The dynamics of planktons in the coastal waters of Ondo State, Nigeria were investigated from April, 2014 to March, 2016. Samples were collected monthly using plankton net of 55 μm-mesh size, preserved in 4% formalin, examined with Olympus microscope and identified using standard guides. Zooplankton was more abundant in the environment in this study with NOI of 55.54% and 53.25% in the wet and dry seasons respectively. A total of twenty-three species of phytoplanktons belonging to two taxonomic groups were recorded. Diatom (65.22%) represented by five phyla consisting of Ochrophyta (7species), Heterokontophyta (6species) and Bacillariophyta (2species) while Dinoflagellates (34.78%) were represented by phylum Dinoflagellata (4species) and Myzozoa (4species). The zooplankton assemblage was composed of twenty-seven species including phylum Arthropoda (10), Rotifera (6), Chordata (3), Chaetognatha (2), Echinodermata (2), Ciliophora (1), Cnidarian (1), Granolereticulosa (1), and Mollusca (1). In wet-season, Fish-eggs (4.69%) (most abundant) Brachionus quadridentatus (3.79%); Coscinodiscus sp (3.67%); Biddulphia mobiliensis (3.31%) and Odonata nymph (3.01%) with Foraminifera (0.60%, the least) while the composition of the dry-season had a descending trend of Asterionellopsis sp (3.25%)>Cerataulina sp and Pseudo-nitzschia sp (3.02%)>Chaetognatha adult (2.86%)>Brachionus falcatus; Ceratium hirundinella sp and Coscinodiscus sp (2.70%)>Gyrodinium spp (2.62%) to Protoceratium reticulatum (0.48%). The Number of Individuals ranged from 26.25±6.86 (Dry-season) to 38.85±16.33 (Wet-Season) while Number of the most abundant species (Nmax) and average number of species (S) (which exhibited seasonal variation at P<0.05) ranged from 2.92±0.87 (Dry-Season) to 3.92±1.18 (Wet-Season) and from 18.10±3.14 (Dry-season) to 20.38±5.72 (Wet-Season) respectively. The Margalef’s Diversity Index and Shannon-Wiener Index which showed no seasonal variation ranged from 5.25±0.70 to 5.35±1.28 and 2.80±0.18 to 2.86±0.29 respectively, while Simpsons-Reciprocal Index ranged from 32.25±12.74 (Wet-Season) to 40.10±15.84 (Dry-season) and exhibited seasonal variation at P<0.05. The abundance and diversity of planktons in this study is an indicator of eutrophic ecosystem that is possibly unstressed.
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