Long-Term Changes in Thecomposition, Abundance and Diversity of Phytoplankton Community of Oshika Rivers State

This study was aimed at determining the long-term changes in composition, abundance, and diversity of the phytoplankton community of the Oshika freshwater biota. Phytoplankton samples at Oshika lake were collected from the same stations and similar months as in the year 1983 study of the 5000 barrels of oil spill. The samples were handled with optimum care and treated according to standard practices in order to preserve the plankton for further analysis and identification. The results of the analysis showed that the phytoplankton yielded a total of 40 species belonging to three groups in 1983 and 36 species belonging to four groups in 2019. Xanthophyceae was the group absent in 1983 but present in 2019, while Chlorophyceae, Cyanophyceae and Bacilariophyceae were common to both 1983 and 2019. Generally, the abundance of phytoplankton in each group in 1983 was significantly different from those of 2019. The Chlorophyceae was the most abundant in both 1983(12.31-186.5) and 2019 (1.27-6.58). The diversity indices result showed that the phytoplankton in 1983 (2.123- 2.563) had lower diversity compared to 2019 (2.381- 2.815). The cluster analysis revealed two groups in 1983 viz the generalist and the dry season specialist, while three groups viz the dry season specialist, wet season specialist and the generalist were observed in 2019. More so, greater data variations were observed in 1983 than 2019. Hence, we infer that there were changes in the phytoplankton community structure between 1983 and 2019. There were changes in the biological community structure between 1983 and 2019. These changes cannot be linked to the oil spill but seems more likely a result of natural variation since results of studies on the hydrocarbon values in water were very insignificant. Since the lake-riverine system is subject to flooding it can be concluded that new plankton from unaffected areas are transported into this area during the flood. The mixing of the water over the years leads to the high variable change. The high variable change brings about high species turnover given the unrestricted hydrodynamics of the River Niger flooding system and its influence on the floodplain Oshika lake.