Background:
Waterfalls generate strong spatial heterogeneity in hydrological and physicochemical conditions, affecting oxygen dynamics, carbon fluxes, nutrients, and aquatic biota. Studies report pronounced gradients in water chemistry, microbial contamination, and biological assemblages, especially in tropical regions. In Nigeria, increasing anthropogenic pressures contrast with limited limnological data. Insufficient integration of water quality and algal indicators hampers understanding of ecosystem functioning and disturbance gradients, necessitating targeted limnological assessments to support biodiversity conservation and sustainable management of waterfall ecosystems.

Objectives:
This study aims to conduct a comprehensive limnological and algal assessment of Erin Ijesha Falls, Nigeria. It hypothesizes that spatial variations in physicochemical parameters create distinct water quality gradients that structure algal communities, and that the Palmer Index can reveal anthropogenic organic pollution not detected by conventional physicochemical indicators.

Methods:
Sampling was conducted during the dry season of 2024 across upper, middle, and lower sections of seven waterfall cascades. Integrated water and periphytic algal samples were collected in triplicate following ISO, APHA, and Nigerian standards. In situ measurements included temperature, pH, conductivity, and dissolved oxygen, while nutrients, major ions, and hardness were analysed in the laboratory. Algae were identified microscopically using standard taxonomic keys, and water quality was evaluated using the Palmer pollution index to assess organic contamination.

Results:
Air and water temperatures at Erin-Ijesha Waterfall showed clear diurnal warming, with air rising from 20 to 33°C and water from 19 to 26.5°C. Turbidity and colour were low to moderate (6–13 NTU; 1–4 PtCoU), while conductivity, TDS, pH, and alkalinity remained low and stable. Dissolved oxygen was high (8–10.5 mg L⁻¹), and nutrients and hardness were generally low, reflecting a well-oxygenated, soft, and minimally impacted freshwater system. The algal community at Erin-Ijesha Waterfall comprised 78 taxa across four divisions, dominated by Chlorophyta (42.3%) and Bacillariophyta (38.5%). Green algae and diatoms indicate generally good water quality, habitat heterogeneity, and moderate nutrient availability, while the Palmer Index (19) suggests slight organic enrichment without severe pollution.

Conclusion:
Spatial variation in physicochemical parameters at Erin Ijesha Falls defines a distinct water quality gradient. Algal communities, dominated by Chlorophyta and Bacillariophyta, effectively track hydrochemical conditions and reveal moderate organic enrichment undetected by physicochemical measures. Integrated indicators indicate moderate disturbance, ecological resilience, and the need for regular monitoring.