Journal of Microbiology and Biotechnology Research Scholars Research Library J. Microbiol. Biotech. Res., 2012, 2 (2):289-297 (http://scholarsresearchlibrary.com/archive.html) ISSN : 2231 –3168 CODEN (USA) : JMBRB4 289 Available online at www.scholarsresearchlibrary.com Seasonal variations in physicochemical regime, bacterioplankton and mycoplankton of Imo River in Etche, Nigeria Dike Henry Ogbuagu* 1 , Adedolapo Abeke Ayoade 2 and Chidiogo Grace Okoli 1 . 1 Department of Environmental Technology, Federal University of Technology, Owerri, Nigeria 2 Department of Zoology, University of Ibadan, Ibadan, Nigeria ______________________________________________________________________________ ABSTRACT We investigated seasonal variations in some physicochemical regime, bacterioplankton and mycoplankton biotypes of Imo River in a Niger Delta region of Nigeria. Seven sampling points were established along the middle reaches of the river and in situ measurements and collection of water samples for laboratory analyses made according to standard methods. Correlation coefficient (r) was used in estimating relationships between variables while pairwise comparisons in seasonal variation were made with the studentized t-test. Large temporal variations were observed in turbidity (11.00-279.00; Mean= 96.70 ± 9.3 NTU), sulphate (0.90-8.10; mean= 4.35 ± 0.25 mg/L) and dissolved oxygen (DO) (4.50-8.81; mean= 6.96 ± 0.14 mg/L). Significant seasonal variations were observed in sulphate ion concentrations at P<0.05 only. More total heterotrophic bacteria (THB) (range= 2.30 x 10 2 -2.8 x 10 2 cfu/mL) and total heterotrophic fungi (THF) counts (range= 5.0 x 10 1 -7.5 x 10 1 cfu/mL) were recorded in the wet, while more hydrocarbon utilizing bacteria (HUB) counts (range= 8.50 x 10 1 -1.00 x 10 2 cfu/mL) were recorded in the dry seasons. There was significant seasonal variations in THB, THF and HUB at P<0.05. At P<0.05, THC (r=0.633) and nitrate (r=0.623) correlated with the HUF. It was concluded that the study ecosystem was stable, with interacting physicochemical characteristics and seasonally variable bacterio- and myco-plankton compositions in a hydrocarbon rich ecozone. Keywords: Bacterioplankton, Mycoplankton, Imo River, Niger Delta, Hydrocarbonoclastic microorganisms. ______________________________________________________________________________ INTRODUCTION Plankton as bioindicators have been extensively used in the establishment of water quality status. Their suitability for theoretical and experimental population ecology studies is conferred by their small sizes, short generation time and relatively homogenous habitats. These autotrophs, which occupy the base level of food chains that lead up to commercially important fisheries play a major role in the biogeochemical cycles of many important elements such as the carbon cycle, nitrification, denitrification, remineralization and methanogenesis among others [1]. A functional group classification of plankton comprises phytoplankton (autotrophic pro- or eukaryotic algae that live in the euphotic zones of water where there is sufficient light to support photosynthesis), zooplankton (small protozoans or metazoans that feed on other plankton, or even some eggs and larvae of larger aquatic animals such as fish, crustaceans and annelids), bacterioplankton (bacteria and archaea, which play important role in remineralization of organic materials down the water column), and mycoplankton (fungi, which are achlorophyllous heterotrophic microscopic plants that also contribute to remineralization in water column [2]. These classifications have narrow boundaries as many phytoplankton are also bacterioplankton and mycoplankton. In the Imo River (one of the major inland freshwaters of the Niger Delta in Nigeria, which serves domestic, economic and recreational purposes, as well as home to fin and shellfish) for example, plankton studies are very few and limited to those carried out for surveillance monitoring purposes only [3]. Again, several other planktological