Influence of Nutrient Enrichment and Temperature on Aquatic Invertebrate Communities in a Tropical Riverine System in Kenya

ABSTRACT: One of the major concerns in freshwater ecosystems is nutrient enrichment from human sources, which causes significant effects on aquatic biodiversity and ecological functioning. This study, therefore, investigated the order and family levels of Odonata, Diptera, and Coleoptera responses to nutrient enrichment along River Kiminini, Trans-Nzoia County, Kenya. Field sampling was conducted from July to September 2023. Duplicate water samples were taken from ten sampling stations labelled (S1–S10), twice a month for three months. The concentrations of nitrate–nitrogen and phosphate–phosphorus were measured calorimetrically using standard procedures; the Ascorbic Acid method for phosphates–phosphorus and the Brucine method for nitrates–nitrogen. A spectrophotometer was used to measure absorbance. A standard kick net was used at each station to collect the macroinvertebrates (Odonata, Diptera, and Coleoptera). Using an updated taxonomic key, the macroinvertebrate samples were sorted and identified to their family level under a dissecting microscope, and abundance counts were made. Direct field observations were used to record human activity at each station. The R programming language was used to conduct all statistical analyses. The water quality status for each station was determined using the Nutrient Pollution Index, which was calculated based on ecologically relevant nutrient thresholds. The Nutrients Pollution Index scores ranged from 9.66 (S5) to 17.2 (S3). All the stations were greater than 6, indicating very high pollution levels and a significant risk of eutrophication. At the family level, Dytiscidae, Coenagrionidae, and Lestidae were abundant in less polluted stations and fewer in highly polluted stations. For instance, Coenagrionidae had the highest numbers in station S1 (32) and declined drastically in the highly polluted station S9 (3). The same trend was observed for Lestidae, with the highest numbers in stations S1 and S2 (21) and declining in station S9 (1). Dytiscidae also had higher numbers in the relatively less polluted station S7 (10) and lowest in the highly polluted station S3 (1). On the other hand, Chironomidae was observed to be tolerant to organic pollution. Chironomidae had their highest numbers in station S9 (30) and declined in moderately cleaner stations S5, S6, and S8, which had a similar record of 6 organisms. According to the findings, it is necessary to enhance biomonitoring and guide riverine management plans under nutrient stress and increasing water temperatures by incorporating functional and taxonomic diversity into water quality assessments.