Abstract

Fluctuation in plankton abundance and diversity of the Imo River in Etche, a Niger Delta region of Nigeria was investigated. Plankton samples were collected with 55 micro meter mesh size plankton net at 7 sampling locations once monthly between March 2007 and February 2009. Samples were preserved with 4% formalin solution in labeled plastic containers in the field. In the laboratory, 1ml of the plankton subsample was withdrawn with a wide-mouthed pipette from field samples and placed on a Sedge-wick rafter-counting chamber for species identification and counts with standard keys through direct microscopy. The studentized t-test of significance was used to partition numerical abundances of plankton biotypes seasonally. Phytoplankton comprised 43 genera and a mean density of 1859 cells/ml. The dominant phytoplankton was the Bacillariophyceae (53.25%), followed in order by Cyanophyceae (21.25%), Chlorophyceae (10.33%), Chrysophyceae (4.84%), Pyrrophyceae (4.57%), Xanthophyceae (3.39%) and Euglenophyceae (2.42%). Zooplankton was made up of 7 taxa and a mean density of 433 organisms/ml. The order of dominance was the Cladocera (25.87%), Copepoda (20.55%), Protozoans (19.17%), Rotifera (18.71%), fish eggs and larvae (9.24%), Crab larvae (4.62%), and Beetle larvae (0.69%). Phytoplankton species showed oscillating as well as stable seasonal patterns of occurrence. Higher Margalef’s diversities were recorded in the dry (3.655; 57% and 1.273; 61%) than wet season (2.732; 43% and 0.810; 39%) for phytoplankton and zooplankton biotypes, respectively. Phyto- and zoo-plankton taxa each showed significant numerical differences between the 2007/2008 and 2008/2009 sampling periods [F_(14.39)>F_crit(4.30) and F_(29.08)>F_crit(4.23), respectively] at P<0.05. The observed seasonal peaking in abundance could be attributed to periods of concentrations of nutrients and stability in growth factors of plankton biotypes.

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