Ecological Assessment of Estuarine Habitat Quality Using Multiple Bioindicator Groups in Ondo State, South South Nigeria

Estuarine systems in Ondo State are under pressure from urban waste discharge, artisanal fisheries, oil pollution, and mangrove clearing, yet their ecological status has rarely been assessed using standardised multimetric approaches. This study conducted an ecological quality assessment of three estuaries in Ondo State using multiple bioindicator groups including benthic macroinvertebrates, phytoplankton, and fish assemblages. Biotic samples were collected at the same time at twelve stations across the three estuaries during wet and dry seasons. Physico-chemical water quality was simultaneously measured. Multimetric indices were developed for each bioindicator group and combined into a composite ecological quality score. Estuarine ecological quality was classified into five status categories following an adapted framework based on European Water Framework Directive principles. Results showed that two estuaries achieved only poor or moderate ecological status, driven by high concentrations of petroleum hydrocarbons and poor dissolved oxygen conditions in lower reaches. The most remote estuary attained good ecological status with diverse benthic and fish communities. Bioindicator groups showed broadly consistent spatial patterns but differed in sensitivity, with benthic macroinvertebrates detecting organic pollution most sensitively and fish assemblages reflecting overall habitat quality at broader scales. The study establishes a baseline multimetric assessment framework applicable to other Nigerian estuaries and recommends routine quarterly bioindicator monitoring to track ecological quality trends in Ondo State coastal waters. Keywords: estuarine ecology, bioindicators, multimetric index, Ondo State, ecological quality.