Abstract

We investigated the spatial relationships between physicochemical variables and planktonic (bacterio- and myco-plankton) assemblages of the middle reaches of the Imo River. In situmeasurements and collection of water samples for laboratory analyses of physicochemical characteristics and enumeration of microbial groups were made at 7 sampling locations along the river between March 2007 and February 2008 according to standard methods. The principal components analysis was used for data reduction in the physicochemical variables, while one-way ANOVA was used to determine variance equality in means of physicochemical variables and planktonic assemblages. Post structure detection was made with means plots and relationships between the physicochemical variables and planktonic groups explored using correlation (r). Surface water temperature (SWT) varied between 24.00-28.10 (26.89 ± 0.12)°C, turbidity between 11.00-279.00 (96.70 ± 9.3) NTU, pH between 6.00-6.70 (6.40 ± 0.02) and total dissolved solids between 19.90-38.00 (28.40 ± 0.56) mg/l. Dissolved oxygen varied between 4.50-8.81 (6.96 ± 0.14) mg/l, biological oxygen demand between 2.10-3.90 (2.85 ± 0.05) mg/l, conductivity between 12.00-300.00 (107.80 ± 9.91) µS/cm and total hydrocarbon (THC) between 0.000-0.038 (0.020 ± 0.001) mg/l. Nitrate varied between 0.10-1.35 (0.54 ± 0.04) mg/l, phosphate between 0.07-0.23 (0.13 ± 0.01) mg/l and sulphate between 0.90-8.10 (4.35 ± 0.25) mg/l. A test of homogeneity of variance in inter-location means revealed significant inequality in both the physicochemical variables [F_(25.81)>F_crit(3.90)] and planktonic groups [F_(86.16)>Fcrit_(3.93)] at P<0.05. The PC analysis revealed two major components- sulphate and nitrate, forming the extraction solution and explaining nearly 62% of the variability in the original 12 variables. PC 2 correlated negatively with HUF (r = -0.687) at P<0.01, even as conductivity correlated negatively with HUF (r = -0.555), and THC with THB (r = -0.635), THF (r= -0.642) and HUB (r = -0.596) at P<0.05. Seven HUB genera (Bacillus sp., Arthrobacter sp., Corynebacter sp., Pseudomonas sp.,Escherichia coli, Micrococcus sp. and Serratia sp.) and 4 HUF genera (Rhizopus sp.,Cladosporium sp., Aspergillus sp. and Mucor sp.) were identified across the sampling locations. Though its comparatively low levels in this study could infer biogenic origin in a hydrocarbon rich region, the contributory significant correlational effect, as well as spatial variation in THC most probably determined planktonic assemblages abundance in the sampling locations.

Key words: Bacterioplankton, spatial variation, biogenic inputs, hydrocarbon utilizing fungi, physicochemistry.