Taxonomic Composition and Diversity of Phytoplankton Community in Kpassa Reservoir (Northen Benin)

Aims: The present study was conducted to determine the taxonomic composition and the diversity of phytoplankton community in Kpassa reservoir.

Place and Duration of Study: The investigation occurred in Kpassa reservoir at 6 stations from August 2014 to May 2016.

Methodology: Microscopic observations and identification keys allowed to index phytoplankton taxa. Diversity was evaluated using species richness index (d), Shannon-Weiner index (Hs) and species equitability index (J). Means ± standard deviation, ANOVA, Kruskal-Wallis, Kolmogorov-Smirnov and Levene test were performed using SPSS 16.0.

Results: Current study recorded 52 phytoplankton taxa grouped as Conjugatophyceae (18 taxa), Diatomophyceae (16 taxa), Euglenophyceae (8 taxa), Cyanophyceae (5 taxa), Chlorophyceae (3 taxa), Dinophyceae (1 taxa) and Xanthophyceae (1 taxa). The highest abundance of phytoplankton (25828 ± 4642 individuals/mL) was recorded in January 2016 (dry season) with the arrival of the cold wind called Harmattan. High values of species richness index ranged from 1,71 to 2,67 at stations St1 and St2 while high values of diversity index varied between 2,16 and 2,98 bits/individuals at stations St1 and St6 and high values of equitability index ranged from 0,50 to 0,64 at stations St1 and St5 during rainy season. . Differences in the phytoplankton abundance, species richness index, Shannon-Weiner index and species equitability index are significant between seasons (p = .00; p = .00; p = .00; p = .00) but the differences between sampling sites are not significant (p = .76; p = .98; p = .97; p = .96).

Conclusion: Diatomophyceae predominance suggests organic pollution and high enriched water of the reservoir. Phytoplankton abundance is influenced by temperature but further work is needed to confirm the link. Same to silica which was also find to be on the basis of high phytoplankton abundance. Phytoplankton abundance is likely directly dependent of Diatomophyceae abundance. Rainy season represents the stability period of the phytoplankton community in Kpassa reservoir. Station St1 close to raw water pumping point of the National Company of Benin Water is the most stable site. Low values of species richness indicate a degraded water quality of the reservoir. Low values of diversity and evenness index reveal also a eutrophic state of the reservoir. Agricultural, pastorals, washing and crockery activities undertaken in the reservoir basin may be the principal cause of the deterioration of the reservoir water quality. The presence of aquatic plants was also found to induce organic pollution. Kpassa reservoir is horizontally homogene. A couple of sites are sufficient to horizontally monitor the reservoir status.