Hydrology-oxygen and detrital enrichment drive oligochaetes turnover in tropical lowland rivers in the Niger Delta under abattoir effluent pressure

Tropical lowland rivers in rapidly urbanizing regions face concurrent hydrological alteration and organic loading, yet management-ready indicators that resolve both spatial and seasonal impacts remain limited. Oligochaetes, sediment-associated, pollution-tolerant microdriles offer a sensitive lens for diagnosing such pressures. We assessed oligochaete bioindicators across five western Niger Delta rivers (Benin, Ikpoba, Obosh, Orogodo, Umaluku) over one year to test how oxygenation, hydrodynamics, and detrital enrichment structure communities across effluent gradients and seasons. Station-level profiling showed enrichment peaks (TOC/DOC, BOD, CPOM) and depressed DO/flow at effluent sites (S2), mirrored by numerical dominance of Tubifex tubifex and strong contributions from naidids ( Nais communis, Chaetogaster spp., Dero spp.). Wet-season abundance increased markedly across rivers (Kruskal–Wallis H = 41.78, p < 1 × 10⁻⁹; Dunn z = 5.64, p < 2 × 10⁻⁸), and community composition shifted significantly between Wet and Dry seasons (PERMANOVA p = 0.005). Constrained ordination (Hellinger–RDA) resolved two opposing, system-wide controls, a DO/flow axis and a CPOM/DOM enrichment axis with pH acting secondarily; the first two canonical axes explained 16.7% of total variance and 20.43% of constrained variance, and the Tubifex centroid aligned with the enrichment vector. Together, these results show that hydrology mediated organic enrichment amplified at abattoir inputs and drives both the numerical dominance of tolerant taxa and the compositional turnover observed across space and season, providing actionable guidance for slaughterhouse wastewater treatment, hotspot remediation, and routine biomonitoring that pairs oligochaete metrics with core water-quality indicators.