, recorded in the lacustrine zone, might be due to stability of certain environmental variables in this zone as a result of reduced water current, restricted movement and higher transparency (Salem, 2011;Adedeji et al., 2015).The lowest abundance in spite of high species richness recorded in April 2013, a period towards the peak of the rainy season, could possibly be an effect of the washing away of many individual phytoplankton through flooding and from littoral vegetation hence species enrichment (Adeniyi and Adedeji, 2007).The observed irregular variation in phytoplankton distribution from surface to bottom across the three sampling stations could be as a result of high mixing and nutrient re-cycling in the reservoir column during the rainy season (Ugwumba and Ugwumba, 1993;Adedeji et al., 2015).The higher planktonic composition recorded during the rainy season may furthermore be due to an increase in ionic dilution during this period as well as an increase in nutrient inflow and introduction of organic matter (Adedeji et al., 2015).
Generally, phytoplankton has been identified to be important in bio-monitoring of trophic status as well as water quality (Townsend et al., 2000;Davies et al., 2009;Achionye-Nzeh and Isimaikaye, 2010;Offem et al., 2011).Notable bio-indicator phytoplankton species recorded were Anabaena circinalis, A. flos-aquae, Microcystis sp., Aphanocapsa litoralis and Microcystis aeruginosa, which have been reported to produce algal toxins such as microcystin, that is a hepatotoxin and can cause serious illness in both humans and some other mammals (WHO, 2009;Ugwumba et al., 2013).These species were noted to be significantly in abundance during the present study.Other pollution indicator species that were recorded in this study include oscillatoria agardhii, Phacus sp., Surirella sp., Closterium sp., Aphanocapsa sp. and Euglena sp.suggesting the likelihood of pollution in the reservoir (Ugwumba et al., 2013).Moreover, high percentage of Chlorophyceae and Cyanobacteria in a water-body, as obtained from this study, with Cyanobacteria being the second most represented taxa, is a clear indication of eutrophication (Taub, 1984;Olasehinde and Abeke, 2012).The elevated abundance of these species might have resulted from the quick increase in the supplied nutrients to the reservoir from several anthropogenic activities from the basin catchment area (Jaji et al., 2007).

Conclusions
Opa Reservoir is rich in phytoplankton, which are mostly members of Bacillariophyta (diatoms), Cyanobacteria (blue-green) and Chlorophyceae (green algae) often recorded in eutrophic lakes.As the hereby study revealed, very high abundance of the algae, recorded as compared to zooplankton abundance resulting from the increase in nutrients through continual inflow and seasonal changes, could lead to the lake deterioration with time.The lake should be therefore monitored closely.

Table 1 .
Outline classification and taxa composition of the phytoplankton flora

Table 2 .
Spatial and temporal abundance distribution of phytoplankton species

Table 3 .
ANOVA statistics showing significant spatial variations in phytoplankton species abundance

Table 4 .
ANOVA statistics showing significant seasonal variation among phytoplankton species