Cyanobacteria abundance and its relationship to water quality in the Mid-Cross River floodplain, Nigeria
Okogwu Okechukwu I.1 & Ugwumba Alex O.2
Cyanobacteria abundance was fairly high between seasons and stations. This may be attributed to the presence of still waters in the several ponds and lakes within the Cross River floodplain with conditions conducive for the proliferation of these plankton groups. Still blackwater was suggested by del Giorgio et al. (1992) to be the likely sources of cyanobacteria bloom in rivers. In line with this, cyanobacteria abundance was remarkably higher in the lakes than in the open water. High cyanobacteria abundance in the lake was mainly attributed to crustacean grazing activities, which depleted the smaller edible algae to the advantage of the cyanobacteria (Okogwu 2008). Cyanobacteria are known to be less palatable and less attractive to zooplankton (especially cladocerans), so they receive little grazing attention (Relevante and Gilmartin 1982, Repka 1996). In cladoceran dominated lakes, cyanobacteria are known to have high densities as the grazing activities of this group of zooplankton effectively eliminate other competing alga from the phytoplankton community. This may explain the higher density of cyanobacteria recorded during the wet season compared to the dry season in most of the stations as cladoceran density attained peak in these lakes during the wet season (Okogwu 2008). The density of green algae and diatom was reported to be very low during this period (wet season) (Okogwu 2008). Grazing zooplankton remove their natural competitors (small alga) releasing nutrients to them (Repka 1996). However cyanobacteria are generally harmful to zooplankton by clogging their feeding apparatus, toxin production and poor nutritional quality of the cells (DeMott et al. 2001, Sterner and Elser 2002, Jang et al. 2003). Therefore, the cyanobacteria abundance recorded in the floodplains of the Cross River should be of concern as these are the breeding grounds of the important fishes of the river notably Chrysichthys nigroditatus, Clarias gariepinus Oreochromis niloticus and Tilapia zilli during the wet season. Fish larvae exposed to cyanobacterial toxin showed reduced feeding and growth rate (Baganz et al. 1998).