A Checklist of the Phytoplankton Flora of a Southern Nigerian Lotic Ecosystem

This study presents a first compilation of phytoplankton species composition of a stretch of fresh and brackish water ecosystem in Southern Nigeria. Samples collection spanned a period of sixteen months (January 2003 to April 2004). Phytoplankton samples were collected monthly in the open water using plankton net of 55 μm mesh size towed horizontally for about five minutes. The net hauls were transferred into 200 mL properly labelled plastic containers and immediately preserved with 4% formation solution. Samples were examined and illustrations made with a Leitz Orthoplan Research Microscope equipped with a tracing and measuring devices at the phycology laboratory in University of Benin, Benin City. A seasonal pattern of phytoplankton variation was observed with a hundred and 154 phytoplankton species recorded. The taxa recorded belong to four divisions namely: Bacillariophyceae (diatoms), Cyanophyceae (blue-green algae), Euglenophyceae (euglenoids) and Chlorophyceae (green algae). The Diatoms were the predominant group and account for 79% of total species composition. Others are Chlorophyceae 19.77%; Cyanophyceae 1% and Euglenophyceae, 0.23%. The brackish water stations recorded relatively higher number of species and number of individuals of each species than the freshwater stations. An array of floristically diverse phytoplankton was observed with notable common and cosmopolitan diatom species like Coscinodiscus and Nitschia spp in abundance.


INTRODUCTION
The term plankton refers to any small biota (usually microscopic) living in the water adrift in the water column and incapable of maintaining its position and at the mercy of currents.Phytoplankton (algae) such as diatoms, grow in the presence of sunlight and nutrients such as nitrogen and phosphorus.They are mostly unicellular, filamentous or aggregate of cells and form the base of the trophic structure in the aquatic food chain.Some of these plants are in turn grazed upon by zooplankton, which is dominated by small crustaceans such as copepods, shrimps and their larvae.The amount of phytoplankton in the water column reflects the influence of a number of environmental factors and processes (Suthers and Rissik, 2009).Some phytoplankton species may be considered as villainsproducing red tides or toxic algae-but there are only a few species responsible.Most phytoplankton is enormously beneficial, such as those used in the aquaculture industry (Suthers and Rissik, 2009).Phytoplankton has immense values as they play a vital role in aquaculture feed since they form the primary food producer.The great fishing grounds of the seas are found in where algae are found in abundance.As the most sensitive organisms, they serve as indicators of water quality with their ability to detect even the subtle changes taking place in their ambient environment (Sithik et al., 2009).
The study is important because it is a pioneer investigation of this nature in Osse River and therefore will contribute to the knowledge of phycological information in Nigeria.

Study area:
The Osse River takes its source from the Akpata hills in Ekiti State, Nigeria (Fig. 1).It flows through Ovia North-East Local Government Area of Edo State and empties into the Benin River through the Gwato Creek which anddrains into the Atlantic Ocean at the Bight of Benin.The climate has the unique features of the humid tropical wet season and dry seasons.In the wet season, the river is characterized by increased flow rate, high turbidity and muddy water especially after heavy rainfall while dry season is characterized by moderate or slow flow rate and clearer water.Several streams and creeks drain into the river.The river is the major source of drinking water for the inhabitants of these communities (Omoigberale and Ogbeibu, 2007).

METHODOLOGY
Phytoplankton samples were collected by towing 55 μm mesh size plankton net against the current at subsurface level for ten minutes.The samples were preserved in a solution of four per cent formaldehyde.Phytoplankton samples were examined in the laboratory using a Leitz Orthoplan Universal Wide-Field Research microscope equipped with tracing and measuring devices.

A CHECKLIST OF PHYTOPLANKTON SPECIES IN OSSE RIVER
In this list, algal species are arranged according to their divisions, classes, orders and families.

DISCUSSION
A total of one hundred and 154 phytoplankton species were recorded in this study.The phytoplankton of Osse River was dominated by diatoms throughout the study.Similar findings have been reported by Onyema (2010), Emmanuel and Onyema (2007), Adesalu and Nwankwo (2008), Uttah et al. (2008), Davies et al. (2008), Olomukoro and Oronsaye (2009), Kadiri (2002b), Mustapha (2010), Onyema et al. (2010) and Nwankwo (1998).The predominance of diatoms (the Bacillariophyceae) is common feature of open lotic waters with fast flowing currents (Uttah et al., 2008).Diatoms (79.00%) were the largest group of the phytoplankton observed in this study with the pennate forms accounting for 67.9% while the centric forms accounted for 32.1%.This indicates that the water body is more fresh than saline as pennate forms are found more in fresh water.The order of dominance of the phytoplankton group was Bacillariophyceae (79.00%) >Chlorophyceae (19.77%) >Cyanophyceae (1.00%) >Euglenophyceae (0.23%).