A systematic review of freshwater microplastics in water and sediments: Recommendations for harmonisation to enhance future study comparisons

Abstract

Microplastics are contaminants of emerging concern that have been widely detected in aquatic environments over the past decades. While in recent years an increasing number of studies has been published on microplastics in freshwater, there have been questions regarding inconsistency of the results due to large variations in methods and analytical techniques used in the literature. In this study, we adopted a systematic quantitative literature review approach to critically analyse 183 publications reporting microplastics in freshwater ecosystems. We also analysed the current approaches used for sampling, processing and analysis of microplastics in water and sediment, and the findings related to forms, polymer types, colours and sizes. Microplastics are frequently detected in various freshwater ecosystems across six continents, including lakes, rivers, estuaries, and wetlands. The reported microplastic concentrations span eight orders of magnitude in freshwater (1.2 × 10−3 to 5.42 × 105 particles/m3) and six orders of magnitude in sediments (8.1 × 10−1 to 9.5 × 105 particles/kg). Various forms, polymer types, colours and sizes of microplastics have been reported in water and sediment However, the heterogeneity in filter sizes, sampling and sample processing methodologies, and units of reporting between different studies make it difficult to bring together all studies and produce a comprehensive picture of the current state of knowledge. The results also highlight a lack of standardised quality assurance and quality control (QA/QC) protocols, which further makes meaningful assessment of the current literature difficult. We recommend using a consistent language for reporting microplastic concentrations and forms as well as providing more detailed information about sizes and chemical composition of microplastics to facilitate comparisons between studies. In addition, a comprehensive QA/QC protocol needs to be implemented during MP monitoring, especially, negative (field and laboratory) and positive controls, to produce reliable results.