Variation in macroinvertebrate assemblages and water quality as a test for different levels of ecological impairment across an Ethiopian highland river system

Abstract

The Ethiopian highlands is an area under high anthropogenic pressure. Despite being a hotspot for freshwater biodiversity, there is no regular monitoring of the ecological status of local rivers. Physicochemical and nutrient water-quality parameters and macroinvertebrate assemblages were sampled monthly from the Megech River and its tributaries from January to March 2021. Sampling locations were designated as being either upstream or downstream of the Gondar City urban area. We tested for temporal (monthly) and spatial (location, river) variation in water quality and spatial (location, river) variation in macroinvertebrate assemblage structure, and examined which water quality parameters were most associated with patterns in macroinvertebrate assemblages. Water quality varied significantly between locations and months, but not between rivers. Water quality differences were particularly driven by higher nutrient, salinity, dissolved solids and conductivity levels downstream, and higher pH and dissolved oxygen levels upstream. Despite macroinvertebrate assemblages being more abundant and diverse downstream, biotic index scores generally indicated slightly increased ecological impairment downstream. There was a clear spatial pattern in macroinvertebrate assemblages between locations. We detected a significant difference between rivers, with pairwise differences between sampling locations, particularly due to high dissimilarity between the upstream and downstream locations of the Angereb, Shinta and Qeha rivers. BIO-ENV Pearson correlations indicated that variation in macroinvertebrate assemblages were mostly explained by dissolved oxygen, salinity and nutrient parameters including nitrate, ammonium, and sulphate. Our findings confirm the need for more rigorous monitoring of Ethiopian highland rivers and improved management of nutrient inputs.