Spatio-temporal Dynamics of Physico-Chemical Hydrology and Aquatic Insect Assemblages in a Western Nigerian River: A Canonical Correspondence Approach to Ecological Integrity

Technical Summary: Spatio-temporal Dynamics of Physico-Chemical Hydrology and Aquatic Insect Assemblages in a Western Nigerian River

Problem Statement
Freshwater ecosystems in West Africa, particularly within the agrarian zones of Western Nigeria, face increasing anthropogenic pressures from urbanization, agricultural expansion, and land-use changes. These stressors, compounded by the region's distinct bimodal tropical climate, create complex, dynamic environmental matrices that challenge the ecological integrity of lotic systems. Traditional water quality monitoring, which relies on isolated chemical snapshots, is often insufficient to capture transient pollution events or the integrated long-term impacts of habitat alteration. Consequently, there is a critical research gap in understanding how localized agricultural pressures interact with extreme seasonal flow changes to reshape the niche space of aquatic insect assemblages in the Omi River (Iwajowa Local Government Area, Oyo State). Specifically, there is a lack of data mapping the direct, multivariate relationships between community structure and environmental variables using constrained ordination techniques in this region.

Methodology
This study employed a spatio-temporal survey design to evaluate the interconnected dynamics of water physico-chemistry and aquatic insect assemblages in the Omi River.

• Study Area: Five sampling stations were established along the river stretch, covering distinct microhabitats within the Iwajowa LGA.
• Sampling Protocol: Data collection occurred over an annual cycle (December 2024 to September 2025) to capture both wet and dry seasons. Sampling was conducted tri-monthly.
• Physico-Chemical Analysis: Hydrological profiling (width, depth, velocity, discharge) and in-situ parameters (temperature, pH, dissolved oxygen, electrical conductivity, total dissolved solids, turbidity) were measured. Water samples were collected for laboratory analysis of biochemical oxygen demand (BOD), nutrients (nitrate, phosphate), and ionic profiles (calcium, magnesium, hardness).
• Biological Sampling: Aquatic insects were collected using D-frame nets and hand-picking methods. Specimens were preserved in 70% ethanol, sorted, and identified to the lowest possible taxonomic level using standard dichotomous keys.
• Statistical Analysis: Spatial and temporal variations were assessed using one-way ANOVA and independent-samples t-tests. Community structure was evaluated using diversity indices (Shannon-Wiener, Simpson, Pielou's evenness, Margalef's richness) and biotic integrity scores (BMWP-SA, ASPT). Canonical Correspondence Analysis (CCA) was utilized to model the relationship between environmental matrices and insect distribution, with significance validated via Monte Carlo permutation tests.

Key Results

• Taxonomic Composition: A total of 1,040 individuals were cataloged, representing 6 orders, 19 families, and 44 genera. The assemblage was dominated by Rhagovelia sp. (24.04%), followed by Chlorocypha sp. (24.04% relative to the total count in Table 3, though the text highlights Rhagovelia as the dominant taxon). Sensitive taxa, including Ephemeroptera (Elassoneuria sp.) and Plecoptera (Neoperla sp.), were present, while pollution-tolerant taxa like Chironomus sp. were scarce.
• Spatial Dynamics: The river exhibited high environmental uniformity across the five stations. Most physico-chemical parameters showed no significant spatial variation. The exception was Biochemical Oxygen Demand (BOD), which was significantly higher at Station 5 ($p < 0.05$), indicating mild downstream organic enrichment.
• Seasonal Dynamics: The bimodal climate drove sharp seasonal fluctuations.
  • Wet Season: Characterized by significant increases in river width, depth, discharge, turbidity, and dissolved oxygen ($p < 0.001$). Total Dissolved Solids (TDS) and Electrical Conductivity (EC) were significantly lower due to dilution.
  • Dry Season: Marked by significantly higher TDS and EC ($p < 0.001$) due to concentration effects. Total abundance and diversity metrics (Shannon-Wiener, Simpson, Margalef) peaked during the stable dry season. Conversely, the number of EPT (Ephemeroptera, Plecoptera, Trichoptera) individuals surged during the wet season, likely due to increased aeration.
• CCA Findings: The Canonical Correspondence Analysis indicated that hydromorphological attributes (discharge, depth, stream width, turbidity) were the primary ecological forces structuring species distribution, outweighing nutrient gradients. However, the Monte Carlo permutation tests revealed that none of the canonical axes reached statistical significance ($p > 0.05$), suggesting that while trends exist, the environmental variables did not exert a strictly deterministic control over the distribution across the sampling grid.

Key Contributions

• Baseline Data: The study provides a crucial baseline framework for the ecological integrity of the Omi River, documenting a diverse macroinvertebrate community in a region often subjected to intensive agricultural activity.
• Methodological Application: It demonstrates the application of Canonical Correspondence Analysis (CCA) to map the complex interactions between hydrological variables and aquatic insect assemblages in a West African tropical river, addressing a gap in regional conservation frameworks.
• Ecological Insight: The research highlights that hydromorphological stability and seasonal flow regimes are more critical drivers of community structure than nutrient gradients in this specific system. It confirms that the river maintains high biological integrity, evidenced by the presence of pollution-sensitive taxa and favorable BOD/DO ratios.

Significance
The paper concludes that the Omi River retains high biological integrity at the time of the study. The presence of sensitive taxa and the dominance of predatory and semi-aquatic insects indicate a balanced aquatic community with active trophic interactions. The study asserts that the river's self-purification capacity is sufficient to handle localized organic loading, and that seasonal hydrological variations act as natural physical filters that redistribute communities without degrading water quality. These findings offer a statistically validated framework to guide targeted river restoration, water policy formulation, and biodiversity conservation efforts in West African lotic ecosystems, emphasizing the need to integrate habitat assessment with water quality monitoring.