Can aquatic plants trap microplastics in chalk rivers and streams?
Postgraduate research student, Perline Bastid, explains her research journey into microplastic pollution — from learning about marine litter to a PhD studying the interactions between aquatic plants and microplastics in rivers.
What inspired you to research the impacts of plastics?
Before joining the University of Portsmouth to study MSc Coastal and Marine Resource Management in 2018, I studied my undergraduate degree in biology and ecology at the University of Montpellier in France.
My studies taught me about the risks posed by human activities on marine ecosystems. The effects of plastic pollution particularly interested and concerned me. During my Master’s degree at Portsmouth, I learned more about the complexity and potential harm posed by microplastics, which made me want to investigate the topic further.
Natural traps for microplastics
During my MSc, I studied how seagrass can act as a microplastics trap in coastal environments by accumulating plastic particles in their beds. While studying seagrass, my attention was drawn to the water-crowfoot (Ranunculus) growing in Hampshire’s chalk streams.
I started wondering whether these aquatic plants might also act as trapping structures for microplastics drifting in rivers. As watercourses play a significant role in the transport of microplastics from land to the oceans, the trapping of plastic particles by aquatic plants could be an important phenomenon influencing downstream discharge of plastics into the sea.
I became really interested in microplastic traps in rivers, and the implications this phenomenon could have. This led me to shift my focus from a marine perspective to freshwater for my PhD. I am now investigating the capacity for the water-crowfoot plant to act as a microplastic retention structure in chalk streams.
The River Meon is a typical Hampshire chalk stream with water crowfoot growing along on the riverbed
Water crowfoot has pretty white flowers and its dense vegetation may trap microplastics
Why are chalk streams important?
Chalk rivers and streams are an important and rare habitat, home to a wide range of plants and animals. They are characterised by their crystal clear water.
Chalk streams are highly vegetated watercourses. Submerged aquatic plants can cover large portions of the stream bed. These plants play an active role in the physical and biological dynamics of the stream. As a result of their abundance, the effect of aquatic plants on microplastic transport is likely to be substantial.
The focus of my research is the River Itchen — a typical chalk river, designated as a Site of Special Scientific Interest (SSSI) and dominated throughout by water-crowfoot. The river flows from mid-Hampshire, arising near the town of New Alresford, through to the city of Southampton, where it meets the Solent. The river passes through other urban areas, such as Winchester, making it a highly relevant location to study microplastic-plant interactions.
Chalk streams in Hampshire are protected habitats due to the amount of wildlife they support
Chalk streams are an important breeding ground for fish, such as brown trout
What threats do microplastics pose to chalk streams?
In chalk streams, water-crowfoot beds create important habitats that support a rich diversity of invertebrates and fish. If these aquatic plants retain microplastics in their shoots or sediments, it could increase local levels of microplastic contamination.
The negative impacts from microplastics have been documented for a number of freshwater species of macroinvertebrates, crustaceans, algae, and fish. Plastic pollutants could have potential adverse effects on the chalk stream’s wildlife and the health of the ecosystem as a whole. We need to better understand the behaviour and presence of microplastics in chalk streams in order to protect these important habitats.
The negative impacts from microplastics have been documented for a number of freshwater species of invertebrates, crustaceans, algae, and fish. Plastic pollutants could have potential adverse effects on the chalk stream’s wildlife and the health of the ecosystem as a whole.
Testing plastics in the University’s hydrology labs
During the first year of my PhD, I defined a set of field and laboratory experiments and analysis.
Chalk streams in Hampshire are protected habitats, noted for their importance for nature conservation. They are protected by several organisations including Natural England and the Test and Itchen Association. I’ve made contact with relevant organisations to inform them of my project and to get the necessary authorizations to do field work and collect samples from the River Itchen.
I identified knowledge gaps by doing a literature review. As a result, the project evolved to take a hydrodynamic perspective. Tests were performed in the flume of the hydrological laboratory at the University. This was to prepare for future simulations of water-crowfoots trapping microplastics under controlled conditions.
The next step is to prepare the material needed to start the lab experiments. For example, the particles used for the trapping simulations in the flume will be made using the cryo-milling equipment at the University’s Centre for Enzyme Innovation.
What impact do you hope your microplastics research will have?
My research will generate information on the effect of the water-crowfoot on microplastic contamination in chalk streams. In particular, the biological and physical conditions that can trigger microplastic trapping by water-crowfoot beds. The study will also reveal the types of plastic particles that become trapped, and whether they are more likely to be trapped during a particular season.
My research could have a direct impact on the development of models on plastic particle transport in vegetated rivers. It could also enhance understanding of the processes influencing microplastic behaviour in streams. Importantly, the results should indicate whether the water-crowfoot can significantly reduce downstream emissions of plastic particles in marine environments.
The research will also develop knowledge on the exposure of fauna and flora to microplastic pollution in chalk streams. This will have implications for the description of habitats most at risk from contamination. It will also provide advice for future field monitoring of plastic debris in rivers.