Microplastics Occurrence and their Potential Toxicity in Aquatic Environment

Abstract

The aims of this study were to determine the microplastic abundance in nearshore and offshore surface water of Rawal Lake, Pakistan, to ascertain if sampling methodology influences the microplastic counts and to investigate the toxic effects of microplastics on zooplanktons. The near shore surface water samples were collected from 7 sites in proximity of human settlements, tourist spots and recharging points, using grab and pump sampling techniques. Average microplastic abundance was 2.07 ± 1.12 and 0.004 ± 0.002 particles L-1 in grab and pump samples respectively. Both sampling methods showed highest microplastic abundance nearby human settlement. The majority of microplastics in both type of samples was white/transparent, blue and within the size range (1-2.8 mm). Polypropylene (PP) was the dominant polymer type in both type of samples as determined by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). Overall, pump sampling method collected more shapes (including foams), colors (including yellow), and small microplastics (0.1-0.9 mm) than grab sampling method. Microplastic pollution was investigated in offshore surface water of Rawal Lake samples by collecting samples from 10 sites; 5 tributaries, 2 human settlements and 3 fishing and boating areas using sampling techniques: 100 µm mesh trawl and 20 L sample through a 45 µm mesh sieve. A significant difference was observed in the abundance of microplastics across two methods with sieve method yielding 2.8 ± 1.44 and trawl yielding 0.025 ± 0.024 particles L-1. Tributaries and boating/fishing area had higher microplastic abundance than residential area. Filaments were the dominant shape of microplastics in both type of samples. Microbeads were only detected in trawl samples. Microplastics within size range 0.1-0.9 mm were mostly fragments (82%). White/transparent and black microplastics were common. ATR-FTIR analysis revealed that PP was the main type of microplastic in offshore surface water (40-74%). Scanning Electron Microscopy (SEM) of microplastics showed cracks, roughness and striations on particles. Energy Dispersive Spectroscopy (EDS) detected heavy metals (Fe, Cu, Ni, Pb, Zn, Co and Cr) in microplastics. Microplastics were detected in all samples of Rawal Lake drinking water treatment plant (DWTP). Microplastic abundance in raw water was 12.2 particles L-1 and fibers (42%), white/transparent microplastics (41%) and particles within size range 0.045-0.1 mm were common. Coagulation/sedimentation process removed 52% of the microplastics, mainly films/fragments and larger particles. Overall, DWTP reduced microplastic contamination by 74% however the concentration of fibers and smaller particles in treated water was > 50%. The presence of microplastics in drinking water may negatively affect human health. Microplastics ultimately end up in oceans where strong hydrodynamic conditions keep microplastics in suspension. Contrastingly, lab scale toxicity studies that are carried out under static condition does not represent real environment. This study addressed this research gap and studied toxicity of polystyrene (PS) microplastics to Artemia salina in both static and dynamic conditions. PS microplastics did not affect the survival of Artemia nauplii in static and dynamic exposure conditions however due to increased interaction with microplastics in dynamic condition, microplastics accumulated in gut. To investigate whether microplastics increase or decrease the bioavailability of co-contaminant (Zinc Zn), Artemia nauplii were exposed to 1) both PS and Zn ions and 2) leftover Zn ions after the removal of microplastics via filtration. Toxicity of co-contaminant (in terms of mortality) decreased from 75% in positive control to 44% when microplastics and Zn co occurred. Exposure to left over Zn ions (after removing microplastics via filtration) caused minimum mortality (35%). Furthermore, toxicity of pristine (Unwashed) and aged, clean (Biofilm-) or fouled (Biofilm+), PS microspheres (3 and 10 μm) was studied, using Washed particles as a reference material, on selective and continuous larval culture of Amphibalanus amphitrite. Exposure to 3 μm Unwashed and Biofilm+ particles for 24 h induced significant mortality (60 and 57% respectively) in stage II larvae. Stage II and VI nauplii showed greater uptake of 3 μm Biofilm- particles. Accumulative exposure to microplastics in continuous larval culture significantly affected naupliar survival, particularly of stage III and IV. Cumulative mortality was >70% after exposure to 3 μm Unwashed and 10 μm Biofilm+ particles. Unwashed particles with increasing concentration and aged particles with increasing size, delayed the development of nauplii to cyprids. The highest concentration of 3 μm Biofilm+ microspheres inhibited settlement and induced precocious metamorphosis in 9% of the cyprids.