An investigation into the occurrence, distribution, and composition of particulate plastics in water and sediment, and their effect on the health of oreochromis mossambicus from the Olifants River System

Abstract

Microplastics (MPs), particularly polyethylene terephthalate (PET), have emerged as contaminants of concern in aquatic ecosystems, yet their impact on freshwater environments and organisms is limited, especially in South Africa. While most research has focused on marine environments, limited attention has been given to rivers, dams, and other inland freshwater systems. This study aimed to address this gap by evaluating both the bioaccumulation and health effects of MPs using the Mozambique tilapia (Oreochromis mossambicus) as a freshwater fish model, using in vivo experiments and field assessments. Laboratory trials consisted of two experiments; with the first investigating the accumulation of PET MPs of varying size in different fish tissues, while the second examined the health effects and survival under varying exposure concentrations. In the first trial, the O. mossambicus specimens 5 – 11 cm in total length were exposed to fluorescent PET particulates, < 45 μm and 45 – 80 μm in size, at a concentration of 20 mg/L over seven days. Tissue samples (gills, brain, gut, liver, muscle) were collected at intervals of 4, 12, 24, 48, and 168 hours to measure MP accumulation via UV-vis spectrophotometry. High concentrations of PET MPs were detected in the liver, followed by those recorded in the gut, gills, brain, and muscle tissue respectively. In the second trial, specimens 9 – 16 cm in length were exposed to non-fluorescent PET MPs at varying concentrations (0.00, 0.02, 0.20, and 2.00 mg/L) for 21 days. The health of fish was assessed using the Health Assessment Index (HAI) protocol and histopathology. A higher mortality (75%) and more severe histopathological lesions were observed for fish exposed to 45 – 80 μm particulates, at a concentration of 2.00 mg/L, with smaller particles causing less harm. Mucous hypersecretion was observed in the gills and severe signs of necrosis in the liver in fish exposed to concentrations ≤ 0.20 mg/L. Fieldwork was conducted at three waterbodies along the Olifants River System (ORS) at sites in the upper (Loskop Dam), middle (Flag Boshielo Dam), and lower reaches (Phalaborwa Barrage) of the river to assess how variations in water and metal content influence MP levels in the environment and in O. mossambicus. Seasonal surveys were undertaken during a period of high river flow (survey 1 during February/March 2023 ) and low river flow (survey 2 in June 2023), whereby samples of water and sediment were collected and analysed for metal and MPs content. Fish were collected using gill nets, and their health examined at the makeshift field laboratory following the HAI protocol. Blood and liver biomarkers such as alanine transaminase (ALT), aspartate transaminase (AST) and alkaline phosphatase (ALP) in combination to histopathology analyses, were analysed to determine how fish from the three impoundments responded to MPs detected in water, sediment and different body tissues of fish. Microplastics were reported in water, sediment, and fish tissues samples collected at all sites. Metal levels in water and sediment were observed to be higher during survey 2, with Phalaborwa Barrage exhibiting the highest levels. High MPs was recorded in sediment collected from Flag Boshielo Dam during survey 1, and from Phalaborwa Barrage during survey 2. From the three sites, a total of 67 O. mossambicus specimens were analysed, and 312 MPs extracted from the various tissues sampled, with values ranging from 0.88 particles/g in the liver Loskop Dam and 4.14 particles/g in the gill tissues of fish sampled from Phalaborwa Barrage. Among the various MP types, only fibres were observed from water and sediment samples collected, consistent with secondary MPs sources such as household textiles. Based on these findings, the gills, gut, and brain were sensitive to short-term changes in water quality, while the liver and muscle reflected long-term metal retention. Overall, Phalaborwa Barrage was the most impacted site, showing the strongest links between tissue and metal concentrations. Fish from Phalaborwa Barrage showed the highest HAI scores, indicating the fish at this site to be in poor health when compared to other sites. Similarly, blood and liver analyses of O. mossambicus reflected site- and season-specific physiological stress associated with metal and MP content. Elevated WBC counts and reduced Hct values, particularly during low flow, indicated immune activation and anaemia associated with MP and metal exposure. Variations in total serum proteins, A/G ratios, and liver enzyme activities further suggested metabolic disruption and sublethal stress. Findings when using CCA model revealed weak associations. This study is among the first to document MPs accumulation and associated health impacts in O. mossambicus in South Africa. The results highlight the vulnerability of this economically and ecologically important species to MPs pollution. Of the methods used to assess fish health, the determination of liver biomarkers such AST and ALT in conjunction with the A/G ratio, proved to be key indicators. In this study the liver consistently showed the highest MP accumulation of all tissues examined, highlighting its direct exposure to pollutants. Similarly, elevated AST and ALT activities, together with altered A/G ratios, reflected sublethal metabolic stress and disruptions in immune and protein homeostasis, rather than overt hepatic failure. Histopathological alterations such as necrosis and inflammation further confirmed liver vulnerability to MPs, making these biomarkers reliable indicators of MP-induced toxicity. Collectively, the liver’s metabolic sensitivity and its role as a primary site for MP accumulation underscore its importance as a sentinel organ for assessing fish health under environmental stress. The study highlighted the need for continuous monitoring of MPs and metal pollution in the ORS, particularly during dry seasons when accumulation is more pronounced. Findings also suggest that regular health assessments of freshwater fish should be integrated into water quality monitoring programmes. Ultimately, these findings call for increased awareness in terms of research and policy interventions to address MPs pollution in South African inland waters to safeguard aquatic biodiversity and to minimise the health risks to humans.