Use of bioindicators and biomarkers to assess aquatic environmental contamination in wetlands of Lake Tana, Ethiopia

Abstract

Anthropogenic pressures on Ethiopian freshwater ecosystems are intensifying due to rapid population growth, industrialization, agriculture, urbanization, and sewage discharge, heightening the need for water quality and biodiversity monitoring. This study employed an integrated ecological approach across six wetlands and four seasons of Lake Tana, combining physicochemical analyses, macroinvertebrate/fish bioindicators, fish histopathology, and health indices to assess ecological health. Seasonality strongly influenced trophic conditions and physicochemical properties. Spatial pollution gradients emerged: highest at Megech River Mouth, moderate at Avaj and Ras Abbay, and lowest at Wonjeta, Zewdie Girar, and Gumara River Mouth. Macroinvertebrate community structure reflected these gradients, aligning with nutrient and water quality shifts. Discrepancies were detected between Carlson’s Trophic State Index (TSI) and the Water Quality Index (WQI) in wetlands dominated by inorganic turbidity rather than algal biomass. While Carlson’s TSI (prioritizing chlorophyll-a, nutrients, and transparency) misclassified some sites as eutrophic, the WQI, integrating broader physicochemical parameters, more accurately reflected ecosystem conditions, highlighting limitations of nutrient-centric indices in sediment-driven turbid systems. Bioindicator responses were predictable: pollution-sensitive macroinvertebrates thrived in cleaner wetlands (e.g., Zewdie Girar), while tolerant taxa dominated degraded areas. Diversity indices confirmed higher ecological stability at Zewdie Girar, Avaj, Wonjeta, and Ras Abbay. Fish assemblages similarly reflected water quality: Oreochromis niloticus and Labeobarbus spp. prevailed in less polluted zones, whereas Clarias gariepinus dominated impacted sites. Fishing pressure reduced diversity at Wonjeta, Avaj, and Ras Abbay, while undisturbed Megech, Zewdie Girar, and Gumara River Mouths maintained richer communities. Histopathology of Labeobarbus spp. revealed pollution-linked tissue damage most severe at Megech and Gumara River Mouths, moderate at Ras Abbay, Wonjeta, and Avaj, and minimal at Zewdie Girar. Fish health indices corroborated these stress gradients. This work underscores the value of incorporating seasonal dynamics and biological indicators particularly macroinvertebrate and fish health assessments for holistic freshwater monitoring. The integrated approach provides critical insights for sustainably managing Ethiopia’s aquatic resources.