The impacts of microplastics on aquatic ecosystems: state of the art, modelling and policy for UK rivers-with a River Thames case study

Written by: Jonas Adjasoo, WSPM student 2019-2020 cohort

On the 2nd June 2020, the Oxford Water Network ran a water quality webinar led by eminent scholars; Prof. Paul Whitehead, Dr Ana Castro-Castellon and Dr Jocelyne Hughes.  The webinar focused on sources of microplastics (MPs) and their size distributions, potential impacts on aquatic ecology, the lack and critical gap in standardization of MPs sampling and analysis, alternative strategies for MPs mitigation based on Integrated Catchments (INCA) model, and policy instruments.

We heard from Prof. Paul Whitehead that sources of MPs in the River Thames are derived from effluent discharges, vehicle tyres, fibres from textiles, agriculture (sludge and fertilizers applied to land), packaging from food products, cosmetics, pharmaceutical products and others. These MPs may then enter into the river through runoff and wind flow.

Dr Ana Castro-Castellon further described the size distributions and shapes of microplastics. Microplastics are solid synthetic particles which range in size from 1 µm to <5 mm of regular or irregular shape, are from primary or secondary manufacturing origin, and are insoluble in water. The shape of microplastics can be pellets, fibres, films, fragments, foams etc. In their research, six classes of microplastic were selected and compared to aquatic organisms: first class (≤ 5µm), second class (5 – ≤ 100µm), third class (100 – ≤ 350µm), fourth class (0.35 – ≤ 1mm), fifth class (1 – ≤ 2mm), and sixth class (2- 5mm).

These MPs act as pollutants and stressors disturbing the natural balancing nature of fresh water ecosystems. The impacts of microplastics on freshwater is interdependent on the microplastics’ physico-chemical characteristics, concentration in aquatic habitats, and the eco-physiology of aquatic organisms. Microplastic types such as polyamide, polystyrene, acrylamide and polyvinyl are in chemical composition when manufactured. The first three classes of MPs are hazardous for lower trophic levels and cells. Through time, the MPs endure fragmentation and recycling, potentially enhancing their ability to leach chemicals or serve as an adsorbent to attract pollutants to their surface. These MPs can also be colonized by biofilms, leading to the formation of aggregates. Due to the hydrodynamic forces of the waterbody and climatological conditions, MPs can be concentrated, re-suspended, or their sink rate can be changed.

The impact of microplastics also depends on organism feeding strategies; either direct (planktivorous – feeding on suspended organisms, or Benthivorous – bottom feeders) or indirect (trophic transfer through the food web to higher trophic levels). The impact of microplastics on an organism depends on the developmental stages of the organisms’s life cycle. For example, larval stages of aquatic invertebrates appear to be more sensitive to the damaging effects of microplastics. When compared to adult stages, they are unharmed but some species show morphological behavouiral changes. Experiments on tadpoles indicate biometric and morphological changes as a result of MPs exposure, with bioaccumulation accelerating mortality rate and inducing changes at cellular and molecular level. That being said, contradictory results from other research on the impact of microplastics in freshwater is evident. Differences have emerged between studies that compare acute toxicity experiments (organisms exposed to microplastics for 48-72 hours indicate no harmful impacts to the organism) to chronic toxicity experiments (organisms exposed to microplastics for 5 days or more resulting in morphological and physiological changes). Research also shows that benthic organisms are more affected by microplastics than planktic organisms. The research  suggests that microplastics affect life cycle and species sensitivity, which in turn might affect biodiversity and ecosystem fuctioning.

The webinar also revealed a lack of standardization of MPs sampling and analysis. It is important to recognise that the range of microplastics size collected in research affects the mean abundance of microplastics when reported. In order to obtain a good estimation of the quantity of microplastics that exist in freshwaters, Dr Ana Castro-Castellon posited that there is a need to harmonize sampling. Researchers must  perform fraction filtering when sampling to give a good representation of microplastics size collected. Also, processing the samples for analysis can potentially alter the microplastic chemical properties, mask the microplastics, and amplify the loss of microplastics in the research process, especially when oxidative acid and alkalise are used. This leads to under estimation of MPs abundance. The enzyme digestion process should be considered to avoid misidentification and underestimation of microplastics during analysis.

Moreover, the lack of methods used for characterization and quantification of microplastics is a challenging issue. Optical analysis can identify MPs shape and measurement, but cannot identify the elemental composition of MPs. The use of combined optical and spectroscopy analysis provides an area per unit volume as well as identification and quantification of MPs. Organic chemistry methods such as Gas Chromatography-Mass Spectrometry (GC-MS) provide more information, but are time consuming and expensive. Automated spectroscopy and image analysis seem to be more popular for MPs identification. FTIR (Fourier-Transform-Infrared-micro imaging-spectroscopy), coupled with Focal Plane Array (FPA-FTIR) and MP Hunter software avoids pre-sorting of MPs, and thereby provides data unbiased by the analyst. This allows the identification of MPs with lower size range.

Prof. Paul Whitehead modelled Microplastic in Thames Basin, UK, using the Integrated Catchment (INCA) model. The model was driven by daily climate data (e.g. rainfall, temperature), with inputs (e.g. effluents) via point sources and diffuse sources. The river system was modelled by dividing the Basin into 8 sub catchments; Eynsham, Oxford, Sutton Courtenay, Days Weir, Reading, Staines, Walton and Kingston upon Thames. The model is able to simulate daily fluxes of materials moving down the river. The model specifies the microplastic concentration, simulates the impact on the river and their movement down the river system. From the simulated model profile, microplastics build up down the river system, peaking at sewage treatment work discharge points. The MPs also drop as sediment onto the river bed. The model was used to investigate mitigation strategies; the best strategy mitigated microplastic loads at sewage treatment works by 50%. The model also estimates the build-up of plastics on the riverbed and the fluxes of microplastics moving down the river system into the Thames Estuary/North Sea.

To end the webinar, Dr Jocelyne Hughes, described the ‘source-pathway-receptor’ framework, to investigate policy, legislative and regulatory approaches to MPs mitigation in UK freshwaters. To mitigate MPs, there is a need: to enhance the circular economy; introduce further legislation and enforcement that focuses on plastics such as the ban on microplastic beads in cosmetics in 2018 and charge on single use plastic bags which has  reduced bag usage by 90%.The new  Environment Bill 2020 will serve as an opportunity to enforce regulation to eliminate avoidable plastic waste by 2042. In addition, the EU Plastic Strategy aims to reduce plastic products, for example, by collecting 77% of plastic bottles by 2025 and 90% by 2029 using deposit return schemes. This Plastic Strategy also considers the replacement of plastics in fertilizers with biodegradable polymers by 2026.

The main takeaway messages for webinar attendees is to improve monitoring and standards as a means of regulating sewage sluge, whilst also incentivising the development of non-plastic alternative materials.

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