Welland catchment native India Grice has just graduated from the University of Sheffield with a 1st class BSc in Geography. During her degree the issue of plastic pollution in freshwater environments caught her attention and she used her thesis to investigate if plastic pollution was a cause for concern in her local river. In this blog post she explains her rationale, methods and results…
Microplastic pollution in freshwater ecosystems is an increasingly alarming environmental issue. It is estimated by researchers that 80% of marine plastic debris reaches the open ocean through riverine transport. It may come as a surprise that the majority of microplastic research focuses on marine rather than freshwater environments when they are considered as such significant contributors of pollution in the open ocean.
Due to a lack of available study data concerning microplastic pollution in freshwater environments, the immediate sources of microplastics are not widely understood. Nevertheless, their identification may be the key to solving one of the greatest environmental problems we face today.
Microplastic pollution sources in river catchments may be grouped simply into rural or urban sources. Some urban sources may be quite obvious – the breakdown of larger plastic debris on riverbanks and the washing of microplastic particles through storm drains by surface water processes are well researched. Others, such as atmospheric transport of clothes fibres and windblown microplastic particles are gaining more attention in environmental research. Rural sources have been explored to a much lesser extent, however the potential impact of the application of sewage sludge as fertiliser on agricultural land is gaining more attention. Although research concerning microplastic pollution sources is ever expanding, there is still much to learn about their impact on freshwater ecosystems.
A lack of understanding of the harmful effects of microplastics is alarming, particularly when considering the impacts on freshwater fauna. There is particular rising concern regarding the ingestion of microplastic particles by fish and the transfer of particles through trophic levels and into the human diet. This problem is exaggerated by the microscopic size of most microplastic particles that in turn affects their bioavailability. Smaller microplastics are much more likely to be ingested by lower trophic organisms due to their reduced selectivity of particles within the water column. It is likely that this results in greater effects further down the food chain, but much more research is required to fully understand the impact across trophic levels of freshwater fauna.
To address these gaps in microplastic research, I conducted a multi-hypotheses investigation of microplastic concentration trends in sediments from the River Welland. This study aimed to investigate trends in microplastic shape, size, mass and distribution, and to use the findings to investigate potential rural and urban microplastic pollution sources in the River Welland catchment.
Sediment samples were collected in triplicate from ten sample sites along the River Welland, beginning in Lubenham and terminating at Ketton. Market Harborough was selected as the focus of “urban activity” for the purpose of the study. One site was selected before Market Harborough and four within; these were determined as “upstream” of urban activity. Urban sites were selected based on their proximity to known urban microplastic sources identified in microplastic research, such as at the River Welland’s confluence with the River Jordan, within the busy Welland Park area and close to the popular St. Mary’s shopping precinct. Five sites were selected “downstream” of Market Harborough to investigate spatial trends of microplastic pollution downstream of urban sources, and to further investigate rural sources. Rural samples were collected along the River Welland as it flowed through Cottingham, Gretton, Harringworth, Wakerley and Ketton respectively.
The research provided evidence that both urban and rural sources have significant impacts on microplastic concentrations in the River Welland catchment. There were very clear trends of greater microplastic masses closer to pollution sources where particles were yet to experience degradation due to their limited exposure to the processes that breakdown microplastics. Small, rounded microplastic particles were the most abundant type and increased in concentrations downstream from urban activity resulting from their degradation during riverine transport. The most likely sources of microplastic pollution in the River Welland catchment are attributed to surface runoff through storm drains in urban areas, atmospheric fallout from domestic waste and clothing fibres and sewage sludge. Interestingly, industrial activity influenced microplastic concentrations to a lesser extent than has been reported in previous studies.
Ultimately, microplastic pollution in freshwater environments is a global issue with harmful effects on aquatic fauna and a known potential to be transported to the open ocean. Microplastic concentrations in the River Welland catchment are influenced by a combination of degradation processes, riverine transport, hydrodynamic effects and sedimentation rates. Through attempts made to identify exact sources of microplastic pollution in river catchments and an increasing awareness amongst authorities and the public, future attempts to mitigate the environmental impact of plastic pollution can begin to be explored.