Phytoplankton dynamics of a tropical river: A dry and rainy season comparison

Abstract

A study was carried out in River Ethiope in the Niger Delta area ofNigeria, to assess the influence of seasonal dynamics on the phytoplankton  composition, density and diversity. Samples were collected in the dry and rainy season. Qualitative phytoplankton samples were collected by towing 55 ìm mesh plankton net while quantitative samples were obtained by sedimenting a known volume of water using Lugol's iodine. River Ethiope is an oligotrophic freshwater river. The taxonomic and qualitative composition of phytoplankton flora showed seasonal differences. The phytoplankton comprised Bacillariophyta (diatoms), Chlorophyta (green algae),  Cyanophyta (blue-green algae) found in the dry season. In addition to these, Euglenophyta ( euglenoids) occurred in the rainy season. The phytoplankton flora was distinguished into three assemblages- rainy season assemblage, dry season assemblage and seasonindifferent assemblage. Centric diatoms were more common in the rainy season than in the dry season while pennate diatoms were equally well represented in both dry and rainy seasons. The season in different groups were green algae and pennate diatoms. Quantitatively, the dominant taxa during the rainy season were Aulacoseira spp. whereas in the dry season,  phytoplankton flora was dominated by a baculifom1 desmid-Desmidium aptogonium var. tetragonium and a filamentous pennate diatom Fragillaria construens. Spatial variation was observed in community structure indices of phytoplankton

Keywords: Freshwater, Phytoplankton, Niger-delta, Diversity, tropical