Spatially Uniform Plankton Assemblages and A Temperature Linked Zooplankton Gradient in a Tidal Peri-Urban River of The Niger Delta: The Egbelu Reach of The New Calabar River, Nigeria

Plankton respond quickly to changes in water quality and are widely used to assess rivers and estuaries. Even so, few Niger Delta studies have related plankton structure to environmental variables measured at the same places and times. We sampled phytoplankton and zooplankton at three stations on the Egbelu reach of the New Calabar River, Rivers State, Nigeria, every month from May to July 2026, and measured eleven physico-chemical variables alongside them. Salinity (0.45–1.88 ppt), conductivity and total dissolved solids differed among stations, while temperature (27.5–28.0 °C) and pH rose over the three months. The phytoplankton count came to 1,171 individuals in 19 taxa from four classes. Bacillariophyceae dominated (68.1%), followed by Cyanophyceae (23.4%), Chlorophyceae (6.3%) and Euglenophyceae (2.1%). Zooplankton numbered 1,124 individuals in 14 taxa: Copepoda (42.6%; five calanoid morphotypes), Cladocera (22.4%), Rotifera (20.2%) and tintinnid Protozoa (14.8%). Both assemblages were diverse and very even (phytoplankton H′ = 2.83–2.85, J′ = 0.96–0.97; zooplankton H′ = 2.60–2.61, J′ = 0.98–0.99), and Bray–Curtis similarity among stations was above 88%. For both groups, cluster analysis set Station 1 apart from Stations 2 and 3. In canonical correspondence analysis, temperature defined the first axis for both groups, but the model was significant only for zooplankton (52.8% of inertia constrained; p = 0.044), not for phytoplankton (p = 0.299). In the early rainy season, plankton composition in this reach followed temperature and sampling month more closely than it followed the spatial differences in water chemistry.