First investigation of micro-plastics pollution in the River Nile in Khartoum, Sudan: using Nile Tilapia (Oreochromis niloticus) as a bio-indicator

Abstract

Microplastics (MPs) have become the bane of the environment for decades, and their impact on freshwater systems is still understudied. In addition to the inevitable and potentially irreversible imbalances in ecosystems caused by particulate matter pollution, environmental standards and coordinated actions are limited by the lack of knowledge about their abundance, distribution, and fate in the ecosystem. This project aimed to generate new data on the occurrence and characteristics of MPs in the River Nile in Khartoum, Sudan, using Nile Tilapia (Oreochromis niloticus) as a bioindicator. The study also aimed to assess the contribution of anthropogenic activities to MP pollution in the River Nile. To achieve this, thirty Nile Tilapia fish samples (freshly caught) were bought from Al-Mawrada fish market. Samples were frozen and transferred to the MPs laboratory at the University of the Witwatersrand, Johannesburg, South Africa. The digestive tracts were removed and individually digested with 10% potassium hydroxide (KOH) to degrade biogenic and natural organic matter that could interfere with MP identification. MP particles were separated according to their density using Sodium iodide (NaI) as a density separator. Nikon light microscope (stereomicroscope) was used to detect and identify MPs based on size, shape, and colour. The surface morphology of MP particles was examined by scanning electron microscope (SEM), and Raman spectroscopy was used to identify the polymer types. The weight of the fish ranged from 0.11 to 1.39 kg with an average of 0.48 ± 0.38 kg. Two-thirds of the fish samples (twenty fish) were identified as males, leaving ten female samples. The values of condition factor (K), except for one sample, were greater than 1, implying that the fish were in good health. A total of 567 particles were recovered from the digestive tracts of the Nile Tilapia, ranging from 5 to 45 MPs/individual fish with an average intensity of 18.90 ± 9.17 MPs/individual. The average abundance of MPs in the fish samples was 72.07 ± 62.06 particles/kg. The examination of the physical characteristics of MPs revealed a predominance of fibers (85.19%), small-sized particles of less than 0.5 mm (30.51%), and coloured MPs (83.60%). However, MPs of other shapes and sizes were also present in considerable percentages, for instance, 25.93% of MPs were detected in the size range of 0.5 - 1 mm, and 27.69% were in the range of 1 - 2 mm. In terms of shape, fragments accounted for 9.52% of the total MPs, while other shapes were present in very small quantities. The chemical characterization of MPs in this study showed two types of polymers, polyethylene (PE) and polypropylene (PP). The high prevalence of fibers and fragments indicates that MP contamination in the River Nile is from secondary sources. These shapes are formed by the fragmentation of macro-plastics. Their potential sources include domestic and industrial effluent, recreational activities, and surface run off from urban centres. This study is the first investigation to assess MPs in the River Nile in Sudan. It provides the first data on their presence and bioavailability, thus providing a benchmark for guiding future monitoring studies and identifying research gaps.