Introduction
Phytoplankton (singular-phytoplankter) is very small (microscopic) free floating plants that drift with water currents. They exist in both freshwater and marine environments (Microsoft Corporation, 2009; Lindsey and Scott, 2010; Encyclopædia Britannica, 2012). Like terrestrial plants, they use carbon dioxide, release oxygen and convert minerals to the form that animals can use. They are responsible for 50 percent of all photosynthetic activity on Earth. They can be divided into two divisions which include: phototrophic bacteria (which is made up of purple sulphur bacteria and green sulphur bacteria) and algae (which is made up of euglenophyta, chrysophyta, phaecophyta, rhodophyta, pyrrophyta and cyanophyta) (Edmondson, 1958). Their composition and abundance is highly dependent on the availability of sunlight, carbon dioxide and nutrients such as nitrate, phosphate and silicate etc. These factors influence their density and distribution throughout the water column. Phytoplankton is very important because they form the base with which the aquatic ecosystem is culminating (Reynolds, 1984). They are a source of food to almost all aquatic life either directly or indirectly. In aquaculture, they serve as a food stock for zooplankton which are in turn fed to fish larvae reared in fish hatcheries (Moncheva and Parr, 2010; Lindsey and Scott, 2010). As beneficial as phytoplankton are, they could also be harmful. Under certain conditions such as pollution of water body with sewage and human excreta, certain harmful phytoplankton predominate and produce bio toxins which affect the taste and colour of water, impact bad taste on fish and harbor diseases thereby causing massive fish kills. Examples of such harmful phytoplankton include: Cyanophyta (Cyanobacteria / blue green algae) which produces geosmin which is a toxin that impacts a bad flavor on fish especially bottom dwellers (such as Cat fish and carp) and rhodophyta which causes red tides following massive fish kills. The beneficial and harmful importance of phytoplankton cannot be undermined. There is therefore need to carry out routine checks on the phytoplankton species composition and abundance of water bodies worldwide.

River Ogun is a perennial river prone to pollution due to the human activities carried out around its borders. This study is therefore aimed at investigating the phytoplankton abundance and species composition of River Ogun.

1 Results
Five taxonomic groups consisting of forty-one species were discovered from Ogun River (Table 1). Chrysophyta was represented by 15 species consisting of 36.6%, Chlorophyta with 14 species consisting of 34.1%, Cyanophyta (7 species) consisting of 17.1%, Euglenophyta with 3 species consisting of 7.3% and Pyrrophyta with 2 species consisting of 4.92%. The abundance of phytoplankton species is presented in Figure 1 and Table 2. Cyanophyta was the most abundant with 41%. This was followed by Chrysophyta (31.4%), Chlorophyta (23.6%), Euglenophyta (2.5%) and Pyrrophyta (1.5%).

2 Discussion
Cyanophyta (also known as Blue green algae, Cyanobacteria and Myxophyceae) surpassed the phytoplankton in terms of abundance while Chrysophyta surpassed in terms of number of species/genus. The dominance of Cyanophyta has been observed by several authors among which are: Sekadende et al., (2004) who observed Cyanophyta dominance in Lake Victoria basin, Ogato, (2007) who observed dominance of Cyanophyta in Lake Bishoftu, Deng et al., (2007) who reported dominance of Cyanobacteria in summer and autumn in Lake Chaohu; Shakila and Natarajan (2012) who also observed dominance in the Temple Pond of Thiruporur, Chennai. The abundance of Cyanophyta observed in this study must have been caused by the polluted nature of the water due to the anthropogenic activities carried out around its shores. The dominance of Cyanophyta is a signal that the river is polluted. Consequently, there is therefore need to regulate the amount of pollutants discharged into the river so as to avoid total ecological collapse and extinction of the populace. It is hereby recommended that relevant laws in respect to sustainability of the river should be implemented.

3 Materials and Methods
3.1 Geography of the river
Ogun State is a state in the South-western region of Nigeria. It has boundary with Lagos State to the South, Oyo and Osun states to the North, Ondo State to the east and the republic of Benin to the west. Abeokuta is the capital and largest city in the state (NBS, 2012). River Ogun is one of the main rivers in the southwestern part of Nigeria with a total area of 2.24×10⁶ m² and a fairly large flow of about 393 m³ secG1 during the wet season (Oketola et al., 2006). It is 3°28'E and 8°41'N from its source in Oyo state to 3°25'E and 6°35'N in Lagos where it enters the Lagos lagoon (Ayoade et al., 2004). Two seasons are distinguishable in Ogun river basin, a dry season from November to March and a wet season between April and October. Mean annual rainfall ranges from 900 mm in the north to 2 000 mm towards the south. The estimates of total annual potential evapotranspiration have been put between 1 600 mm and 1 900 mm (Bhattacharya and Bolaji, 2010). The water is used for agriculture, transportation, human consumption, various industrial activities and domestic purposes. Along its course, it constantly receives effluents from breweries, slaughterhouses, dyeing industries, tanneries and domestic wastewater before finally discharging to Lagos lagoon (Ayoade et al., 2004; Oketola et al., 2006). A 100 square kilometer area around River Ogun has an approximate population of 3 637 013 (0.03637 persons per square meter) and an average elevation of 336 m above the sea (Travel Journals, 2012). Numerous human activities such as bathing, washing of clothes, fishing, locust bean processing, mangrove cutting, and transportation were noticed. These are potential sources of pollution to the environment. Indiscriminate dumping of human excreta was observed at two of the sampling stations. Four sampling stations were established along the length of Ogun River.

3.2 Station 1 (Ibẹrẹkodo)
It is located just downwards the Ogun State Water Works Corporation at Arakanga. The water is cured by the corporation and dispensed through underground pipes to respective homes. The river here is characterized by a dam/spillway and high concrete dykes. The water is generally very clear and has good aesthetic quality. The activities here are majorly fishing, bathing, dam maintenance and fishing gear mending. No farmland was observed at this station.

3.3 Station 2 (Agọ ika)
It is located close to the FADAMA III supported ferry for transportation to Lafenwa. The vegetation around the banks is very dense. The activities here includes: locust bean processing, bathing, washing of clothes, refuse dumping and transportation by ferry. The activities of the ferry men could lead to siltation of the water body causing high turbidity conditions. Indiscriminate discharge of human wastes into the river was noticed.

3.4 Station 3 (Ẹnu gada)
It is located some few meters after the bridge connecting to Lafenwa. It is characterized by less dense vegetation and slow flowing water. Activities carried out here includes: washing of clothes, farming, indiscriminate dumping of refuse and human excreta.

3.5 Station 4 (Off Pepsi bus stop)
It is characterized by rocky outcrops with fast flowing water. The vegetation consists of terrestrial vegetation which is less dense. The water is clear and has good aesthetic quality. Activities carried out here includes: bathing, washing of clothes and drying of fishing gear on the rocks.

3.6 Sample collection
Samples for phytoplankton analysis were collected by the hand trawling method. This involves trawling horizontally at the littoral zone (i.e. at the shore) with 55 µm mesh size plankton net. The net content of the plankton net was put into a 120 ml plastic bottle. This was preserved with 4% formalin solution within 5 minutes of sample collection. The sample was then taken to the Laboratory for identification and enumeration.. A drop of the sample was placed onto the glass slide of the binocular microscope and was covered with a cover slip for identification and enumeration. Phytoplankton was identified at magnifications of 10x, 40x and 100x using suitable keys. Enumeration of phytoplankton was carried out following the drop count method adapted from Verlecar and Desai (2004) and was reported as units or organisms per drop.

3.7 Statistical analysis
Descriptive Statistics in the form of tables and bar charts were used in the presentation of the data. MS EXCEL statistical package was used for the analysis.

Acknowledgement
All the members of staff especially Dr. (Mrs.) F.O.A. George of the Department of Aquaculture and Fisheries Management, College of Environmental Resources Management, Federal University of Agriculture, Abeokuta, Ogun state, Nigeria are appreciated for their various contributions. I also thank Mrs. Binyotubor of the National Institute of Freshwater Fisheries Research for her technical support. May God be with you all.

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