EFFECTS OF SAND DREDGING ON PHYTOPLANKTON DIVERSITY AND TROPHIC STRUCTURE OF THE OBIOKPOK NSIT RIVER, NIGER DELTA, NIGERIA

Abstract

The study assessed the influence of sand dredging on phytoplankton diversity and water quality in the Obiokpok Nsit River, Akwa Ibom State, Nigeria. Monthly sampling was conducted from August 2023 to January 2024 across three stations representing different dredging intensities: upstream (undredged), midstream (moderately dredged), and downstream (intensively dredged). Physicochemical parameters were measured using standard methods, while phytoplankton samples were collected using plankton nets and identified microscopically. Significant spatial variations (p < 0.05) were observed in several environmental parameters, with dredged stations showing reduced transparency and dissolved oxygen but elevated turbidity, total suspended solids, and phosphate concentrations. A total of 1,684 phytoplankton individuals belonging to five taxonomic classes were recorded, with Cyanophyceae dominating the assemblage (83.14%). Taxonomic richness was highest at the upstream station (20 taxa) and lowest at the moderately dredged station (15 taxa). Canonical correspondence analysis indicated that nutrient enrichment and suspended solids were the major environmental drivers influencing phytoplankton distribution. The results demonstrate that sand dredging alters water quality conditions and shifts phytoplankton community structure toward disturbance tolerant taxa, potentially affecting the trophic integrity of the river ecosystem. These findings highlight the need for effective regulation and environmental monitoring of dredging activities in tropical freshwater systems.